protozoa


Introduction
The taxonomic kingdom Protista is a collection of single celled organisms that do not fit into any other category. Protists are a group made up of protozoa, unicelular algae and slime molds. We will concentrate on the animal portion of this group: the protozoa (proto=first, zoa=animals). Protozoa are the oldest known group of heterotrophic life-that consume and transforms complex food particles into energy. Although protozoans are only made up of a single cell these organisms manage to perform all the basic tasks of life.

The protozoa are divided into four major groups: the ciliates, the flagellates, the heliozoans, and the amoebas.
Morphology (General)
Ciliates:Ciliophora

Structure in this group is fairly diverse but almost all species retain a few basic characters which helps to identify them as ciliates. The first character is the presence of cilia on at least one developmental stage of the organism. Modifications from the full body covered norm include a single ring of cilia, grouped ciliary organelles called cirri, and restriction of cilia to feeding tentacles. Most species also bear toxicysts that are most likely used to capture and stun prey. These toxicysts can be found around the mouth, along the length of tentacles or anywhere else on the surface of the cell body.
Flagellates Zoomastigophora)

Flagellates are characterized by having one or more flagella. Parasitic species generally have more flagella than those that are free living.
Amoebas: Sarcodina

Amoebas can reach a maximum size of 2 mm in diameter. These protozoans are constantly changing shape; they look and move much like balloons half filled with water. When manipulating a water balloon you can force most of the water to one end or hold it so different sections squeeze out between your fingers. Amoebas change shape like that, only the forces are internal. They can create extensions of their body wall called pseudopodia that help them locomote or capture prey or simply churn up their insides to distribute nutrients. The shape of a pseudopod is generally reflective of the family grouping to which it belongs.
Freshwater radiolarans: Heliozoa

The most identifiable characteristic of the heliozoans is the presence of axopodia. This is a type of pseudopod strengthened by tiny microtubules that extend into solid protective rods. Some marine heliozoans (radiolarians) have a protective exoskeleton of silica, but freshwater species just have tiny silica scales or a perforated capsule.
Morphology (Structures)


All five groups of protozoans include some sessile species but most are swimmers. Ciliates use their many tiny cilia, in controlled waves, to propel themselves through the water. Flagellates have a single posterior flagella that pushes them forward in much the same way as a motor boat uses its propeller. Amoebas locomote by shifting cytoplasm inside their bodies to create pseudopods which slowly pull the organisms along. Finally, heliozoans combine the efforts of cilia and axiopods to maneuver their way through the water.

All protozoans have chemical or tactile senses to detect other members of their own species for sexual reproduction, but many of these chemicals have not yet been studied in detail. A sensory structure has been identified in ciliates. Kineties (found beneath the surface of the cell membrane at the base of each cilia), are organized in a brushlike formation at the mouth, are used for prey recognition.
Metabolism

Because they are so tiny, protozoans do not need any specialized organelle, such as red blood cells, to meet their oxygen demand. In fact, many can live in water with very low concentrations of oxygen. Some ciliates have specially adapted green algae living inside them. In higher light conditions these algae convert the carbon dioxide produced by the ciliate into oxygen, ensuring an abundant internal supply of oxygen for the ciliate. On the flip side, a few groups are anaerobic and intolerant of oxygenated water. These organisms are often endosymbionts living in the digestive system of multi-celled animals.

Protists use contractile vacuoles to remove excess water from their cells. If the contractile function of a cell is compromised, the cell swells until it ruptures. The same will also happen to a marine protozoan when placed in freshwater; marine members have no contractile vacuoles. Ciliates have permanent contractile vacuole pathways and pores where amoebas will release them from any point along the surface of its body.
Reproduction/Development

Many protozoans reproduce both asexually and sexually during their lifetime. The move to sex is often either controlled by an internal clock or by the arrival of harsh environmental conditions.

The majority of protozoans reproduce asexually by binary fission. However, some are endosymbionts (species that live within another organism) often engage in multiple fission with many tiny cells produced from a single parent cell released to search out a new host.

Sexual reproduction is common in ciliates, but rare in heliozoans and amoebas, and absent in flagellates. The three basic types of sex are gametogamy, autogamy, and conjugation—all of which are explained on the reproduction strategies page.

Ciliates reproduce sexually through conjugation which involves the exchange of haploid nuclei between two joined protists. Once the genetic information is exchanged each of the ex-conjugants clones itself. These resulting daughter cells go through a long period of "sexual immaturity" where they will only reproduce asexually.
Ecology

Flagellates employ their flagella for both swimming and acquiring food. Sessile or colony forming members of the collared flagellates use their flagella to create a water current to draw small food particles, such as water-borne bacteria. These food particles are then trapped on mucous-coated microvilli (peaks and valleys on the cell membrane which increase the surface area of the cell for the purpose of absorption).

Large amoebas eat algae, other protists and some tiny multicelled animals, while smaller amoebas feed on bacteria. Amoebas ingest particles by phagocytosis. They wrap themselves around the food particle and once enclosed it is embedded within a food vacuole for digestion. Amoebas can capture food with pseudopods made of any outer area of the cells, so their whole body surface is a potential mouth! The same is true for pinocytosis (the "drinking" of organic substances) and the release of wastes that are contained in contractile vacuoles.

Ciliates have toxicysts which they fire at their prey to subdue it. Sessile forms (e.g. Suctioria) use haptocysts on feeding tentacles to snag smaller ciliate prey and then suck out the nutritious cytoplasm.

Heliozoans engulf any organism ranging from picoplankton to copepods. Extrusomes at the base of the axiopoda secrete cytoplasm over the axiopoda. Food sticks to the cytoplasm, and the flow of the liquid brings the food towards the cell where pseudopods reach out to grab it.
Idiosyncratic inverts

Researchers working with the ciliate Paramecium have discovered that protozoans have a very high mortality rate after conjugation. This is attributed to the large number of mutations which accumulate during long bouts of asexual reproduction. Also the exchange of this information is stressful to the acceptor. Some conjugants will also transmit microscopic prokaryotic endoparasites to the acceptor. These endoparasites allow the new host to clone for a limited amount of time, in order for the prokaryotes to distribute in new hosts. Immediately afterwards, the parasite kills the host.

pdf notes for biology
http://decapoda.arthroinfo.org/pdfs/25253/25253.pdf

__________________
LivE lIke ALI
dIe liKe HUSSAIN

The nematodes (pronounced /ˈnɛmətoʊdz/) or roundworms (phylum Nematoda) are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described,[1] of which over 16,000 are parasitic. It has been estimated that the total number of nematode species might be approximately 1,000,000.[2] Unlike cnidarians or flatworms, roundworms have a digestive system that is like a tube with openings at both ends.

Taxonomy and systematics

The group was originally defined by Karl Rudolphi in 1808[4] under the name Nematoidea, from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'like'). The vernacular word "nematode" is a corruption of this taxon, reclassified as family Nematodes by Burmeister in 1837[4] and order Nematoda by K. M. Diesing in 1861.[4]

At the origin, the "Nematoidea" included both roundworms and horsehair worms. Along with Acanthocephala, Trematoda and Cestoidea, it formed the group Entozoa.[5] The first differentiation of roundworms from horsehair worms, though erroneous, is due to von Siebold (1843) with orders Nematoidea and Gordiacei (Gordiacea). They were classed along with Acanthocephala in the new phylum Nemathelminthes (today obsolete) by Gegenbaur (1859). Then the taxon Nematoidea has been promoted to the rank of phylum by Ray Lankester (1877) including the family Gordiidae (horsehair worms). In 1919, Nathan Cobb proposed that roundworms should be recognized alone as a phylum. He argued that they should be called nema(s) in English rather than "nematodes"[6] and defined the taxon Nemates (Latin plural of nema). For ITIS, the taxon Nematoda is invalid.[7] Since Cobb was the first to exclude all but nematodes from the group, the valid taxon should be Nemates Cobb 1919 or Nemata Cobb 1919.

Phylogeny
The relationships of the nematodes and their close relatives among the protostomian Metazoa are unresolved. Traditionally, they were held to be a lineage of their own, but in the 1990s it was proposed that they form a clade together with moulting animals such as arthropods. This group has been named Ecdysozoa. However, the monophyly of the Ecdysozoa was never unequivocally accepted: while most researchers consider at least the placement of arthropods as more distant relatives of annelids — with which they were formerly united — to be warranted, the presumed close relationships of the nematodes and relatives with the arthropods has been a major point of contention.[8]

Even though the amount of data since accumulated in regard to this problem is staggering, the situation seems if anything less clear these days. DNA sequence data, initially strongly supporting the Ecdysozoa hypothesis, has become rather equivocal on ecdysozoan monophyly, and is simply unable to refute either a close or a more distant relationship between the arthropod and nematode lineages. That the roundworms have a large number of peculiar apomorphies and in many cases a parasitic lifestyle confounds morphological analyses. Genetic analyses of roundworms[citation needed] suggest that — as is also indicated by their unique morphological features — the group has been under intense selective pressure during its early radiation, resulting apparently in accelerated rates of both morphological and molecular evolution. Furthermore, no distinctive apomorphies of Ecdysozoa are known; even moulting has recently been confirmed to occur outside the presumed clade.[8]

Conversely, the identity of the closest living relatives of the Nematoda has always been considered to be well resolved. Morphological characters and molecular phylogenies agree with placement of the roundworms as sister taxon to the parasitic horsehair worms (Nematomorpha); together they make up the Nematoida. Together with the Scalidophora (formerly Cephalorhyncha), the Nematoida form the Introverta. It is entirely unclear whether the Introverta are, in turn, the closest living relatives of the enigmatic Gastrotricha; if so, they are considered a clade Cycloneuralia, but there is much disagreement both between and among the available morphological and molecular data. The Cycloneuralia or the Introverta — depending on the validity of the former — are often ranked as a superphylum.[8]


Nematode systematics
Due to the lack of knowledge regarding many nematodes, their systematics is contentious. Traditionally, they are divided into two classes, the Adenophorea and the Secernentea, and initial DNA sequence studies[verification needed] suggested the existence of five clades:[9]

* Dorylaimia
* Enoplia
* Spirurina
* Tylenchina
* Rhabditina

As it seems, the Secernentea are indeed a natural group of closest relatives. But the "Adenophorea" appear to be a paraphyletic assemblage of roundworms simply retaining a good number of ancestral traits. The old Enoplia do not seem to be monophyletic either but to contain two distinct lineages. The old group "Chromadoria" seem to be another paraphyletic assemblage, with the Monhysterida representing a very ancient minor group of nematodes. Among the Secernentea, the Diplogasteria may need to be united with the Rhabditia. while the Tylenchia might be paraphyletic with the Rhabditia.[10]

The understanding of roundworm systematics and phylogeny as of 2002 is summarised below:

Phylum Nematoda

* Basal order Monhysterida
* Class Dorylaimea
* Class Enoplea
* Class Secernentea
o Subclass Diplogasteria (disputed)
o Subclass Rhabditia (paraphyletic?)
o Subclass Spiruria
o Subclass Tylenchia (disputed)
* "Chromadorea" assemblage


Anatomy
Nematodes are slender, worm-like animals, typically less than 2.5 millimetres (0.10 in) long. The smallest nematodes are microscopic, while free-living species can reach as much as 5 centimetres (2.0 in) and some parasitic species are larger still. The body is often ornamented with ridges, rings, warts, bristles or other distinctive structures.[11]

The head of a nematode is relatively distinctive. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical, with sensory bristles and, in many cases, solid head-shields radiating outwards around the mouth. The mouth has either three or six lips, which often bear a series of teeth on their inner edge. An adhesive caudal gland is often found at the tip of the tail.[11]

The epidermis is either a syncytium or a single layer of cells, and is covered by a thick collagenous cuticle. The cuticle is often of complex structure, and may have two or three distinct layers. Underneath the epidermis lies a layer of muscle cells. Projections run from the inner surface of these cells towards the nerve cords; this is a unique arrangement in the animal kingdom, in which nerve cells normally extend fibres into the muscles rather than vice versa.[11]

The muscle layer surrounds the body cavity, which is filled with a fluid that lacks any form of blood cells. The gut runs down the centre of the cavity.[11]

Digestive system
The oral cavity is lined with cuticle, which is often strengthened with ridges or other structures, and, especially in carnivorous species, may bear a number of teeth. The mouth often includes a sharp stylet which the animal can thrust into its prey. In some species, the stylet is hollow, and can be used to suck liquids from plants or animals.[11]

The oral cavity opens into a muscular sucking pharynx, also lined with cuticle. Digestive glands are found in this region of the gut, producing enzymes that start to break down the food. In stylet-bearing species, these may even be injected into the prey.[11]

There is no stomach, with the pharynx connecting directly to the intestine that forms the main length of the gut. This produces further enzymes, and also absorbs nutrients through its lining. The last portion of the intestine is lined by cuticle, forming a rectum which expels waste through the anus just below and in front of the tip of the tail. The intestine also has valves or sphincters at either end to help control the movement of food through the body.[11]

Excretory system
Nitrogenous waste is excreted in the form of ammonia through the body wall, and is not associated with any specific organs. However, the structures for excreting salt to maintain osmoregulation are typically more complex.[11]

In many marine nematodes, there are one or two unicellular renette glands that excrete salt through a pore on the underside of the animal, close to the pharynx. In most other nematodes, these specialised cells have been replaced by an organ consisting of two parallel ducts connected by a single transverse duct. This transverse duct opens into a common canal that runs to the excretory pore.[11]

Nervous system
Four nerves run the length of the body on the dorsal, ventral, and lateral surfaces. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.[11]

At the anterior end of the animal, the nerves branch from a dense circular nerve ring surrounding the pharynx, and serving as the brain. Smaller nerves run forward from the ring to supply the sensory organs of the head.[11]

The body of nematodes is covered in numerous sensory bristles and papillae that together provide a sense of touch. Behind the sensory bristles on the head lie two small pits, or amphids. These are well supplied with nerve cells, and are probably chemoreception organs. A few aquatic nematodes possess what appear to be pigmented eye-spots, but is unclear whether or not these are actually sensory in nature.[11]
Reproduction
Most nematode species are dioecious, with separate male and female individuals. Both sexes possess one or two tubular gonads. In males, the sperm are produced at the end of the gonad, and migrate along its length as they mature. The testes each open into a relatively wide sperm duct and then into a glandular and muscular ejaculatory duct associated with the cloaca. In females, the ovaries each open into an oviduct and then a glandular uterus. The uteri both open into a common vagina, usually located in the middle of the ventral surface.[11]

Reproduction is usually sexual. Males are usually smaller than females (often much smaller) and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm. Nematode sperm is thought to be the only eukaryotic cell without the globular protein G-actin.

Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. A few species are known to be ovoviviparous. The eggs are protected by an outer shell, secreted by the uterus. In free-living roundworms, the eggs hatch into larvae, which appear essentially identical to the adults, except for an under-developed reproductive system; in parasitic roundworms, the life cycle is often much more complicated.[11]

Nematodes as a whole possess a wide range of modes of reproduction.[12] Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida: intrauterine birth causing maternal death.[13] Some nematodes are hermaphroditic, and keep their self-fertilized eggs inside the uterus until they hatch. The juvenile nematodes will then ingest the parent nematode. This process is significantly promoted in environments with a low or reducing food supply.[13]

The nematode model species Caenorhabditis elegans and C. briggsae exhibit androdioecy, which is very rare among animals. The single genus Meloidogyne (root-knot nematodes) exhibit a range of reproductive modes including sexual reproduction, facultative sexuality (in which most, but not all, generations reproduce asexually), and both meiotic and mitotic parthenogenesis.

The genus Mesorhabditis exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum. Contact with the sperm is essential for the ovum to begin dividing, but because there is no fusion of the cells, the male contributes no genetic material to the offspring, which are essentially clones of the female.[11]

Agriculture and horticulture
Depending on the species, a nematode may be beneficial or detrimental to plant health.

From agricultural and horticulture perspectives, there are two categories of nematode: predatory ones, which will kill garden pests like cutworms, and pest nematodes, like the root-knot nematode, which attack plants and those that act as vectors spreading plant viruses between crop plants.

Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control.

There are some fungi that are predators on soil nematodes. They set enticements for the nematodes, set traps for them in the form of lassos and eventually capture and devour them.

Rotations of plants with nematode resistant species or varieties is one means of managing parasitic nematode infestations. For example, marigolds, grown over one or more seasons (the effect is cumulative), can be used to control nematodes.[15] Another is treatment with natural antagonists such as the fungus gliocladium roseum. Chitosan is a natural biocontrol that elicits plant defense responses to destroy parasitic cyst nematodes on roots of sobyean, corn, sugar beets, potatoes and tomatoes without harming beneficial nematodes in the soil.[16] Furthermore soil steaming is an efficient method to kill nematodes before planting crop.

CSIRO has found[17] that there was 13- to 14-fold reduction of nematode population densities in plots having Indian mustard (Brassica juncea) green manure or seed meal in the soil.

Hundreds of Caenorhabditis elegans were featured in a research project on NASA's STS-107 space mission (which ended in the Space Shuttle Columbia Disaster).[18]

__________________
LivE lIke ALI
dIe liKe HUSSAIN

Annelid

The annelids (also called "ringed worms"), formally called Annelida (from French annelés "ringed ones", ultimately from Latin anellus "little ring"[2]), are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments. Although most textbooks still use the traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech-like species, research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of protostomes that also includes molluscs, brachiopods, flatworms and nemerteans.

The basic annelid form consists of multiple segments, each of which has the same sets of organs and, in most polychaetes, a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly-defined or absent in some, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles to act as hearts. The septa of these species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been studied. The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvae, which live as plankton and then sink and metamorphose into miniature adults. Oligochaetes are full hermaphrodites and produce a ring-like cocoon round their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.

Earthworms support terrestrial food chains both as prey and by aerating and enriching soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-letting is no longer in favor with doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.

Since annelids are soft-bodied, their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about 299 million years ago. Scientists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest certain fossils of the group appear in the Tertiary period, which began 65 million years ago.
Classification and diversity

There are over 17,000 living annelid species,[3] ranging in size from microscopic to the Australian giant Gippsland earthworm, which can grow up to 3 metres (9.8 ft) long.[4][5] Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids,[6][7] most textbooks use the traditional classification into the following sub-groups:[4][8]

* Polychaetes (about 12,000 species[3]). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes have parapodia that function as limbs, and nuchal organs ("nuchal" means "on the neck") that are thought to be chemosensors.[4] Most are marine animals, although a few species live in fresh water and even fewer on land.[9]

An earthworm's clitellum

* Clitellates (about 5,000 species[3]). These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") round their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch.[8][10] The clitellates are sub-divided into:[4]
o Oligochaetes ("with few hairs"), which

includ

*
o es earthworms. Oligochaetes have a sticky pad in the roof of the mouth.[4] Most are burrowers that feed on wholly or partly decomposed organic materials.[9]
o Hirudinea, whose name means "leech-shaped" and whose best known members are leeches.[4] Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators.[9] They have suckers at both ends of their bodies, and use these to move rather like inchworms.[11]

The Archiannelida, minute annelids that live in the spaces between grains of sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes.[8] Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:

* Pogonophora / Siboglinidae were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate phylum, Pogonophora, or as two phyla, Pogonophora and Vestimentifera. More recently they have been re-classified as a family, Siboglinidae, within the polychaetes.[9][12]
* The Echiura have a checkered taxonomic history: in the 19th century they were assigned to the phylum "Gephyrea", which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but a molecular phylogenetics analysis in 1997 concluded that Echiurans are annelids.[3][12][13]
* Myzostomida live on crinoids and other echinoderms, mainly as parasites. In the past they have been regarded as close relatives of the trematode flatworms or of the tardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes.[9] However, another analysis in 2002 suggested that myzostomids are more closely related to flatworms or to rotifers and acanthocephales.[12]

Distinguishing features
No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent.[4][8] Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen[4] and does not molt[14] – on the other hand arthropods' cuticles are made of the more rigid α-chitin,[4][15] and molt until the arthropods reach their full size.[16] Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.[14]
Description
Segmentation

Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external chaetae (Greek χαιτα, meaning "hair") and, in some species, appendages. However, the frontmost and rearmost sections are not regarded as true segments as they do not contain the standard sets of organs and do not develop in the same way as the true segments. The frontmost section, called the prostomium (Greek προ- meaning "in front of" and στομα meaning "mouth") contains the brain and sense organs, while the rearmost, called the pygidium (Greek πυγιδιον, meaning "little tail") contains the anus, generally on the underside. The first section behind the prostomium, called the peristomium (Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not a true segment, but in some polychaetes the peristomium has chetae and appendages like those of other segments.[4]

The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called teloblastic growth.[4] Some groups of annelids, including all leeches,[11] have fixed maximum numbers of segments, while others add segments throughout their lives.[8]

The phylum's name is derived from the Latin word annelus, meaning "little ring".[3]
Body wall, chetae and parapodia
1 O Nephridiopore
2 Nephridium
3 Cuticle
4 Circular muscle
5 Longitudinal muscle
6 Peritoneum
7 Gut
8 Blood vessel
9 Nerve cord(s)
10 Coelom
Cross-section through a typical annelid[4][8]

Annelids' cuticles are made of collagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and mucus-secreting glands in the epidermis protect their skins.[4] Under the epidermis is the dermis, which is made of connective tissue, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of the coelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips,[14] whose contractions make the segment shorter and fatter.[4] Some annelids also have oblique internal muscles that connect the underside of the body to each side.[14]

The chetae ("hairs") of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The parapodia ("limbs") of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked.[4] Chetae are made of moderately flexible β-chitin and are formed by follicles, each of which has a chaetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming microvilli, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta.[4] Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.[4]

Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia) and gills.[14]
Nervous system and senses

The brain generally forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.[4] The brains of polychaetes are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the peristomium.[24] In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.[14] The rest of the central nervous system is generally "ladder-like", consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body.[4] However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling genus Owenia the single nerve chord has no ganglia and is located in the epidermis.[8][25]

As in arthropods, each muscle fiber (cell) is controlled by more than one neuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. Vertebrates have a different system, in which one neuron controls a group of muscle fibers.[4] Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement.[4]

The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body.[4] Nuchal ("on the neck") organs are paired, ciliated structures found only in polychaetes, and are thought to be chemosensors.[14] Some polychaetes also have various combinations of ocelli ("little eyes") that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images.[25] The compound eyes probably evolved independently of arthropods' eyes.[14] Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withraw into their tubes.[25] Some burrowing and tube-dwelling polychaetes have statocysts (tilt and balance sensors) that tell them which way is down.[25] A few polychaete genera have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers".[14]
Coelom, locomotion and circulatory system

Most annelids have a pair of coeloms (body cavities) in each segment, separated from other segments by septa and from each other by vertical mesenteries. Each septum forms a sandwich with connective tissue in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side. Each mesentery is similar except that the mesothelium is the lining of each of the pair of coeloms, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.[4] The mesothelium is made of modified epitheliomuscular cells,[4] in other words their bodies form part of the epithelium but their bases extend to form muscle fibers in the body wall.[26] The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates: producing and storing glycogen and fat; producing the oxygen-carrier hemoglobin; breaking down proteins; and turning nitrogenous waste products into ammonia and urea to be excreted.[4]

Peristalsis moves this "worm" to the right

Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body),[4] or flex the body while using parapodia to crawl or swim.[27] In these animals the septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon".[4] However, the septa are often incomplete in annelids that are semi-sessile or that do not move by peristalsis or by movements of parapodia – for example some move by whipping movements of the body, some small marine species move by means of cilia (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it.[4]

The fluid in the coeloms contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a respiratory pigment – red hemoglobin in some species, green chlorocruorin in others[14] – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the blood plasma. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of capillaries (fine blood vessels) in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients. Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of many earthworms some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly-developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen.[4]

However, leeches and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the Clitellata.[11] In leeches there are no septa, the connective tissue layer of the body wall is so thick that it occupies much of the body, and the two coeloms are widely separated and run the length of the body. They function as the main blood vessels, although they are side-by-side rather than upper and lower. However, they are lined with mesothelium, like the coeloms and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move like inchworms. The anus is on the upper surface of the pygidium.[11]
Respiration

In some annelids, including earthworms, all respiration is via the skin. However, many polychaetes and some clitellates (the group to which earthworms belong) have gills associated with most segments, often as extensions of the parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.[14]
Feeding and excretion

Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular pharynx that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two families, the Eunicidae and Phyllodocidae, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. Filter feeders use "crowns" of palps covered in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.[14]

The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the anus on the underside of the pygidium.[4] However, in members of the tube-dwelling family Siboglinidae the gut is blocked by a swollen lining that houses symbiotic bacteria, which can make up 15% of the worms' total weight. The bacteria convert inorganic matter – such as hydrogen sulfide and carbon dioxide from hydrothermal vents, or methane from seeps – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.[14]

Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia.[4] Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials as urine. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first – in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia.[28] In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.[4]
Reproduction and life cycle
Asexual reproduction
This sabellid tubeworm is budding

Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.[4][29] Some oligochaetes, such as Aulophorus furcatus, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding.[8][30] However, leeches have never been seen reproducing asexually.[8][31]

Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.[8][29] Annelids are the most complex animals that can regenerate after such severe damage.[32] On the other hand leeches cannot regenerate.[31]
Sexual reproduction
Apical tuft (cilia)
Prototroch (cilia)
Stomach
Mouth
Metatroch (cilia)
Mesoderm
Anus
/// = cilia
Trochophore larva[33]

It is thought that annelids were originally animals with two separate sexes, which released ova and sperm into the water via their nephridia.[4] The fertilized eggs develop into trochophore larvae, which live as plankton.[34] Later they sink to the sea-floor and metamorphose into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head); a small area round the trochophore's anus becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth).[4]

However, the lifecycles of most living polychaetes, which are almost all marine animals, are unknown, and only about 25% of the 300+ species whose lifecycles are known follow this pattern. About 14% use a similar external fertilization but produce yolk-rich eggs, which reduce the time the larva needs to spend among the plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch – some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch. These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have penes that inject sperm into the female.[34] There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.[34]

Some polychaetes breed only once in their lives, while others breed almost continuously or through several breeding seasons. While most polychaetes remain of one sex all their lives, a significant percentage of species are full hermaphrodites or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent gonads, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn.[34]

Most mature clitellates (the group that includes earthworms and leeches) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and all copulate. Earthworms store their partners' sperm in spermathecae ("sperm stores") and then the clitellum produces a cocoon that collects ova from the ovaries and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae.[34]
Ecological significance

Charles Darwin's book The Formation of Vegetable Mould through the Action of Worms (1881) presented the first scientific analysis of earthworms' contributions to soil fertility.[35] Some burrow while others live entirely on the surface, generally in moist leaf litter. The burrowers loosen the soil so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating the decomposition of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily.[36][37] Earthworms are also important prey for birds ranging in size from robins to storks, and for mammals ranging from shrews to badgers, and in some cases conserving earthworms may be essential for conserving endangered birds.[38]

Marine annelids may account for over one-third of bottom-dwelling animal species round coral reefs and in tidal zones.[35] Burrowing species increase the penetration of water and oxygen and water into the sea-floor sediment, which encourages the growth of populations of bacteria and small animals alongside their burrows.[39]

Although blood-sucking leeches do little direct harm to their victims, some transmit flagellates that can be very dangerous to their hosts. Some small tube-dwelling oligochaetes transmit myxosporean parasites that cause whirling disease in fish.[35]
Interaction with humans

Earthworms make a significant contribution to soil fertility.[35] The rear end of the Palolo worm, a marine polychaete that tunnels through coral, detaches in order to spawn at the surface, and the people of Samoa regard these spawning modules as a delicacy.[35] Anglers sometimes find that worms are more effective bait than artificial flies, and worms can be kept for several days in a tin lined with damp moss.[40] Ragworms are commercially important as bait and as food sources for aquaculture, and there have been proposals to farm them in order to reduce over-fishing of their natural populations.[39] Some marine polychaetes' predation on molluscs causes serious losses to fishery and aquaculture operations.[35]

Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.[35]

Accounts of the use of leeches for the medically dubious practise of blood-letting have come from China around 30 AD, India around 200 AD, ancient Rome around 50 AD and later throughout Europe. In the 19th century medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports, and Hirudo medicinalis is treated as an endangered species by both IUCN and CITES. More recently leeches have been used to assist in microsurgery, and their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which also prevents tumors from spreading.[35]

Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are biomineralized with calcium salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusual proteins that bind strongly to zinc.[41]
Evolutionary history
Fossil record

Since annelids are soft-bodied, their fossils are rare.[42] Polychaetes' fossil record consists mainly of the jaws that some species had and the mineralized tubes that some secreted.[43] Some Ediacaran fossils such as Dickinsonia in some ways resemble polychaetes, but the similarities are too vague for these fossils to be classified with confidence.[44] The small shelly fossil Cloudina, from 549 to 542 million years ago, has been classified by some authors as an annelid, but by others as a cnidarian (i.e. in the phylum to which jellyfish and sea anemones belong).[45] Until 2008 the earliest fossils widely accepted as annelids were the polychaetes Canadia and Burgessochaeta, both from Canada's Burgess Shale, formed about 505 million years ago in the early Cambrian.[46] Myoscolex, found in Australia and a little older than the Burgess Shale, was possibly an annelid. However, it lacks some typical annelid features and has features which are not usually found in annelids and some of which are associated with other phyla.[46] Then Simon Conway Morris and John Peel reported Phragmochaeta from Sirius Passet, about 518 million years old, and concluded that it was the oldest annelid known to date.[44] There has been vigorous debate about whether the Burgess Shale fossil Wiwaxia was a mollusc or an annelid.[46] Polychaetes diversified in the early Ordovician, about 488 to 474 million years ago. It is not until the early Ordovician that the first annelid jaws are found, thus the crown-group cannot have appeared before this date and probably appeared somewhat later.[1] By the end of the Carboniferous, about 299 million years ago, fossils of most of the modern mobile polychaete groups had appeared.[46] Many fossil tubes look like those made by modern sessile polychaetes, but the first tubes clearly produced by polychaetes date from the Jurassic, less than 199 million years ago.[46]

The earliest good evidence for oligochaetes occurs in the Tertiary period, which began 65 million years ago, and it has been suggested that these animals evolved around the same time as flowering plants in the early Cretaceous, from 130 to 90 million years ago.[47] A trace fossil consisting of a convoluted burrow partly filled with small fecal pellets may be evidence that earthworms were present in the early Triassic period from 251 to 245 million years ago.[47][48] Body fossils going back to the mid Ordovician, from 472 to 461 million years ago, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.[47][49]
Family tree
Annelida







some "Scolecida" and "Aciculata"










some "Canalipalpata"




Sipuncula, previously a separate phylum




Clitellata





some "Oligochaeta"






Hirudines (leeches)




some "Oligochaeta"






some "Oligochaeta"





Aeolosomatidae[50]






some "Scolecida" and "Canalipalpata"








some "Scolecida"




Echiura, previously a separate phylum





some "Scolecida"









some "Canalipalpata"






Siblonginidae, previously phylum Pogonophora




some "Canalipalpata"







some "Scolecida", "Canalipalpata" and "Aciculata"


Annelid groups and phyla incorporated into Annelida (2007; simplified).[6]
Highlights major changes to traditional classifications.

Traditionally the annelids have been divided into two major groups, the polychaetes and clitellates. In turn the clitellates were divided into oligochaetes, which include earthworms, and hirudinomorphs, whose best-known members are leeches.[4] For many years there was no clear arrangement of the approximately 80 polychaete families into higher-level groups.[6] In 1997 Greg Rouse and Kristian Fauchald attempted a "first heuristic step in terms of bringing polychaete systematics to an acceptable level of rigour", based on anatomical structures, and divided polychaetes into:[51]

* Scolecida, less than 1,000 burrowing species that look rather like earthworms.[52]
* Palpata, the great majority of polychaetes, divided into:
o Canalipalpata, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes.[52]
o Aciculata, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae).[52]

Also in 1997 Damhnait McHugh, using molecular phylogenetics to compare similarities and differences in one gene, presented a very different view, in which: the clitellates were an off-shoot of one branch of the polychaete family tree; the pogonophorans and echiurans, which for a few decades had been regarded as a separate phyla, were placed on other branches of the polychaete tree.[53] Subsequent molecular phylogenetics analyses on a similar scale presented similar conclusions.[54]

In 2007 Torsten Struck and colleagues compared 3 genes in 81 taxa, of which 9 were outgroups,[6] in other words not considered closely related to annelids but included to give an indication of where the organisms under study are placed on the larger tree of life.[55] For a cross-check the study used an analysis of 11 genes (including the original 3) in 10 taxa. This analysis agreed that clitellates, pogonophorans and echiurans were on various branches of the polychaete family tree. It also concluded that the classification of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the members of these alleged groups were scattered all over the family tree derived from comparing the 81 taxa. In addition, it also placed sipunculans, generally regarded at the time as a separate phylum, on another branch of the polychaete tree, and concluded that leeches were a sub-group of oligochaetes rather than their sister-group among the clitellates.[6] Rouse accepted the analyses based on molecular phylogenetics,[8] and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.[7]

In addition to re-writing the classification of annelids and 3 previously independent phyla, the molecular phylogenetics analyses undermine the emphasis that decades of previous writings placed on the importance of segmentation in the classification of invertebrates. Polychaetes, which these analyses found to be the parent group, have completely segmented bodies, while polychaetes' echiurans and sipunculan offshoots are not segmented and pogonophores are segmented only in the rear parts of their bodies. It now seems that segmentation can appear and disappear much more easily in the course of evolution than was previously thought.[6][53] The 2007 study also noted that the ladder-like nervous system, which is associated with segmentation, is less universal than previously thought in both annelids and arthropods.[6]
Bilateria



Acoelomorpha (Acoela and Nemertodermatida)






Deuterostomia (Echinoderms, chordates, etc.)

Protostomia



Ecdysozoa
(Arthropods, nematodes, priapulids, etc.)

Lophotrochozoa



Bryozoa






Annelida




Sipuncula




Mollusca




Phoronida and Brachiopoda




Nemertea




Dicyemida




Myzostomida

Platyzoa



Other Platyzoa






Gastrotricha




Platyhelminthes








Relationships of Annelids to other Bilateria:[54]
(Analysis produced in 2004, before Sipuncula were merged into Annelida in 2007[6])

Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals – the other is the deuterostomes, which includes vertebrates.[54] Within the protostomes, annelids used to be grouped with arthropods under the super-group Articulata ("jointed animals"), as segmentation is obvious in most members of both phyla. However, the genes that drive segmentation in arthropods do not appear to do the same in annelids. Arthropods and annelids both have close relatives that are unsegmented. It is at least as easy to assume that they evolved segmented bodies independently as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla.[54] The current view is that annelids are grouped with molluscs, brachiopods and several other phyla that have lophophores (fan-like feeding structures) and/or trochophore larvae as members of Lophotrochozoa.[56] Bryzoa may be the most basal phylum (the one that first became distinctive) within the Lophotrochozoa, and the relationships between the other members are not yet known.[54] Arthropods are now regarded as members of the Ecdysozoa ("animals that molt"), along with some phyla that are unsegmented.[54][57]

The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids, molluscs, nemerteans and flatworms, follow a similar pattern in the fertilized egg's development. When their cells divide after the 4-cell stage, descendants of these 4 cells form a spiral pattern. In these phyla the "fates" of the embryo's cells, in other words the roles their descendants will play in the adult animal, are the same and can be predicted from a very early stage.[58] Hence this development pattern is often described as "spiral determinate cleavage".[59]

__________________
LivE lIke ALI
dIe liKe HUSSAIN

Mollusca

The Mollusca, common name molluscs or mollusks,[note 1] is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. This is the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat.

The phylum Mollusca is typically divided into nine or ten taxonomic classes, of which two are extinct. The gastropods (snails and slugs) include by far the most classified species, accounting for 80% of the total. Cephalopod molluscs such as squid, cuttlefish and octopus are among the most neurologically advanced invertebrates. Either the giant squid or the colossal squid is the largest known species of animal without a backbone.

The two most universal features of the body structure of molluscs are a mantle with a significant cavity used for breathing and excretion, and the organization of the nervous system. Because of the great range of anatomical diversity, many textbooks base their descriptions on a hypothetical "generalized mollusc", with features common to many but not all classes within the Mollusca.

There is good evidence for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 542 to 488.3 million years ago. However, the evolutionary history both of the emergence of molluscs from the ancestral group Lophotrochozoa, and of their diversification into the well-known living and fossil forms, is still vigorously debated. The most abundant metallic element in molluscs is calcium.[2]

Molluscs have for many centuries been the source of important luxury goods, notably pearls, mother of pearl, Tyrian purple dye, and sea silk. Their shells have also been used as money in some pre-industrial societies.

There is a risk of food poisoning from toxins that accumulate in molluscs under certain conditions, and many countries have regulations that aim to minimize this risk. Blue-ringed octopus bites are often fatal, and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated.[3] Stings from a few species of large tropical cone shells can also kill. However, the sophisticated venoms of these cone snails have become important tools in neurological research and show promise as sources of new medications.

Schistosomiasis (also known as bilharzia, bilharziosis or snail fever) is transmitted to humans via water snail hosts, and affects about 200 million people. A few species of snails and slugs are serious agricultural pests, and in addition, accidental or deliberate introduction of various snail species into new territory has resulted in serious damage to some natural ecosystems.
Diversity

Estimates of accepted described living species of molluscs vary from 50,000 to a maximum of 120,000 species.[1] In 2009 Chapman estimated the number of described living species at 85,000.[1] Haszprunar in 2001 estimated about 93,000 named species,[4] which include 23% of all named marine organisms.[17] Molluscs are second only to arthropods in numbers of living animal species[7]—far behind the arthropods' 1,113,000 but well ahead of chordates' 52,000.[18] It has been estimated that there are about 200,000 living species in total,[1][19] and 70,000 fossil species,[20] although the total number of mollusc species that ever existed, whether or not preserved, must be many times greater than the number alive today.[21]

Molluscs have more varied forms than any other animal phylum. They include snails, slugs and other gastropods; clams and other bivalves; squids and other cephalopods; and other lesser-known but similarly distinctive sub-groups. The majority of species still live in the oceans, from the seashores to the abyssal zone, but some form a significant part of the freshwater fauna and the terrestrial ecosystems. Molluscs are extremely diverse in tropical and temperate regions but can be found at all latitudes.[22] About 80% of all known mollusc species are gastropods.[7] Cephalopoda such as squid, cuttlefish and octopus are among the neurologically most advanced of all invertebrates.[23] The giant squid, which until recently had not been observed alive in its adult form,[24] is one of the largest invertebrates. However a recently caught specimen of the colossal squid, 10 metres (33 ft) long and weighing 500 kilograms (0.49 LT; 0.55 ST), may have overtaken it.[25]

Freshwater and terrestrial molluscs appear exceptionally vulnerable to extinction. Estimates of the numbers of non-marine molluscs vary widely, partly because many regions have not been thoroughly surveyed. There is also a shortage of specialists who can identify all the animals in any one area to species. However, in 2004 the IUCN Red List of Threatened Species included nearly 2,000 endangered non-marine molluscs. For comparison, the great majority of molluscs species are marine but only 41 of these appeared on the 2004 Red List. 42% of recorded extinctions since the year 1500 are of molluscs, almost entirely non-marine species.
A "generalized mollusc"

Because of the enormous variations between groups of molluscs, many text books start the subject by describing a "generalized mollusc", which some suggest may resemble very early molluscs and which is rather similar to modern monoplacophorans.[5][10][22][30]

The generalized mollusc has a single, "limpet-like" shell on top. The shell is secreted by a mantle that covers the upper surface. The underside consists of a single muscular "foot".[30] The visceral mass, or visceropallium, is the soft, non-muscular metabolic region of the mollusc. It contains the body organs.[29
Mantle and mantle cavity
The mantle cavity is a fold in the mantle that encloses a significant amount of space. It is lined with epidermis. It is exposed, according to habitat, to sea, fresh water or air. The cavity was at the rear in the earliest molluscs but its position now varies from group to group. The anus, a pair of osphradia (chemical sensors) in the incoming "lane", the hindmost pair of gills and the exit openings of the nephridia ("kidneys") and gonads (reproductive organs) are in the mantle cavity.[30] The whole soft body of bivalves lies within an enlarged mantle cavity.[29]
Shell

The mantle edge secretes a shell (secondarily absent in a number of taxonomic groups, such as the nudibranchs[29]) that consists of mainly chitin and conchiolin (a protein) hardened with calcium carbonate),[30][35] except that the outermost layer in almost all cases is all conchiolin (see periostracum).[30] Molluscs never use phosphate to construct their hard parts,[36] with the questionable exception of Cobcrephora.[37] While most mollusc shells are composed mainly of aragonite, those gastropods that lay eggs with a hard shell use calcite (sometimes with traces of aragonite) to construct the eggshells.[38]

The shell consists of three layers : the outer layer (the periostracum) made of organic matter, a middle layer made of columnar calcite and an inner layer consisting of laminated calcite, that is often nacreous.[29]
Foot
The underside consists of a muscular foot, which has adapted to different purposes in different classes.[39]:4 The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus as a lubricant to aid movement. In forms that have only a top shell, such as limpets, the foot acts as a sucker attaching the animal to a hard surface, and the vertical muscles clamp the shell down over it; in other molluscs, the vertical muscles pull the foot and other exposed soft parts into the shell.[30] In bivalves, the foot is adapted for burrowing into the sediment;[39]:4 in cephalopods it is used for jet propulsion,[39]:4 and the tentacles and arms are derived from the foot.[40]
Multiple functions of organs
Molluscs organs are used for multiple functions. For example: the heart and nephridia ("kidneys") are important parts of the reproductive system[citation needed] as well as the circulatory and excretory systems; in bivalves, the gills both "breathe" and produce a water current in the mantle cavity, which serves both excretion and reproduction.
Circulation

Molluscs' circulatory systems are mainly open. Although molluscs are coelomates, their coeloms are reduced to fairly small spaces enclosing the heart and gonads. The main body cavity is a hemocoel through which blood and coelomic fluid circulate and which encloses most of the other internal organs. These hemocoelic spaces act as an efficient hydrostatic skeleton.[29] The blood contains the respiratory pigment hemocyanin as an oxygen-carrier. The heart consists of one or more pairs of atria (auricles), which receive oxygenated blood from the gills and pump it to the ventricle, which pumps it into the aorta (main artery), which is fairly short and opens into the hemocoel.[30]

The atria of the heart also function as part of the excretory system by filtering waste products out of the blood and dumping it into the coleom as urine. A pair of nephridia ("little kidneys") to the rear of and connected to the coelom extracts any re-usable materials from the urine and dumps additional waste products into it, and then ejects it via tubes that discharge into the mantle cavity.[30]
Respiration
Most molluscs have only one pair of gills, or even only one gill. Generally the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so that water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart.[30]
Eating, digestion, and excretion

Most molluscs have muscular mouths with radulae, "tongues" bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. The radula primarily functions to scrape bacteria and algae off rocks. This radula is associated with the odontophore, a cartilaginous supporting organ[29]

Molluscs mouths also contain glands that secrete slimy mucus, to which the food sticks. Beating cilia (tiny "hairs") drive the mucus towards the stomach, so that the mucus forms a long string.[30]

At the tapered rear end of the stomach and projecting slightly into the hindgut is the prostyle, a backward-pointing cone of feces and mucus, which is rotated by further cilia so that it acts as a bobbin, winding the mucus string onto itself. Before the mucus string reaches the prostyle, the acidity of the stomach makes the mucus less sticky and frees particles from it.[30]

The particles are sorted by yet another group of cilia, which send the smaller particles, mainly minerals, to the prostyle so that eventually they are excreted, while the larger ones, mainly food, are sent to the stomach's cecum (a pouch with no other exit) to be digested. The sorting process is by no means perfect.[30]

Periodically, circular muscles at the hindgut's entrance pinch off and excrete a piece of the prostyle, preventing the prostyle from growing too large. The anus is in the part of the mantle cavity that is swept by the outgoing "lane" of the current created by the gills. Carnivorous molluscs usually have simpler digestive systems.[30]

As the head has largely disappeared in bivalves, their mouth has been equipped with labial palps (two on each side of the mouth) to collect the detritus from its mucus.[29]
Nervous system

Molluscs have two pairs of main nerve cords (three in bivalves) the visceral cords serving the internal organs and the pedal ones serving the foot. Both pairs run below the level of the gut, and include ganglia as local control centers in important parts of the body. Most pairs of corresponding ganglia on both sides of the body are linked by commissures (relatively large bundles of nerves). The only ganglia above the gut are the cerebral ganglia, which sit above the esophagus (gullet) and handle "messages" from and to the eyes. The pedal ganglia, which control the foot, are just below the esophagus and their commissure and connections to the cerebral ganglia encircle the esophagus in a nerve ring.[30]

The brain, in species that have one, encircles the esophagus. Most molluscs have a head with eyes, and all have a pair of sensor-containing tentacles, also on the head, that detect chemicals, vibrations and touch.
Reproduction

The simplest molluscan reproductive system relies on external fertilization, but there are more complex variations. All produce eggs, from which may emerge trochophore larvae, more complex veliger larvae, or miniature adults. Two gonads sit next to the coelom, a small cavity that surrounds the heart and shed ova or sperm into the coloem, from which the nephridia extract them and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some molluscs use internal fertilization and/or are hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems.[30]

The most basic molluscan larva is a trochophore, which is planktonic and feeds on floating food particles by using the two bands of cilia round its "equator" to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus. New tissue grows in the bands of mesoderm in the interior, so that the apical tuft and anus are pushed further apart as the animal grows. The trochophore stage is often succeeded by a veliger stage in which the prototroch, the "equatorial" band of cilia nearest the apical tuft, develops into the velum ("veil"), a pair of cilia-bearing lobes with which the larva swims. Eventually the larva sinks to the seafloor and metamorphoses into the adult form. Whilst metamorphosis is the usual state in molluscs, the cephalopods differ in exhibiting direct development: the hatchling is a 'miniaturized' form of the adult.[42]

__________________
LivE lIke ALI
dIe liKe HUSSAIN