Amphibians

The ancestors of today's amphibians were the first chordates to venture onto land (sometime during the Devonian, about 350 million years before present). Although many changes in the anatomy, physiology, and behavior were required for terrestrial colonization, several preadaptations among the early amphibians eased their colonization of terrestrial habitats. Based on fossil and anatomical evidence, most zoologists believe that amphibians evolved from primitive lobe-finned fish. These fishes had fins supported by bones that set them away from their bodies. Such structures undoubtedly were useful for pulling them around on land when their ponds dried up. Limbs such as these, combined with more efficient lungs, permitted the first amphibians to crawl on land. Two barriers, however, keep most amphibians from a totally terrestrial lifestyle: respiration and reproduction.

Today's amphibians have a moist, glandular skin with no scales (with a few exceptions covered below). Most have two pair of limbs adapted for walking and/or swimming. Their hearts have three chambers (two auricles and one ventricle) and both oxygenated and deoxygenated blood pass through the Ventricle. Respiration is by the skin, lining of the mouth, gills, and/or lungs, depending on the species or stage in their life history. Most cannot survive away from water for very long because they lose too much moisture through their thin respiratory surfaces. In addition, most amphibians require water for reproduction and have an aquatic larval stage. Amphibian eggs are usually enveloped by a gelatinous mass that, unlike the leathery covering of reptile eggs or the hard calcareous shell of bird eggs, provides little protection from drying out. Fertilization may be internal or external and most are oviparous.

There are three surviving orders of amphibians: Caudata (tailed amphibians), Anura (frogs and toads), and Gymnophiona (caecilians). Caecilians are wormlike creatures found in tropical forests. They make their livings by burrowing through the soft soil searching for worms and other tasty invertebrates. Several adaptations are related to their subterranean habits: They have no legs, are almost totally blind (atrophied eyes are hidden under the skin), and have sensory tentacles. Their skin is annulated, and some have minute dermal scales. Their coloration is usually drab. This combination of characteristics makes them look very much like overgrown earthworms.

Caudatans (salamanders and newts) have poorly developed limbs and retain a tail as adults (the name Caudata refers to the presence of a tail). They prey on worms, insects, and small mollusks. Some species have no lungs and depend entirely on cutaneous respiration. Others, such as the mudpuppy (Necturus) and the axolotl (Ambystoma), retain the larval gills as adults.

Frogs and toads belong to the order ("Anura" refers to the lack of a tail in adults). They differ from the caudates by having a more complex skeletal system with stronger limbs and developmental metamorphosis (from a tailed and limbless polliwog to a tailless limbed adult). Of all the amphibians, anurans have been most successful in their occupation of terrestrial habitats (including trees).  

Mudpuppies (Necturus) live in lakes and rivers of central and eastern United States. Like other caudatans, mudpuppies undergo a metamorphic development, starting life as a fish-like gilled larva. Unlike most other salamanders, the mudpuppy retains the gills (and two pair of gill slits) when it reaches sexual maturity. If an animal retains larval characteristics as an adult, this condition is called neoteny. A similar situation exists in the Mexican axolotl (Ambystoma). Other salamanders show varying degrees of neoteny. The hellbender, for example, retains gill openings as an adult. In some amphibians the neotenous condition is caused by an interruption in thyroid metabolism. Axolotl, for example, will "complete" their metamorphosis if provided with the hormone thyroxine (this doesn't work with Necturus).

No single trait distinguishes frogs from toads, but toads tend to be more terrestrial in their habits, have drier skin, and possess parotid glands behind the eyes that can secrete noxious or poisonous substances. Besides being tailless as adults, anurans differ from the salamanders in their ability to jump.  During development the frog passes from an aquatic stage to a semiaquatic existence. Coupled with this change in lifestyle are alterations in their feeding, digestion, respiration, and behavior. Newly emergent tadpoles lack mouths and breathe through external gills (actually just skin outgrowths). These gills are supplied with fine capillaries and are ciliated. Tadpoles at this stage do not swim, but remain attached for a day or so to the substratum. Eventually a mouth develops, surrounded by large lips with horny papillae. The tadpoles use these "lip teeth" to scrape algae and other incrustations from rock surfaces. Since their diet is based almost entirely on plant material, the intestine is long and coiled to deal with the bulky food (visible on the ventral surface). The external gills finally disappear and are replaced by internal gills. The gill aperture is covered by an operculum that, unlike that of fish, is fused posteriorly leaving a single opening or spiracle on one side of the animal. Respiratory water first passes into the mouth, then the pharynx. From there it moves into the branchial chamber formed by the operculum, then over the gills and out the spiracle. As the animal increases in size it becomes carnivorous, and the digestive system shrinks to suit its new lifestyle. About this time the hindlegs make their first appearance as limb buds, which then elongate and grow into the muscular hindleg of the adult frog. The forelegs grow beneath the opercula and remain hidden until they tear their way free. At about the third month of development the legs have completely formed and the animal begins a shift from a totally aquatic to a semiaquatic lifestyle. At this time it breaths by both gills and lungs (it makes occasional trips to the surface to gulp air). Finally, the gills disappear and the animal looks froglike except for its tail and more rounded head. The tail is eventually absorbed, and the head flattens to take on the adult form.

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General view of the mudpuppy.	Complete skeleton of the mudpuppy.	Ventral (Left) and dorsal (right) views of the chondocranium of the mudpuppy
Lateral view of the chondocranium of the mudpuppy	Euryops

Frog Gutz

Frog arteries

Frog Veins

Frog heart

Frog Skeleton

Necturis Skull