The most powerful saurians of the present day—the crocodile the gavial and the alligator—have left to the dolphins, the sharks, and other monstrous or swiftly-swimming cetaceans and fishes the dominion of the seas, and now merely infest the rivers and swamps of the tropical zone. The lizards also have long since retired from the scene where they once abounded, and the ocean at present harbours no other reptiles in its bosom than turtles and sea-snakes.
Most of the animals belonging to this class are either dangerous or of a disgusting appearance. Few creatures are objects of such universal abhorrence as the crocodile—the very type of brutal cold-blooded ferocity; as the venomous snake—the emblem of perfidy and ingratitude; or as the loathsome, but innocent toad, to which, on account of its ugliness, noxious properties have been ascribed which the poor animal does not possess. The frogs, lizards, and turtles alone seem to have escaped this general detestation, either from their more active habits, or their well-known harmlessness, or their various utility to man.
The anatomy of the turtle offers many points of interest; its vertebræ, ribs, and breast-bone growing together so as to form a bony envelope round the whole animal. This harness is covered by the skin, which in its turn is bedecked with large scales, while all the muscles and other soft parts are enclosed in the inner cavity. Only the head, feet, and tail protrude through openings between the upper and under carapace, and these can, by the land tortoises at least, be withdrawn entirely under the former. This is the only protection which Nature has afforded these animals against their enemies, for they have neither swiftness of flight, nor any offensive weapon at their command. But as soon as anything suspicious approaches, they conceal themselves under their massive cover, and oppose to every attack by tooth or nail the passive resistance of an impenetrable shield. Most of their enemies find it, besides, no easy task to turn them on their back, as many species attain a very considerable weight, so that their mere bulk constitutes a good defence. It might be supposed that this protection could only avail for a short time, as the want of air must soon force the animal to stretch its head out of its hiding-place, and this indeed would be the case, if kind Nature had not taken her measures against this emergency, by giving the creature a cold blood, so that it can remain a very long time without breathing; long enough, at least, to tire the patience of the most obstinate foe.
Skeleton of Tortoise.
A, superior maxilla; B, inferior maxilla; C, ossiculum auditus; D, os hyoides; E, cervical vertebræ; F, dorsal vertebræ; G, sacrum; H, caudal vertebræ; I, dorsal ribs; K, marginal scales; N, scapula; O, coracoid bone; P, os humeri; Q, radius; R, ulna; S, bones of the carpus; T, metacarpal bones; U, digital phalanges; V, pelvis; W, femur; X, tibia; Y, fibula; Z, tarsus; Æ, metatarsus; A.V., phalanges of the foot.
But how comes it, the reader may ask, that respiration, which pours a warm current through our veins, fails in raising the temperature of the turtle's blood?
Without entering into a lengthened description of the human heart, I shall merely observe that it consists of two halves (each half being again subdivided into two separate chambers), and that the right half, which receives venous blood and pours it into the lungs, is completely separated by a partition from the left half, which receives arterial or aërated blood from the lungs, and propels it into every part of the body. Thus the two different kinds of blood are completely separated, so that an unmixed venous blood flows into the lungs, where it is converted by the oxygen of the air into arterial blood. But this connection, like most chemical processes, takes place under an evolution of heat, which is so considerable that our internal temperature constantly maintains itself at the height of 98° F.
Theoretic Representation of the Circulation in Mammals and Birds.