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and the body correspondingly long and snake-like; but in other cases parts of the vertebræ are reduced in number, and the body is rather short and thick. In the frogs and toads this reduction reaches its culmination, for only nine distinct vertebræ are present, the tail vertebræ, corresponding to those of the salamanders, being represented by a rod-like bone, the urostyle, made of segments grown together.
179. Digestive and other systems.—In its main characters the digestive tract of the amphibian (Fig. 110) resembles
duo. Fig. 110.-Dissection of toad (Bufo). an., anal opening; au., auricle ; bl., bladder;
duo., duodenum ; Ing., lung; lr., liver; pn., pancreas ; rct., rectum; spl., spleen; st., stomach ; V., ventricle.
that of the fishes and the squirrel. The mouth is usually large, and the teeth are very small, as in the frog or salamander, or are lacking completely, as in the common toad. In many salamanders the tongue, like that of a fish, is fixed and incapable of movement. In most of the frogs and toads it is attached to the front of the mouth, leaving its hinder portion free, and capable of being thrown over and outward for a considerable distance. In the throat region gill-clefts may persist, but they usually close as the lungs reach their development. The succeeding portions of the canal are comparatively straight in the elongated forms, or more or less coiled in the shorter species. In some cases no well-marked stomach exists, but ordinarily the different portions, as they are shown in Fig. 110, are well defined.
As noted above, the circulation in the tadpole is the same as in fishes, then lungs arise, and for a time respiration is effected both by gills and lungs, and the circulation resembles in its essential points that of the lung-fishes. This may continue throughout life, but more frequently the gills and their vessels disappear, and the circulation approaches that of the reptiles. In such forms the heart consists of two auricles and one ventricle. Into the left auricle pours the pure blood from the lungs; into the right the impure blood from the body. To some extent these mix as they are forced into the general circulation by the single ventricle. The amount of oxygen carried is therefore smaller than in the higher air-breathers, the amount of energy is proportionately less, and hence it is that all are cold-blooded and of comparatively sluggish habits.
In some species of salamanders the lungs may also disappear, and breathing is carried on by the skin, as it is to a certain extent in all amphibians. In the frogs and toads lungs are invariably present, and vocal organs are situated at the opening of the windpipe in the throat. These produce the characteristic croaking and shrilling, which in many species are intensified through the agency of one or two large sacs communicating with the mouth-cavity.
Although the brain is small in the amphibians, it is more complex in several respects than it is in fishes. The eyes are also usually well developed, but in some of the cave and burrowing salamanders they are concealed beneath the skin, and are rudimentary. The ear varies considerably in complexity in the different species, but in the possession of semicircular canals and labyrinth resembles that of the fishes. In the frogs and toads, as one may readily discover, the drum or tympanum is external, appearing as a smooth circular area behind the eye. Organs of touch, smell, and taste are likewise developed in varying degree of perfection.
180. Breeding-habits.—While the great majority of amphibians mate in the spring and deposit their eggs in the water, often to the accompaniments of croakings and pipings almost deafening in intensity, several species, for various reasons, have adopted different methods. Some of the salamanders bring forth young alive, and several species of toads and frogs are known in which the young are cared for by the parent until their metamorphosis is complete. In one of the European toads (Alytes) the male winds the strings of eggs about his body until the tadpoles are
ready to hatch ; and in a few species of tree-toads the eggs are stored in a great pouch on the back of the parent until the early stages of growth are over. In the Surinam toad of South America the eggs are placed by the male on the back of the female, and each sinks into a cavity in the spongy skin. Here they pass through the tadpole stage without the usual attendant dangers, and emerge with the form of the adult.
Sunlight and warmth are apparent necessities for speedy development. Tadpoles kept in captivity where the conditions are generally unfavorable may require years to assume the adult form. As mentioned above, the tiger salamander (Amblystoma tigrinum) occurs in most parts of the United States and Mexico. In the East this species drops its gills in early life as other salamanders do, and assumes the adult form, but in the cold water of high mountain lakes, in Colorado and neighboring States, it may never become adult, always remaining as in Fig. 111. This peculiar form is locally known as axolotl. In this condition it breeds. It is thus one of the very few examples of animals whose undeveloped larvæ are able to produce their kind. Owing to this trait it was at first considered a distinct species, and many years elapsed before its relationship to the true adult form was discovered.
181. General characteristics.—In all the reptiles the general shape of the body, and to some extent the internal plan, is not materially different from that seen among the amphibians. In spite of external resemblance the actual relationship is not very close. It appears to be true that ages ago the ancestors of the modern reptiles were aquatic animals, possibly somewhat similar to some of the salamanders; but they have become greatly changed, and are now, strictly speaking, land animals. At no time in their development after leaving the egg do we find them living in the water and breathing by gills. Some species, such as the turtles, lead aquatic or semiaquatic lives, but the modifications which fit them for such an existence render them only slightly different from their land-inhabiting relatives. The skin bears overlapping scales or horny plates, united edge to edge, as in the turtles, enabling them to withstand the attacks of enemies and the effects of heat and dryness. Indeed, it is when heat is greatest that reptiles are most active. In no other class of vertebrates, and very few invertebrates, do normal activities of the body appear to be so directly dependent upon external warmth. In the presence of cold they rapidly grow sluggish, and sink into a dormant state.
As in the case of all animals, habits depend upon structure, and accordingly among the reptiles we find many remarkable modifications, enabling them to lead