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Evolution of the vertebrate excretory system

Studies of the embryonic development of primitive vertebrates, such as the dogfish shark, clearly show that the excretory system arises from a series of tubules, one pair in every segment of the body between the heart and the tail. This continuous series of tubules constitutes the archinephros, the name implying that the kidney of the ancestral vertebrate had some such form as this. Each tubule opens internally to the body cavity and may, in the remote past, have opened separately to the exterior; but in all living vertebrates the tubules open on each side into a longitudinal duct, the archinephric duct. At the posterior end of the body cavity the two archinephric ducts unite before opening to the exterior. Later in development, Bowman’s capsule arises as a diverticulum of each tubule, subsequently becoming indented by the glomerulus. Eventually, the tubules usually lose their internal openings to the body cavity. The most anterior tubules of the archinephros (pronephros) usually degenerate in the adult.
These ducts and tubules also subserve the reproductive function, and for this reason they are also called the urogenital system. The extent to which the ducts and tubules are shared is greater in the male than in the female. In the male the spermatic tubules of the testis connect with the kidney tubules in the middle region of the archinephros (mesonephros), and in some vertebrates (e.g., the frog) where there is no development of the posterior region (metanephros), the tubules of the mesonephros serve to convey both urine and sperm. In the reptiles, birds, and mammals there is greater separation of function, the mesonephros being exclusively genital and the metanephros being exclusively urinary.
In the female, even in the lower vertebrates, the two systems are confluent only at the posterior end. It has been held that the oviduct is a derivative of the archinephric duct, but the evidence for this is not compelling.
In primitive marine animals the blood is almost identical with seawater in composition; in typical freshwater animals the concentration of the blood is about half that of seawater. Many originally marine animals have evolved the ability to live in fresh water; relatively few animals, after having thus evolved, have returned to the sea, and in none of them has the blood returned to its original “seawater” concentration. The earliest fossil vertebrates are found in marine deposits, but the fossil record shows clearly that the early evolution of fishes took place in fresh water. It is assumed that the blood of early freshwater fishes, like that of other freshwater animals, was osmotically equivalent to half-strength seawater. The sharks and rays returned to the sea during the Carboniferous Period, and no doubt at that time they evolved the device of urea retention. The bony fishes returned to the sea later, in the Mesozoic Era, and solved their problem by swallowing seawater and rejecting excess salt at the gills.

We have discussed in detail the mammalian excretory structure,this is additional material regarding excretion in Birds,reptiles,amhibians and fishes.




Birds and reptiles

The main excretory product of birds and reptiles is uric acid. Since their glomeruli are relatively small, so also is their daily volume of urine. Not highly concentrated by mammalian standards—although it may be turbid with crystals of uric acid—the urine of birds and reptiles is conducted not to a urinary bladder but to the terminal portion of the alimentary canal, the cloaca; from the cloaca it is voided with the feces. Like mammals, and unlike the lower vertebrates, birds and reptiles have skins impermeable to water and thus are well adapted to terrestrial life. The relative inability of the kidney to produce concentrated urine is compensated for in birds that possess salt glands, which remove excess salt from their bodies. These organs are modified tear glands that discharge a concentrated solution of sodium chloride through the nostrils. Salt glands enable marine birds to drink seawater with no ill effects.

Amphibians

Direct evidence for the occurrence of filtration at the glomerulus was first provided by experiments on the amphibian kidney. Although amphibians are formally given the status of terrestrial animals, they are poorly adapted to life on land. They excrete nitrogen in the form of urea and cannot produce urine more concentrated than the blood. Their skins are permeable to water. On land amphibians are liable to lose water very rapidly by evaporation. In fresh water they suffer entry of water by osmosis, which is counteracted by the excretion of a large volume of dilute urine. The urine is stored in a large bladder before being voided, providing a reserve of water the animal can use when it comes on land.
When an amphibian leaves the water, a number of physiological adjustments are made that have the effect of conserving water. The rate of glomerular filtration is reduced by restriction of the blood supply, and this together with an increased release of antidiuretic hormone results in the production of a small volume of urine of the same concentration as the blood. Antidiuretic hormone (ADH, also known as vasopressin, which increases the permeability of the distal and collecting tubules to water) also increases the permeability of the bladder to water and allows the stored urine to be reabsorbed into the body.

Fishes

The homeostasis problem is the same for freshwater fishes as for other freshwater animals. Water enters the body by osmosis and salts leach out. To compensate, the kidney (which has large glomeruli) produces a relatively large amount of dilute urine (about 20 percent of the body weight per day). This serves to remove the water but by itself is insufficient to prevent gradual loss of salts. Extremely diluted salts are taken up from the fresh water and transported directly into the blood by certain specialized cells in the gills. Nitrogenous excretion is no problem: some ammonia is carried away in the large volume of dilute urine, but most of it simply escapes to the external medium by diffusing through the gills.
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Excretory Products in vertebrates

Metabolism produces toxic by-products. Perhaps the most troublesome is the nitrogen-containing waste from the metabolism of proteins and nucleic acids. Nitrogen is removed from these nutrients when they are broken down for energy or when they are converted to carbohydrates or fats. The nitrogenous waste product is ammonia, a small and very toxic molecule. Some animals excrete their ammonia directly; others first convert it to less toxic wastes such as urea or uric acid . We shall see that the form of nitrogenous waste an animal excretes depends on both the animal's evolutionary history and its habitat.


Ammonia

Most aquatic animals excrete NITROGENOUS WASTES . As ammonia. Ammonia molecules are small and very toxic.

NITROGENOUS WASTES .. Ammonia is a toxic by-product of the metabolic removal of nitrogen from proteins and nucleic acids. Most aquatic animals get rid of ammonia by excreting it in very dilute solutions. Most terrestrial animals convert the ammonia to urea or uric acid, which conserves water because these less toxic wastes can be transported in the body in more concentrated form. soluble in water, so they easily permeate membranes. In soft-bodied invertebrates, ammonia diffuses across the whole body surface into the surrounding water. In freshwater fishes, most of the ammonia is lost as ammonium ions (NH4+) across the epithelium of the gills, with kidneys playing only a minor role in excretion of nitrogenous waste. The epithelium of the gills takes up Na + from the water in exchange for NH4 +, which helps freshwater fishes maintain Na + concentrations much higher than that in the surrounding water.


Urea

Ammonia excretion, though it works in water, is unsuitable for disposing of nitrogenous waste on land. A terrestrial animal would have to urinate copiously to get rid of ammonia, because a compound so toxic could only be transported in the animal and excreted in a very dilute solution. Instead, mammals and most 1 amphibian excrete urea. (Many marine fishes land turtles, which have the problem of conserving water in their hyperosmotic environment, also excrete) This substance can be handled in much more concentrated form because it is about 100,000 times less toxic than ammonia. Urea excretion enables the animal to sacrifice less water to discard its nitrogenous waste, an important adaptation for living on land.
Urea is produced in the liver by a metabolic cycle that combines ammonia with carbon dioxide. The cir-culatory system carries the urea to the kidneys. As mentioned earlier, mammalian kidneys excrete not all urea immediately; some of it is retained in the kidneys, where it contributes to osmoregulation by helping to maintain the osmolarity gradient that functions in water reabsorption. Sharks, remember, also produce urea, which is retained at a relatively high concentration in the blood, which helps balance the osmolarity of body fluids with the surrounding seawater.
Amphibians that undergo metamorphosis generally switch from excreting ammonia to excreting urea during the transformation from an aquatic larva, the tadpole, to the terrestrial adult. This biochemical modification, however, is not inexorably coupled to metamorphosis. Frogs that remain aquatic, such as the South African clawed toad (Xenopus), continue excreting ammonia after metamorphosis. But if these animals are forced to stay out of water for several weeks, they begin to produce urea. Similarly, African lungfish switch from ammonia to urea excretion if their habitat dries up and they are forced to burrow in the mud and become inactive.


Uric Acid

Land snails, insects, birds, and some reptiles excrete uric acid as the major nitrogenous waste. Because it is thousands of times less soluble in water than either ammonia or urea, uric acid can be excreted as a precipitate after nearly all the water has been reabsorbed from the urine. In birds and reptiles, the paste like urine is excreted into the cloaca and eliminated along with feces from the intestine.
Uric acid and urea represent two different adaptations that enable terrestrial animals to excrete NITROGENOUS WASTES . With a minimal loss of water. One factor that seems to have been important in determining which of these alternatives evolved in a particular group of animals is the mode of reproduction. Soluble wastes can diffuse out of a shell-less amphibian egg or be carried away by the mother's blood in the case of a mammalian embryo. The vertebrates that excrete uric acid, however, produce shelled eggs, which are permeable to gases but not to liquids. If an embryo released ammonia or urea within a she]Jed egg, the soluble waste would accumulate to toxic concentrations. Uric acid precipitates out of solution and can be stored within the egg as a solid that is left behind when the animal hatches.
In grouping the various vertebrates according to the NITROGENOUS WASTES . they excrete, the boundaries are not drawn strictly along phylogenetic lines but depend also on habitat. Among reptiles, for instance, lizards, snakes, and terrestrial turtles excrete mainly uric acid; crocodiles excrete ammonia in addition to uric acid; and aquatic turtles excrete both urea and ammonia. In fact, individual turtles modify their NITROGENOUS WASTES . When their environment changes. A tortoise that usually produces urea can shift to uric acid production when the temperature increases and water becomes less available.

This is another example of how response to the environment occurs on two levels: Evolution determines the limits of physiological responses for a species, but individual organisms make adjustments within that range as the r environment changes. This principle also applies to the regulation of body temperature


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Scales in Fishes

In most biological nomenclature, a scale (Greek lepid, Latin squama) is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran (butterfly and moth) species, scales are plates on the surface of the insect wing, and provide coloration. Scales are quite common and have evolved multiple times with varying structure and function.
Scales are generally classified as part of an organism's integumentary system. There are various types of scales according to shape and to class of animal. Although the meat and organs of some species of fish are edible by humans, the scales are usually not eaten.

Fish scales

Fish scales are dermally derived, specifically in the mesoderm. This fact distinguishes them from reptile scales paleontologically. Genetically, the same genes involved in tooth and hair development in mammals are also involved in scale development.

Cosmoid scales

True cosmoid scales can only be found on the extinct Crossopterygians. The inner layer of the scale is made of lamellar bone. On top of this lies a layer of spongy or vascular bone and then a layer of dentine-like material called cosmine. The upper surface is keratin. The coelacanth has modified cosmoid scales that lack cosmine and are thinner than true cosmoid scales.
Ganoid scales

Ganoid scales can be found on gars (family Lepisosteidae) and bichirs and reedfishes (family Polypteridae). Ganoid scales are similar to cosmoid scales, but a layer of ganoin lies over the cosmine layer and under the enamel. They are diamond-shaped, shiny, and hard.
Placoid scales

Placoid scales are found on cartilaginous fish including sharks. These scales, also called denticles, are similar in structure to teeth.
Leptoid scales

Leptoid scales are found on higher order bony fish and come in two forms, ctenoid and cycloid scales.
As they grow, cycloid and ctenoid scales add concentric layers. The scales of bony fishes are laid so as to overlap in a head-to-tail direction, like roof tiles, allowing a smoother flow of water over the body and therefore reducing drag.
Cycloid scales

Cycloid scales have a smooth outer edge, and are most common on more primitive fish with soft fin rays, such as salmon and carp
Ctenoid scales

Ctenoid scales have a toothed outer edge, and are usually found on more derived fishes with spiny fin rays, such as bass and crappie

Do all fishes have scales?

No. Many species of fishes lack scales. All the clingfishes (family Gobiesocidae) for example, are scaleless. Their bodies are protected by a thick layer of mucous.
Why do fish have scales?

The primary purpose of scales is to give the fish external protection.
How many types of scales are there?

There are four main kinds of scales and numerous variations of each kind.
  1. Placoid
  2. Cosmoid
  3. Ganoid
  4. Cycloid and Ctenoid
Different fishes, different scalation

It is interesting to think about the lifestyle and habitat of a fish, then look at its scales. In the Shark Scale Brain Teaser, the scales of five shark species are shown, two are slow swimming bottom-dwelling sharks, one is a generalist predator, and two are fast swimming pelagic species. Can you work out which scale belongs to each shark?
Are all scales the same size?

No. Scale sizes vary greatly between species. Some fishes, such as the freshwater eels have tiny embedded scales. Fishes such as the tunas have tiny scales often found in discrete areas of the body. Many fishes such as the Coral Snappers have medium sized scales whereas the scales of others such as the Tarpon, Megalops cyprinoides are large enough to be used in jewelery. The scales of the Indian Mahseer, Tor tor are known to reach over 10 cm in length.
How old is a fish scale?

As cycloid and ctenoid scales increase in size, growth rings called circuli become visible. These rings look a little like the growth rings in the trunk of a tree. During the cooler months of the year the scale (and otoliths) grows more slowly and the circuli are closer together leaving a band called an annulus. By counting the annuli it is possible estimate the age of the fish. This technique is extensively used by fisheries biologists.
Can a fish have more than one type of scale?

Yes. Some species of flatfishes (flounders, soles, etc) have ctenoid scales on the eyed side of the body and cycloid scales on the blind side.
Can scale type vary with sex?

Yes. In some species of flatfishes, males have ctenoid scales and females have cycloid scales.

Functions

Protection from predators.




Placoid Scales


Ganoid Scales






Cycloid Scales




Ctenoid Scales






Last edited by Viceroy; Monday, May 25, 2009 at 12:01 PM. Reason: Merger
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Fins


The fins are the most distinctive features of a fish, composed of bony spines protruding from the body with skin covering them and joining them together, either in a webbed fashion, as seen in most bony fish, or more similar to a flipper, as seen in sharks. These usually serve as a means for the fish to swim. Fins can also be used for gliding or crawling, as seen in the flying fish and frogfish. Fins located in different places on the fish serve different purposes, such as moving forward, turning, and keeping an upright position.
Spines and rays

In bony fish, most fins may have spines or rays. A fin can contain only spiny rays, only soft rays, or a combination of both. If both are present, the spiny rays are always anterior. Spines are generally stiff and sharp. Rays are generally soft, flexible, segmented, and may be branched. This segmentation of rays is the main difference that separates them from spines; spines may be flexible in certain species, but they will never be segmented.
Spines have a variety of uses. In catfish, they are used as a form of defense; many catfish have the ability to lock their spines outwards. Triggerfish also use spines to lock themselves in crevices to prevent them being pulled out.
Types of fin
  • Dorsal fins are located on the back. A fish can have up to three of them. The dorsal fins serve to protect the fish against rolling, and assists in sudden turns and stops.
    • In anglerfish, the anterior of the dorsal fin is modified into an illicium and esca, a biological equivalent to a fishing pole and a lure.
    • The bones that support the dorsal fin are called Pterygiophore. There are two to three of them: "proximal", "middle", and "distal". In spinous fins the distal is often fused to the middle, or not present at all.
Caudal Fins
  • The caudal fin is the tail fin, located at the end of the caudal peduncle.
    • The tail can be heterocercal, which means that the vertebrae extend into a larger lobe of the tail or that the tail is asymmetrical
      • Epicercal means that the upper lobe is longer (as in sharks)
      • Hypocercal means that the lower lobe is longer (as in flying fish)
    • Protocercal means that the caudal fin extends around the vertebral column, present in embryonic fish and hagfish. This is not to be confused with a caudal fin that has fused with the dorsal and anal fins to form a contiguous fin.
    • Diphycercal refers to the special, three-lobed caudal fin of the coelacanth and lungfish where the vertebrae extend all the way to the end of the tail.
    • Most fish have a homocercal tail, where the vertebrae do not extend into a lobe and the fin is more or less symmetrical. This can be expressed in a variety of shapes.
      • The tail fin may be rounded at the end.
      • The tail fin may be truncated, or end in a more-or-less vertical edge (such as in salmon).
      • The fin may be forked, or end in two prongs.
      • The tail fin may be emarginate, or with a slight inward curve.
      • The tail fin may be lunate, or shaped like a crescent moon.
  • The anal fin is located on the ventral surface behind the anus. This fin is used to stabilize the fish while swimming.
  • The paired pectoral fins are located on each side, usually just behind the operculum, and are homologous to the forelimbs of tetrapods.
    • A peculiar function of pectoral fins, highly developed in some fish, is the creation of the dynamic lifting force that assists some fish, such as sharks, in maintaining depth and also enables the "flight" for flying fish.
Bigeye tuna Thunnus obesus showing finlets and keels.
Drawing by Dr Tony Ayling
    • In many fish, the pectoral fins aid in walking, especially in the lobe-like fins of some anglerfish and in the mudskipper.
    • Certain rays of the pectoral fins may be adapted into finger-like projections, such as in sea robins and flying gurnards.
      • The "horns" of manta rays and their relatives are called cephalic fins; this is actually a modification of the anterior portion of the pectoral fin.
  • The paired pelvic or ventral fins are located ventrally below the pectoral fins. They are homologous to the hindlimbs of tetrapods. The pelvic fin assists the fish in going up or down through the water, turning sharply, and stopping quickly.
    • In gobies, the pelvic fins are often fused into a single sucker disk. This can be used to attach to objects.
  • The adipose fin is a soft, fleshy fin found on the back behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but is found in Salmonidae, characins and catfishes.
  • Some types of fast-swimming fish have a horizontal caudal keel just forward of the tail fin. This is a lateral ridge on the caudal peduncle, usually composed of scutes (see below), that provides stability and support to the caudal fin. There may be a single paired keel, one on each side, or two pairs above and below.
  • Finlets are small fins, generally behind the dorsal and anal fins (in bichirs, there are only finlets on the dorsal surface and no dorsal fin). In some fish such as tuna or sauries, they are rayless, non-retractable, and found between the last dorsal and/or anal fin and the caudal fin.
For every fin, there are a number of fish species in which this particular fin has been lost during evolution.








types of caudal fin :
(A) - Heterocercal, (B) - Protocercal,
(C) - Homocercal, (D) - Diphycercal













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The swim bladder

The swim bladder (also called the gas bladder or air bladder) is a flexible-walled, gas-filled sac located in the dorsal portion of body cavity. This organ controls the fish's buoyancy and in some species is important for hearing. Most of the swim bladder is not permeable to gases, because it is poorly vascularised (has few blood vessels) and is lined with sheets of guanine crystals.
A fish swimming in the water expends less energy if it is neutrally buoyant (that is, it neither sinks nor floats). If this fish starts to descend, the increased pressure from the water surrounding the fish results in a compression of the gas inside the swim bladder. The fish becomes negatively buoyant and will tend to sink. Conversely, if a fish swims into shallower water, there is a decrease in water pressure and so the gas in the swim bladder expands, and the fish tends to float upwards. The swim bladder helps to solve the problems associated with variations of pressure, and thus buoyancy.
If the fish becomes positively buoyant, and starts to float upwards, gas diffuses out of the swim bladder into the blood. This occurs at a site known as the oval. The gas in the blood is then removed from the body into the surrounding water at the gills.
Conversely if the fish becomes negatively buoyant, and starts to sink, air enters the swim bladder at a region called the gas gland. The way the fish does this involves three processes; the acidification of the blood, an increase in the concentration of lactate and hydrogen ions and the movement of blood through a complex structure called the rete mirabile (literally, the wonderful network). These complex processes are not discussed here. Refer to the reference below for more information.
Not all fishes have a swim bladder. Sharks for example do not have a swim bladder, and many species such as the Grey Nurse Shark, use a different strategy which includes having a large oily liver and specialised body shape to maintain buoyancy.
Buoyancy organ possessed by most bony fish. The swim bladder is located in the body cavity and is derived from an outpocketing of the digestive tube. It contains gas (usually oxygen) and functions as a hydrostatic, or ballast, organ, enabling the fish to maintain its depth without floating upward or sinking. It also serves as a resonating chamber to produce or receive sound. In some species the swim bladder contains oil instead of gas. In certain primitive fish it functions as a lung or respiratory aid instead of a hydrostatic organ. The swim bladder is missing in some bottom-dwelling and deep-sea bony fish (teleosts) and in all cartilaginous fish (sharks, skates, and rays).


The gas bladder (also fish maw, less accurately swim bladder or air bladder) is an internal gas-filled organ that contributes to the ability of a fish to control its buoyancy, and thus to stay at the current water depth without having to waste energy in swimming.
Gas bladders are only found in ray-finned fish. In the embryonic stages some species have lost the swim bladder again, mostly bottom dwellers like the weather fish. Other fishes like the Opah and the Pomfret use their pectoral fins to swim and balance the weight of the head to keep a horizontal position. The normally bottom dwelling sea robins can use their pectoral fins to produce lift while swimming. The cartilaginous fish (e.g. sharks and rays) do not have gas bladders. They can control their depth only by swimming (using dynamic lift); others store fats or oils for the purpose.



Structure and function
The gas bladder consists of two gas-filled sacs located in the dorsal portion of the fish. It has flexible walls that contract or expand according to the ambient pressure. The walls of the bladder contain very few blood vessels and are lined with guanine crystals, which make them impermeable to gases. By adjusting the gas pressure using the gas gland or oval window the fish can obtain neutral buoyancy and ascend and descend to a large range of depths. Due to the dorsal position it gives the fish lateral stability.
In physostomous gas bladders, a connection is retained between the gas bladder and the gut, the pneumatic duct, allowing the fish to fill up the gas bladder by "gulping" air and filling the gas bladder. In more derived varieties of fish, the physoclisti, the bladder has a gas gland that can introduce gases (usually oxygen) to the bladder to increase its volume and thus increase buoyancy. To reduce buoyancy, gases are released from the bladder into the blood stream and then expelled into the water via the gills.
In order to introduce gas into the bladder, the gas gland excretes lactic acid and produces carbon dioxide the resulting acidity causes the hemoglobin of the blood to lose its oxygen (Root effect) which then diffuses partly into the gas bladder. The blood flowing back to the body first enters the rete mirabile where virtually all the carbon dioxide and oxygen produced in the gas gland diffuse back to the arteries supplying the gas gland. Thus a very high gas pressure of oxygen can be obtained, which can even account for the presence of gas in the swim bladders of deep see fish like the eel, requiring a pressure of hundreds of bar. Elsewhere, at a similar structure known as the oval window, the bladder is in contact with blood and the oxygen can diffuse back. Together with oxygen other gasses are salted out in the gas bladder which accounts for the high pressures of other gasses as wel.
The combination of gases in the bladder varies; in shallow water fish, the ratios closely approximate that of the atmosphere, while deep sea fish tend to have higher percentages of oxygen. For instance, the eel Synaphobranchus has been observed to have 75.1% oxygen, 20.5% nitrogen, 3.1% carbon dioxide, and 0.4% argon in its gas bladder.
Physoclist gas bladders have one important disadvantage: they prohibit fast rising, as the bladder would burst. Physostomes can "burp" out gas, though this complicates the process of re-submergence.
In some fish, mainly freshwater species, the gas bladder is connected to the labyrinth of the inner ear by the Weberian apparatus, which provides a precise sense of water pressure (and thus depth), and improves hearing.



Evolution
Gas bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the gas bladders of the ray-finned fish.In embryonal development, both lung and gas bladder originate as an outpocketing from the gut; in the case of gas bladders, this connection to the gut continues to exist as the pneumatic duct in more "primitive" teleosts, and is lost in the more derived orders. There are no animals which have both lungs and a gas bladder.
The cartilaginous fish (e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and gas bladders, suggesting that these structures evolved after that split. Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.













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Default chordates

Mechanics of biting

The reserve or successional teeth, which are always present just behind or on the side of the functional fang of all venomous snakes, are in no way connected with the duct until called upon to replace a fang that has been lost. It could not be otherwise, since the duct would require a new terminal portion for each new fang; and as the replacement takes place alternately from two parallel series, the new venom-conveying tooth does not occupy exactly the same position as its predecessor.
Two genera, Doliophis among the elapids and Causus among the viperids, are highly remarkable for having the venom gland and its duct of a great length, extending along each side of the body and terminating in front of the heart. Instead of the muscles of the temporal region serving to press out the venom into the duct, this action is performed by those of the side of the body.
When biting, a viperid snake merely strikes, discharging the venom the moment the fangs penetrate the skin, and then immediately lets it go. A proteroglyph or opisthoglyph, on the contrary, closes its jaws like a dog on the part bitten, often holding on firmly for a considerable time. The venom, which is mostly a clear, limpid fluid of a pale straw or amber colour, or rarely greenish, sometimes with a certain amount of suspended matter, is exhausted after several bites, and the glands have to recuperate.

Mechanics of spitting

Venom can be ejected otherwise than by a bite, as in the so-called spitting cobras of the genera Naja and Hemachatus. Some of these deadly snakes, when irritated, are capable of shooting venom from the mouth, at a distance of 4 to 8 feet. These snakes' fangs have been modified for the purposes of spitting: inside the fangs of a spitting cobra is a channel which makes a ninety degree bend to the lower front of the fang. When the snake is threatened the muscles of the venom gland squeeze the venom sack and as a result venom is projected forward. Spitters may spit thirty or forty times in succession, and even then the snake is still able to deliver a fatal bite.
Spitting is a defensive reaction only. The snake tends to aim for the eyes of a perceived threat; a direct hit can cause temporary shock and blindness through severe inflammation of the cornea and conjunctiva. While there are no serious results if the venom is washed away at once with plenty of water, the blindness caused by a successful spit can become permanent if left untreated. Contact with the skin is not in itself dangerous, but open wounds may become envenomed.


Some Effects
There are three distinct types of venom that act on the body differently.
  • Hemotoxic venoms act on the heart and cardiovascular system.
  • Neurotoxic venom acts on the nervous system and brain.
  • Cytotoxic venom has a localized action at the site of the bite.
It is noteworthy that the size of the venom fangs is in no relation to the virulence of the venom.
But how dangerous really are venomous snakes?

Amongst venomous Colubrids, only one African species, the boomslang (Dispholidus) has fangs big enough and a potent venom to harm a human being, even if it is a non aggressive species difficult to provoke. The rear fanged snakes have to chew their prey while swallowing it in order to administrate the paralyzing venom which initiates the digestion and they usually eat small prey, like lizards, frogs. Moreover, their venom in most cases is weak.

Cobra type venomous snake tends to bite and keep a hold on, as many have shorter fangs and some coral snakes even chew. But vipers, with their highly efficient injecting mechanism, effectuate the bite sometimes in 1/40 of a second and then retreat.

The snake venom varies a lot between different groups. Some venoms are neurotoxic and paralyze the muscles. The victim dies of suffocation and heart attack. All cobra-related snakes have extremely potent neurotoxic venom, but also some rattlesnakes, moccasins and pitless vipers.

Some venoms, like cobra's, produce anaphylactic shock. Other venoms action like a powerful digesting juice, producing necrosis and hemorrhage. Most vipers possess a powerful venom of this type and with their huge fangs can introduce large amounts of venom into the wounds.

The bushmaster (Lachesis muta) , a pit viper from tropical America produces general necrosis, generalized internal hemorrhage and external hemorrhage through all the orifices.

Sea snake venom provokes elimination of mioglobin (muscle protein) through the kidney and necrosis into the muscles, but these snakes are not known to have ever bitten a person.

Some animals are very sensitive to snake venom, like birds, others not, like mongooses that consume venomous snakes and some species, like snake eating snakes (king cobra, king snakes), are totally immune to snake venom. Many snakes have venom specialized for their prey, like an aquatic Colubrid, Fordonia, whose venom is toxic only for the crabs. Ultimately, the aggressiveness is important on appreciating the danger posed by a snake species, and most Elapids and Viperids are both aggressive and highly toxic.









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Default chordates

ADAPTATIONS FOR FLIGHT

- Presence of feathers separates birds from all other living vertebrates. Feathers are associated with flight - the major characteristic feature of birds is the capacity for flight.

- Not all birds are capable of flight, but flightless birds are evolutionarily derived from flying ancestors.

- The entire anatomy and physiology of birds is adapted for efficient flight, so most birds are morphologically very similar because they are constrained by the demands of flight. Differences used to separate different taxonomic Orders of birds would only separate families in other vertebrate groups.

- The 2 major requirements for flight (includes insects, birds, bats, airplanes) are high power and low weight. Birds meet these requirements by several adaptations of their anatomy and physiology.


I. ADAPTATIONS TO REDUCE WEIGHT

1) Hollow, strutted, or spongy bone. Many bones are pneumatic (contain air sacs within hollow insides).
2) Fusion of many bones, including vertebrae appendages, and girdles - also provides rigidity to skeleton that is necessary for flight.
3) Reduction in some bones (e.g., tail) and loss of teeth
4) Feathers - lightweight yet well-adapted for aerodynamics and insulation.
5) Urogenital Adaptations:
a. No urinary bladder or urethra - kidneys produce mainly uric acid, excreted as semisolid paste so this also contributes to water conservation.
b. All lay eggs (no live-bearers) - only vertebrate Class for which this is true.
c. Only left ovary persists in most adult females
d. sex organs in both sexes are enlarged and functional only during the breeding season; when breeding is complete, gonads regress.


II. POWER-PRODUCING ADAPTATIONS

1) Endothermic Homeotherms - maintain highest Tb of any vertebrate, 40 - 43oC (about 37-38oC for mammals); since most aerobic biochemical reactions are temperature dependent, a high Tb --> fast reaction rates --> high and rapid sustainable power production. Birds with the highest aerobic MR/size of any vertebrate.
2) Efficient digestion to acquire fuel to power metabolism
3) Very large breast muscle (pectoralis)
- powers downstroke of wing. Makes up about 15-25% (up to 40%) of total body weight.
- keeled sternum for attachment of high volume of flight muscle, mostly supracoracoideus which powers upstroke. Absent in ratites (flightless) and Archaeopteryx.
4) 4-chambered heart for efficient circulation and rapid blood delivery
5) Large and complicated respiratory system- very efficient at extracting 02 from inspired air.

III. NERVOUS SYSTEM ADAPTATIONS

1) Large eyes and optic lobes
2) Large cerebellum for optimal muscle coordination.


 


Adaptations for Flight


T
he evolution of flight has endowed birds with many physical features in addition to wings and feathers. One of the requirements of heavier-than-air flying machines, birds included, is a structure that combines strength and light weight. One way this is accomplished in birds is by the fusion and elimination of some bones and the "pneumatization" (hollowing) of the remaining ones. Some of the vertebrae and some bones of the pelvic girdle of birds are fused into a single structure, as are some finger and leg bones -- all of which are separate in most vertebrates. And many tail, finger, and leg bones are missing altogether. Not only are some bones of birds, unlike ours, hollow, but many of the hollows are connected to the respiratory system. To keep the cylindrical walls of a bird's major wing bones from buckling, the bones have internal strut-like reinforcements.

The pneumatization of bird bones led to the belief that birds had skeletons that weighed proportionately less than those of mammals. Careful studies by H. D. Prange and his colleagues have shown this not to be the case. More demands are placed on a bird's skeleton than on that of a terrestrial mammal. The bird must be able to support itself either entirely by its forelimbs or entirely by its hindlimbs. It also requires a deep, solid breastbone (sternum) to which the wing muscles can be anchored. Thus, while some bones are much lighter than their mammalian counterparts, others, especially the leg bones, are heavier. Evolution has created in the avian skeleton a model of parsimony, lightening where possible, adding weight and strength where required. The results can be quite spectacular: the skeleton of a frigatebird with a seven-foot wingspan weighs less than the feathers covering it!
Not all birds have the same degree of skeletal pneumatization. To decrease their buoyancy and make diving easier, some diving birds, such as loons and auklets, have relatively solid bones. Those birds are generally less skillful fliers than ones with lighter skeletons.

Birds have found other ways to lighten the load in addition to hollowing out their bones. For instance, they keep their reproductive organs (testes, ovaries and oviducts) tiny for most of the year, greatly enlarging them only during the breeding season.

The respiratory system of birds is also adapted to the demands of flight. A bird's respiratory system is proportionately larger and much more efficient than ours -- as might be expected, since flight is a more demanding activity than walking or running. An average bird devotes about one-fifth of its body volume to its respiratory system, an average mammal only about one-twentieth. Mammalian respiratory systems consist of lungs that are blind sacs and of tubes that connect them to the nose and mouth. During each breath, only some of the air contained in the lungs is exchanged, since the lungs do not collapse completely with each exhalation, and some "dead air" then remains in them.

In contrast, the lungs of birds are less flexible, and relatively small, but they are interconnected with a system of large, thin-walled air sacs in the front (anterior) and back (posterior) portions of the body. These, in turn, are connected with the air spaces in the bones. Evolution has created an ingenious system that passes the air in a one-way, two-stage flow through the bird's lungs. A breath of inhaled air passes first into the posterior air sacs and then, on exhalation, into the lungs. When a second breath is inhaled into the posterior sacs, the air from the first breath moves from shrinking lungs into the anterior air sacs. When the second exhalation occurs, the air from the first breath moves from the anterior air sacs and out of the bird, while the second breath moves into the lungs. The air thus moves in one direction through the lungs. All birds have this one-way flow system; most have a second two-way flow system which may make up as much as 20 percent of the lung volume.

In both systems, the air is funneled down fine tubules which interdigitate with capillaries carrying oxygen-poor venous blood. At the beginning of the tubules the oxygen-rich air is in close contact with that oxygen-hungry blood; farther down the tubules the oxygen content of air and blood are in equilibrium. Birds' lungs are anatomically very complex (their structure and function are only barely outlined here), but they create a "crosscurrent circulation" of air and blood that provides a greater capacity for the exchange of oxygen and carbon dioxide across the thin intervening membranes than is found in mammalian lungs.

Contrary to what was once believed, the rhythm of a bird's respiratory two-cycle pump is not related to the beats of its wings. Flight movements and respiratory movements are independent. The heart does the pumping required to get oxygenated blood to the tissues and to carry deoxygenated blood (loaded with carbon dioxide) away from them. Because of the efficiency of the bird's breathing apparatus, the ratio of breaths to heartbeats can be quite low. A mammal takes about one breath for every four and one-half heartbeats (independent of the size of the mammal), a bird about one every six to ten heartbeats (depending on the size of the bird).

A bird's heart is large, powerful, and of the same basic design as that of a mammal. It is a four-chambered structure of two pumps operating side by side. One two-chambered pump receives oxygen-rich blood from the lungs and pumps it out to the waiting tissues. The other pump receives oxygen-poor blood from the tissues and pumps it into the lungs. This segregation of the two kinds of blood (which does not occur completely in reptiles, amphibians, and fishes) makes a bird's circulatory system, like its respiratory system, well equipped to handle the rigors of flight.

The flight muscles of most birds are red in color ("dark meat") because of the presence of many fibers containing red oxygen-carrying compounds, myoglobin and cytochrome. They are also richly supplied with blood and are designed for sustained flight. Lighter-colored muscles ("white meat"), with many fewer such fibers, are found in pheasants, grouse, quail, and other gallinaceous birds. These are also well supplied with blood, are apparently capable of carrying a heavy work load for a short time, but fatigue more rapidly. If a quail is flushed a few times in a row, it will become so exhausted it will be incapable of further flight.

Finally, of course, it does little good to be able to sustain flight or fly rapidly if you are always crashing into things. Although birds have found many ways to streamline, lighten, or totally eliminate unnecessary parts (like urinary bladders), they have not stinted on nervous systems. Birds have brains that are proportionately much larger than those of lizards and comparable, in fact, with those of rodents. The brain is connected to sharp eyes, and has ample processing centers for coordinating the information received from them. A bird's nerves can rapidly transmit commands of the brain to the muscles operating the wings. It is the combination of visual acuity, quick decision making, and high-speed nerve transmission along short nerves that permits a Golden-crowned Sparrow to weave rapidly among the branches of a thicket, escaping the clutches of a pursuing Sharp-shinned Hawk.





 
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Default chordates

Bird feathers types

All birds have feathers and only birds have feathers even if the feathers are highly modified as on penguins.
While most feathers share a common overall structure, there are several different kinds of feathers adopted for specialized roles. Changes in feather structure provide the adaptations necessary for feathers to be used in many different ways.
Feathers also support the behavior of the bird within its environment and its lifestyle. Feathers that support the soaring flight of an eagle have a much different role than the feathers that protect an American Dipper, which spends much of its time in fast-flowing streams.
This page primarily covers feather types and feather topography. It is intended for the serious student.
Feather topography:

A typical wing feather consists of a central, stiff shaft with the softer vanes on each side. The leading edge of the feather during flight is called the outer vane. The opposite vane is wider than the outer vane and is referred to as the inner vane.
In greater detail, feathers are broken down into the following structural elements.







Central shaft:
The central shaft of a feather is divided into two regions.
The calamus is the part of the shaft closest to the bird's body. It is hollow and does not contain any vanes.
The distal end of the central shaft is referred to as the rachis. The rachis is solid and is defined as the area to which vanes are attached.
Vanes:
The vanes extend from each side of the feather. A series of parallel branches called barbs make up the vane.
Extending from the barbs are a series of short branchlets called barbules. Tiny hooklets tie the barbules, and ultimately the barbs, together. This somewhat complex arrangement creates the strong but light structure of the feather.

Feather Tracts:
Feathers are not attached to birds in a random manner over the entire body of the bird. Instead they are usually found in often linear tracts celled pterylae. The spaces on the bird's body without feather tracts are referred to as apteria. The densest area for feathers is often on the bird's head and neck.
Types of feathers:

Contour feathers:
When you look at a bird the contour feathers are the outermost feathers, or the ones you see. They provide the color and the shape of the bird. The wing feathers are strong and stiff, supporting the bird during flight. The contour feathers tend to lie on top of each other, much like shingles on a roof. The feathers therefore tend to shed rain, keeping the body dry and well insulated.
Each contour feather can be controlled by a set of specialized muscles which control the position of the feathers, allowing the bird to keep the feathers in a smooth and neat condition.
Remiges:
The largest contour feathers are often the large flight feathers, which are collectively called the remiges. Since they are responsible for supporting the bird during flight, remiges are attached by ligaments or directly to the bone. The outer remiges are referred to as the primaries and are the largest and strongest of the flight feathers. They are attached to the skeletal equivalent of the "hand" of the bird.
The inner remiges are called the secondaries and are attached to the "forearm" of the bird. They are located between the body of the bird and the primaries. The secondaries provide lift in both soaring and flapping flight.
Rectrices:
The tail feathers are used to provide stability and control. They are referred to collectively as rectrices. The rectrices are connected to each other by ligaments, with only the innermost feathers attaching directly to the tailbone.
Coverts:
Bordering and overlaying the edges of the remiges and the rectrices on both the lower side and upper side of the body are rows of feathers called coverts. The coverts help streamline the shape of the wings and tail while providing the bird with insulation.
Afterfeather:
Attached to the lower shaft of some contour feathers are the typically much smaller afterfeathers. The afterfeathers resemble the main feather and provide an extra layer of warmth.








In North American birds, the afterfeathers of grouse are especially well developed for their life in seasonally cold and arctic regions.
The flightless Emu of Australia has specially adapted afterfeathers that are as large as the main feather and provide protection as the bird moves through the thick brush of its natural habitat. Relatively recent additions to the Emu's normal range are four-foot high sheep fences topped with barbed wire. Seemingly unperturbed, the flightless Emu crosses these fences by running straight into them, resulting in a high-speed somersault over the fence. The tuft of feathers left behind is a sure indication of an Emu crossing and a testament to the amount of protection the feathers offer.








Bristles:
Highly specialized feathers, bristles are small contour feathers which lack barbs on the outermost part and have an especially stiff rachis.
Rictal bristles project from the beak of many insect-eating birds, including flycatchers, nightjars and even the American Robin. They are believed to provide protection for the bird's eyes as it consumes its wriggly prey. The bristles may also provide tactile feedback, like the whiskers on a dog or cat.



Specialized feathers:

Down feathers:
Up to the challenge of keeping birds warm are the down feathers. In down feathers, the rachis is either missing completely or substantially reduced in length. The barbules lack hooks, which combined with the lack of rachis, results in a very soft and fluffy feather. Without the hooks, the barbs and barbules create a puffy tangle of insulating air pockets.
Natal downs are present on some birds at the time they hatch. They are responsible for giving baby chicks and ducklings their fluffy appearance. Natal downs are typically found on well developed hatchlings that can almost immediately walk or swim independently of their parents.
The young of most Passerine species, such as Blue Jays, are totally helpless and virtually naked at birth. It is thought that the baby birds save energy by not producing down and are able to absorb the body heat of the parent bird more easily.
Semiplumes:
Semiplumes are found between other feathers, providing an additional layer of warmth and helping to maintain the smooth, streamlined shape of the bird. Semiplumes are a cross between down feathers and contour feathers. They do have a supportive rachis, as on contour feathers, but lack the hooks that hold the barbs together. The resulting feather does not form a vane and has a downy feather look.
Filoplumes:
The simplest feather is the filoplume. It consists primarily of the rachis with no barbs or only a few isolated barbs at the tip. These relatively stiff and hair-like feathers lack specific feather muscles but have sensory receptors next to the base of the feathers. Filoplumes lie under the contour feathers and are thought to provide the bird with feedback on contour feather activity.
Powder downs:
Found only in certain taxonomic groups such as pigeons and herons, powder down feathers are never molted. Instead, they grow continuously but disintegrate at the tips into something like a fine talcum powder. The powder permeates the other feathers, presumably to provide waterproofing, although the exact function is not well understood.
Feathers and flight:

The combination of light weight, strength, and shape--combined with precision control--is largely responsible for giving birds their special ability of sustained flight.
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Default chordates

Bird Beaks

Bird beaks, in particular, reflect the variation not only in the types of foods these animals consume but in where and how they get their food. Some birds, for example, catch their prey in water. Stilts, herons, spoonbills, and oystercatchers wade in shallow water, searching for fish, crustaceans, and mollusks. Stilts and herons have long, pointed beaks to snatch fish they've spotted, while spoonbills move their broad, rounded beaks through the water to catch their prey by feel. As their name implies, oystercatchers eat oysters, clams, and mussels. Their beaks, not surprisingly, are very strong and stiff, enabling them to pry open mollusk shells and extract their contents in a matter of seconds.

The size of a bird's prey also presents tremendous challenges. Pelicans and bald eagles, for instance, both rely on the element of surprise, ambushing fish swimming just below the water's surface. The pelican typically dives into the water from great heights and uses its gaping mouth and the pouch at the base of its lower jaw as a fishnet. When it surfaces, the bird tips its beak down to drain the water out, and then up to swallow the fish whole. Bald eagles also grab fish just below the surface but do so with the sharp talons on their feet instead of with their beaks. Because the eagle's prey is often too large to swallow whole, it uses its sharp, hooked beak to tear off bite-sized strips.

Some bird beaks, like those of the woodpecker, the flycatcher, and the bunting, are pincerlike, perfect for grasping tiny objects (usually insects) with great precision. These beak types are specialized further still, relative to each bird's method of foraging. The woodpecker's beak is tough and chisel-like, enabling the bird to excavate holes in trees in search of insects inside. The flycatcher's beak is long, with a slight hook at the tip and flanked with stiff hairlike feathers, all of which help the flycatcher capture insects on the wing. The bunting's beak is less specialized. It allows this species to eat a wider variety of foods, including insects and seeds.

The beak of the crossbill is one of the most specialized of all bird beaks. The upper and lower portions of its beak are curved in opposite directions and cross each other when the beak is closed, making the bird look deformed and the beak unusable. This shape, however, is perfectly suited to prying open the cones of conifers for the seeds they contain.


The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, probing for food, courtship and feeding young. The term also refers to a similar mouthpart in some cephalopods, cetaceans, pufferfishes, turtles, Anurantadpoles and sirens.
Anatomy

Beaks vary significantly in size and shape from species to species. The beak is composed of an upper jaw, called the maxilla, and a lower jaw, called the mandible. The jaw is made of bone, typically hollow or porous to conserve weight for flying. The outside surface of the beak is covered by a thin horny sheath of keratin called the rhamphotheca. Between the hard outer layer and the bone is a vascular layer containing blood vessels and nerve endings. The rhamphotheca can includes knob, which is found above the beak of some swans, such as the Mute Swan, and some domesticated Chinese geese (pictured).
The beak has two holes called nares (nostrils) which connect to the hollow inner beak and thence to the respiratory system.The nares are usually directly above the beak. In some birds, they are in a fleshy, often waxy structure at the base of the beak called the cere (from Latin cera, meaning wax). The cere is an indicator of the reproductive cycle of budgerigars.
Petrels and albatrosses have external horny sheaths called naricorns that protect the nares. These are separately placed on either side of the base of the upper mandible in albatrosses, but fused, with an internal septum, on the top of the base of the upper mandible in petrels.. In the mallard, and perhaps in other ducks, there is no cere, and the nostrils are in the hard part of the beak, as a soft cere would be liable to injury when the duck dredges for food among submerged debris and stones.
On some birds, the tip of the beak is hard, dead tissue used for heavy-duty tasks such as cracking nuts or killing prey. On other birds, such as ducks, the tip of the bill is sensitive and contains nerves, for locating things by touch. The beak is worn down by use, so it grows continually throughout the bird's life.

Uses of beaks

As noted by Darwin in his observations on Galapagos Finches, birds' beaks have evolved to suit the ecological niche they fill: Raptors have decurved (downward curving) beaks for ripping up meat. Hummingbirds have long thin beaks for reaching nectar. The spoonbills' beaks allow them to filter-feed in shallow water. Unlike jaws with teeth, beaks are not used for chewing. Birds swallow their food whole, and it is broken up in the gizzard








Anna's Hummingbirds have long, tubular bills that resemble straws, which they use to sip nectar from flowers.








Acorn Woodpeckers have a strong, long, chisel-like bill to make holes in oak trees.








Vermilion Flycatchers have a wide bill surrounded by a net of bristles that works to funnel flying insects into it's mouth while the flycatcher is in the air.








Mergansers have a long bill with serrated edges and a hooked point, adapted for grabbing fish.







The edges of a Mallard's bill are fringed to strain plants, seeds, and small animals from mud and water







Western Meadowlarks use their long pointed bills to probe for insects in the ground.







The Evening Grosbeak has a thick, conical beak, which is necessary for opening the hard outer shells of seeds to reach the nutritious interior.







Eagles tear prey, such as mice, into bite-sized pieces with their strong, hooked bill






Many shore birds have long, thin probing bills. These bills come in a variety of sizes to jab at different depths in the muck, allowing many species to live together without directly competing for food.









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Default chordates

Bird Feet Adaptation

Feet carry birds to their food and some help deliver food to the bird. They are designed for running, perching, grasping, wading, paddling and even more.
To hold onto a twig, a bird needs feet with opposing toes that wrap around the branch. Why don't perching birds fall off when they sleep? When perching birds sit, their feet automatically lock on the limb. With feet locked, sleeping birds don't fall. As the bird stands up its feet release.


Long-legged birds can wade in shallow water to reach prey buried in the mud in marshes. The Great Blue Heron's long toes give support for walking on mucky stream and lake bottoms.






Birds with webbed feet can paddle through the water and walk on mud. As a duck pushes its feet back, the web spreads out to provide more surface to thrust the water. Then, as the duck draws its foot forward and brings the toes together, the web folds up so there is less resistance to the water.







In open grasslands, most species walk or hop on the ground to find food.





Parrots use their feet to handle food just like we use our hands. Their nimble toes hold the food and bring it to the beak.





Hawks and owls capture, kill, and carry prey with their feet




Chickens use their strong feet to scratch the dirt and leaf litter to uncover seeds and insects.




Strong-legged flightless birds, like the cassowary, protect themselves by kicking with their powerful feet and sharp claws




Jacanas walk on floating plants without sinking because they have extremely long toes that spread the body weight over a wide area, just like using snowshoes on powdery snow.











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