Phylum Annelida

Characteristics
Annelids were the first animals to evolve a complete coelom (an internal body cavity lined with epithelial tissue). They are segmented and have most comlete body systems.
Classification
Members of the class Polychaeta are marine worms. Members of the class Oligocheata are earthworms. Members of the class Hirudinea are parasitic worms.
Movement
Annelids have various muscle groups and simple appendages. They use setae and parapodia for movement.
Feeding & Digestion
Marine worms are filter feeders or scavengers. Earthworms squeeze organic material out of the earth. The digestive systems for all three classes are well-developed and use division of labor.
Circulation
Annelids are the first to have a closed system of blood vessels -- making pumping more efficient.
Excretion
Annelids have one pair of nephridia per segment for excretion.
Respiration
Respiration occurs through diffusion.
Nervous System
Members of this phylum have a simple brain located in the anterior end with ganglia in every segment. They can sense light, moisture, and chemicals.
Reproduction
Since annelids are hermaphroditic, sexual reproduction occurs through the exchange of sperm packets.

Segmented worms are placed in the phylum Annelida, which consists of over 12,000 species of segmented worms grouped into three classes. The three classes comprise freshwater worms and earthworms, marine worms, and leeches. Annelids have the unique ability to inhabit a variety of unusual places such as inside of other spineless or invertebrate animals (such as sponges) and they even live in cylinders or tubes that are actually produced by their own systems and discharged around their bodies. The Feather Duster Worm, for example, secretes a spacious leather-like tube while the Calcareous Tube Worm secretes a hard calcium carbonate tube. Annelids also live in other protected environments such as sand, muddy areas and rock chasms.
Annelid bodies are characterized by segments. Each segment contains a compartment of the coelom or body cavity, with its internal organs, and a part of the body wall. The body wall consists of tough muscles and short rigid hairs collectively called “setae.” These tough muscles are used for swimming from place to place and for crawling while the setae help to keep the annelid inside its cylinder or they help to grip dirt, sand and soil.
You can’t tell by looking at them, but Annelids do have a circulatory system, a digestive system and a nervous system. The blood of an annelid can be either red or green in color or no color at all - it depends on the species. The digestive system or gut can be found on the underside of annelids and it extends down the middle from the mouth to the head. Waste is released through openings or ducts throughout the body cavity. The nervous system is made up of sense organs including tentacles and taste buds as well as eyes and statocysts (organ of balance). Non-aquatic annelids do not have a respiratory system as respiration takes place through the body wall and some aquatic annelids do have gills for breathing.
So how do annelids reproduce? This can be a little confusing. Depending on the species, annelids can reproduce asexually or sexually. Asexual reproduction takes place through a method called fission. During fission, the rear end of the body splits from the rest of the body to form a new annelid. Earthworms do not have the ability to reproduce this way, but they do have the ability to regenerate their posterior segments. Sexual reproduction takes place by species that are hermaphroditic (some earthworms and many leeches) or species that have separate females and males. During sexual reproduction, fluids are transferred from the male pore to the female ovipore. Annelids tend to mate when conditions are moist or following a rain, meaning they may mate quite frequently throughout the year.

Phylum annelida

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The coelom is a fluid filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size. Coeloms are only ever present in triploblastic animals, though coelom is sometimes (incorrectly) used to refer to any developed digestive tract.
Functionally, a coelom can absorb shock or provide a hydrostatic skeleton. It also allows organs to grow independently of the body wall. This can be seen in the digestive tract of earthworms and other annelids, which is suspended within the body in a mesentery derived from a mesoderm-lined coelom. In mammals, the coelom forms the peritoneal, pleural, and pericardial cavities.
In the past, zoologists grouped animals based on characters related to the coelom. The presence or absence of a coelom and the way in which it was formed was believed to be important in understanding the phylogenetic relationships of animal phyla. However, recent molecular phylogenies have suggested this characteristics is not as informative as previously believed. Indeed, the coelom may have arisen twice, once in protostomes and once among the deuterostomes.

Which are the morphological features that differentiate the beings of the phylum Annelida from nematodes and platyhelminthes?
Platyhelminthes are worms with flat bodies (flatworms), nematodes are worms with cylindrical but not segmented bodies (roundworms). Annelids are cylindrical worms with segmented bodies (they are metameric).



What is the main evolutionary novelty presented by annelids?
The main evolutionary novelty presented by the beings of the phylum Annelida is the coelom, the internal body cavity totally covered by mesoderm, a feature also present in arthropods, molluscs, echinoderms and chordates. Platyhelminthes are acoelomate and nematodes are pseudocoelomate (their internal cavity is partially covered by mesoderm).
Another important evolutionary novelty of the annelids is the closed circulatory system.



What is the morphological characteristic that evolutionarily approximates the beings of the phylum Annelida to arthropods?
The metameric feature, i.e., the body segmentation in metameres, approximates annelids to arthropods since these animals are segmented beings too. (Bristles present in oligochaete and polychaete annelids are also covered with chitin, the same substance of the arthropod exoskeleton.)


How does digestion in beings of the phylum Annelida work and which type of digestive system do they have?
Digestion in beings of the phylum Annelida is extracellular. These animals have a complete digestive system, with mouth and anus.



Which are the characteristics and organs of the digestive system of earthworms related to the type of diet of these animals?
Earthworms eat decomposing organic material and small organisms ingested together with soil particles. The digestive tubes of earthworms have special structures, like a muscular wall and a gizzard, that triturate the food and scratch it against the ingested soil particles. Since annelid digestion is exclusively extracellular earthworms also present in the posterior part of their digestive system structures like the cecum and the typhlosole that have the function of increasing the absorption surface of the intestine.



The vascular lesions caused by leeches upon the blood vessels of their host cause blood naturally to coagulate. How does the leech solve this problem since it could be expected that the ingested blood would coagulate inside its body?
Ingested blood does not coagulate inside the leech (Hirudo medicinalis) because in its saliva there is a potent anticoagulant substance, a protein called hirudin.
In the past leeches were largely used as medical treatment. Nowadays hirudotherapy is being used in patients with extensive and chronic inflammation of the skin, in prevention against tissue necrosis after some surgeries and in several others fields of Medicine.

How is the respiratory system of beings of the phylum Annelida characterized?
Respiration in annelids can be cutaneous or branchial. Cutaneous respiration occurs due to the rich vascularity under the epidermis. The gills, present in aquatic annelids, are located in the parapodia (false claws) that have an extensive capillary net.

What is meant when it is said that beings of the phylum Annelida are vascular beings? From which other phyla of the animal kingdom does this feature differentiate them?
The classification of these beings as vascular beings means that they have a circulatory system, with vessels that distribute substances throughout the body.
Poriferans, cnidarians and flatworms do not have a circulatory system. In nematodes there is circulation of gases and nutrients through the pseudocoelom fluid.



How are the circulatory systems of animals classified?
A circulatory system is classified as open or closed. In open circulatory systems blood gets out of vessels and flows also to large cavities that perfuse the tissues to be irrigated. In closed circulatory systems blood circulates only within blood vessels and through the heart.

What is the type of circulatory system present in annelids?
In beings of the phylum Annelida the circulatory system is closed, i.e., blood circulation takes place only within specialized vessels.



Is there a respiratory pigment in the annelid blood?
The blood in beings of the phylum Annelida contains the respiratory pigment hemoglobin (the same found in chordates) and other pigments too.



How can the presence, localization and function of muscular tissue in beings of the phylum Annelida be explained?
In these beings there are a longitudinal muscular layer under the epidermis and, internally juxtaposed and perpendicular to it, another circular (radial to the axis) muscular layer. The circular muscle layer has the function of elongating the body while the longitudinal shortens it. By alternating actions both promote movement.



How can the excretory system of annelids be described?
In each segment (metamere) of the being a pair of complete excretory structures called metanephridium exists. The metanephridium has an extremity, the nephrostoma, which collects residuals from the coelom, filtering them and causing reabsorption along its extension (similar to human nephron tubules). The material to be excreted goes out through a pore, the nephridiopore, which opens in the body surface.

How is the nervous system characterized in beings of the phylum Annelida? How can one compare cephalization in annelids to cephalization in nematodes and platyhelminthes?
Annelids have a nervous system made of two ventral chords and one relatively big nervous cell concentration in its anterior portion resembling a primitive brain.
Nematodes have an anterior neural ring connected to two neural chords, a ventral and a dorsal one, while in planarias (platyhelminthes) there are only two small anterior “cerebral” ganglia from which neural chords split. Cephalization in annelids thus is more outstanding than in nematodes or in flatworms.






What is the clitellum of earthtworms and where it is located?
The clitellum is a special region of the annelid constituted by rings (metameres) with reproductive function. It can be found in the anterior portion of the animal and it is characterized by a lighter color in comparison to the normal color of the other segments.



Concerning the occurrence of separated sexes how are the beings of the phylum Annelida classified?
These beings may be dioecious (the majority of polychaetes) or hermaphrodite monoecious (oligochaetes and hirudineans).



Is the embryonic development in earthworms direct or indirect?
In earthworms there is no larval stage, so the embryonic development is direct.



What is the name of the larval stage of polychaetes?
Among the annelid classes only polychaetes present a larval stage. Their larva is called trocophore.



What is the ecological role of earthworms?
Earthworms have an important ecological role as they eat decomposing organic material. They also dig tunnels in the subsoil allowing the entrance of gases and nutrients that are useful for plant roots and other living beings. So they act as decomposers and as fertilizers too.



Into which classes is the phylum Annelida divided?
The phylum is divided into three classes: oligochaetes (for example, earthworms), hirudineans (e.g., leeches) and polychaetes (these are mostly marine aquatic with parapodia, like nereis).

Excretion.
The basic units of the annelid excretory system are either protonephridia, which have tubules (solenocytes) that end blindly within cells, contain flagella (whiplike projections), and are joined to a common duct that drains to the outside; or metanephridia, which are funnel-shaped structures containing cilia (short, hairlike processes) that open to the outside.
Ammonia is the chief nitrogen-containing end product of protein metabolism in aquatic annelids; earthworms, adapted to living in the soil, excrete more of another nitrogen-containing compound, urea, probably as part of a mechanism to control salt and water balance in the worm. The sea mouse Aphrodita, a polychaete, excretes 80 percent of its nitrogen as ammonia, which is also the primary nitrogenous excretory product in leeches (smaller amounts of urea also are excreted). Part of the ammonia excreted by leeches may come from bacteria in part of the leech’s excretory system (nephridial capsules). The ability of leeches to withstand high concentrations of ammonia is believed to result from a protective effect provided by high levels of calcium in their cells.
Three aspects of nephridial function in annelids correspond to those of the vertebrate kidney—filtration, resorption, and secretion. Coelomic fluid filters through solenocytes. The ciliated funnels of metanephridia retain minute particles and those of moderate size. In oligochaetes, whose coelomic fluid contains proteins, particles are actively absorbed in the ciliated region of the tubule. The tubules of earthworms also resorb inorganic ions such as sodium and calcium and can selectively eliminate excretory products from both the coelomic fluid and the bloodstream.

Phylum mollusca

"Mollusk" sounds like it ought to be the name of some prehistoric animal, but it isn’t. Mollusks are a huge division of animals without back bones, and they range from snails to clams, and oysters to octopuses. They differ in size from almost in visible in creatures to giant squids 15 meters long! They may live in the tropics or in the arctic, in deep seas or on land! But even though there are more than 60,000 different species of mollusks, they have certain characteristics in common. All mollusks have soft, slimy, boneless bodies which are covered with big folds of flesh called "mantles." In many mollusks, this mantle is covered with a hard shell, such as that of the oyster, while others have no protecting shell at all. Almost all mollusks have a kind of "foot," which is an extension of the mantle and which helps them move about. It may help them swim or walk, burrow in the mud, or tunnel through wood, depending on the species. There are five groups of mollusks, and the members of three of them are very well known to everybody.
The first of these common groups is called "Gastropoda," which means "stomach feet." Among the gastropods are snails, slugs, and periwinkles, all of which have one large "foot" on their stomachs. All gastropods have a head on which are eyes and feelers, and many of them carry single spiral shells on their backs. The second common group of mollusks is the bivalve group. In this group you will find oysters, clams, mussels, scallops, and many others. All bivalves have shapeless bodies protected by double, hinged shells. All of them are water creatures. The last common group of mollusks is called the "Cephalopoda," which means "head-footed." The members of this group have many arms, or tentacles, surrounding their mouths. This group includes the octopus, cuttlefish, squid, nautilus, and others. They are the aristocrats of the mollusk world, with a nervous system that raises them far above other mollusks. All mollusks lay eggs, but some lay only a few and others, many. In some the young hatch out as larvae; in others, they are tiny reproductions of their parents.

Phylum mollusca

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Torsion (gastropod)

Torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180˚ with respect to the head and foot of the gastropod. This brings the mantle cavity and anus to an anterior position above the head.
In some groups of gastropods there is a degree of secondary detorsion or rotation towards the original position, this may be only partial detorsion or full detorsion (see Opisthobranchia.)
There are two different developmental stages which cause torsion. The first stage is caused by the development of the asymmetrical velar/foot muscle which has one end attached to the left side of the shell and the other end has fibres attached to the left side of the foot and head. At a certain point in larval development this muscle contracts, causing an anticlockwise rotation of the visceral mass and mantle of roughly 90˚. This process is very rapid taking from a few minutes to a few hours. After this transformation the second stage of torsion development is achieved by differential tissue growth of the left hand side of the organism compared to the right hand side. This second stage is a much slower stage and rotates the visceral mass and mantle a further 90˚. Detorsion is brought about by reversal of the above phases.
During torsion the visceral mass remains almost unchanged anatomically, there are however other important changes to other internal parts of the gastropod. Before torsion the gastropod has an euthyneural nervous system, where the two visceral nerves run parallel down the body. Torsion results in a streptoneural nervous system, where the visceral nerves cross over in a figure of eight fashion. As a result the parietal ganglions end up at different heights. Because of differences between the left and right hand sides if the body, there are different evolutionary pressures on left and right hand side organs and as a result in some species there are considerable differences. Some examples of this are: in the ctenidia (equivalent to lungs or gills) in some species, one side may be reduced or absent; or in some hermaphrodite species the right hand renal system has been transformed into part of the reproductive system.

Evolutionary roles

The original advantage torsion gave gastropods is unclear. It is further complicated by the fact that torsion brought with it a number of problems. A particular problem gastropods had to overcome come was the location where wastes were excreted – above the head which can potentially lead to fouling of the mouth and sense organs. Nevertheless, the diversity and success of the gastropods suggests torsion is very advantageous indeed.
One role which is seems a likely candidate for the original purpose is that of defence against predators in adult gastropods. By moving the mantle cavity over the head, the gastropod can retract its vulnerable head into its shell. Some gastropods close the entrance to their shell with a tough operculum which is attached to their foot. In evolutionary terms the appearance of an operculum occurred shortly after that of torsion, a possible link with the role of torsion, though there is not sufficient evidence either for or against this theory.
There are many other advantages torsion provided gastropods. For aquatic gastropods the anterior positioning may be useful for preventing sediment getting into the mantle cavity, which is more likely with a posterior positioning due to sediment being stirred up by the motion of the gastropod. In terrestrial species, ventilation is better with anterior positioning. This is due to the back and forth motion of the shell during movement which would tend to block the mantle opening against the foot if it was in a posterior position. Another possible advantage for aquatic species is the osphradium (olfactory sense organs) are moved to an anterior position and are able to sample the water the gastropod is moving into to rather than from, this may help the gastropod locate food or avoid predators.
Evolution of an asymmetrical conispiral shell allowed gastropods to grow larger but resulted in an unbalanced shell. Torsion allows repositioning of the shell, bringing the centre of gravity back to the middle of the gastropod’s body and so helps prevent the animal or shell from falling over.
Whatever the original role of torsion, it is clear that further adaptations linked to torsion provide modern gastropods with many advantages.

Economic importance of Molluscs.
Molluscs are indirectly harmful to man but most of them are beneficial. Molluscs are of great important in various ways. There are some benefits of molluscs:

1. The harmful molluscs ate slugs and shipworms. Slugs are injurious in gardens and cultivations. They not only eat leaves but also destroy plants by cutting up their roots and stems. Teredo, a shipworm damages wooden parts of ship.

2. Many mollusks are great source of food for man in many parts of world. Large quantity of calms, oysters and mussels are eaten in Fareast, Europe and America. Oysters are regarded as delicacy.

3. Shell of fresh water mussels is used in button industry.

4. The shell of oyster are mixed with tar for making roads in America.

5. Shells in certain parts of world are also used for making ornaments.

6. Some oysters also make valuable pearls e.g. the pearl oyster.

7. Some pearls are used for making jewellery.

8. Some animals including in this phyla are use to eat in some countries,mussels.


9.Some of the shelled barnacles get attached to the ships and cause problems for shipping industry.

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