There are about 5500 species of living reptiles, of which over 300 are found in the United States. Although, as compared with amphibians, their legs lengthen and strengthen, “reptiles” (re per e, to crawl) are named with an eye to the legless crawling snakes. The group includes not only snakes but also lizards, turtles, and alligators, as well as Sphenodon, a New Zealand genus containing a single species. Also included is a vast company of forms now extinct, many of which were gigantic, that dominated the Mesozoic world throughout a dynasty that endured for ages.
Reptiles are the first true land vertebrates freed from the necessity of returning to the water to breed. “Things that before swam in the water now went upon the ground” (Wisdom of Solomon). This saying is true even of alligators, certain turtles, and water snakes which, although they spend much of the time in water, come out upon the land to lay their eggs. Each egg is usually fertilized and then covered with a shell while still inside the body of the female.
As mentioned in the discussion of amphibians, each embryo of a reptile, bird or mammal is surrounded by an amnion during most of its development (Fig. 34). The astonishingly rapid growth of any developing embryo necessitates a protective covering for the extremely delicate cells and tissues that does not involve hampering adaptations for withstanding exposure to dry air or mechanical shocks during their tumultuous multiplication. Growing embryos of fishes and amphibians have such a provision in the surrounding medium of water in which they are immersed, but reptiles and all other conquerors of the land who cannot cradle their growing youngsters in open water, depend upon a protective antenatal robe that the young embryo forms about itself. This is the amnion, a thin enveloping sac filled with a secreted watery fluid in which the embryo floats. It may be trulysaid, therefore, that in a certain sense every vertebrate passes its early life submerged in water. Because all reptiles, birds and mammals have an embryonic amnion, they are collectively known# as Amniota; while all lower vertebrates in which no amnion develops, are called Anajnnia.
In the closed amnion sac neither gills nor lungs, nor even body surface, can serve as respiratory organs. Hence, contemporary with the amnion is an allantois, an outgrowth from the posterior part of the digestive tract which serves as a temporary breathing and excreting organ. This emergency organ, bearing a rich network of blood vessels, grows out into the space between the amnion and the thin, inner egg-shell membrane, or chorion, so that through the latter and the porous egg-shell there is effected the interchange of gases between the blood and the outside world essential to breathing and excretion. In the case of mammals, the egg has no shell, but develops into a fetus that is parasitically attached to the uterine walls of the mother. The capillary-laden allantois, coming into intimate contact with the richly vascularized wall of the uterus, by interdigitations, forms the placenta. It is through this organ that the young animal breathes and excretes until it is born into independence.
Reptiles have a rather thick integument with few glands and many scales. The lungs are usually well developed in adults. The excretory organs (metanephroi) are of a more advanced type than those (mesonephroi) of fishes and amphibians. On the ends of the digits are claws. The skull bears only one occipital condyle for articulation with the first vertebra. In modern species, the pelvic appendages are connected with two sacral vertebrae. A partial or complete, partition develops between the right and left sides of the ventricle of the heart. Like the lower vertebrates, however, they are cold-blooded.
Extinct reptiles with their fossil remains wrote a long and dramatic chapter in the history of living things upon the earth for modern man to read. Several entire orders, the flying pterosaurs, aquatic ichthyosaurs, and long-necked plesiosaurs, for example, have, so far as is known, left no living descendants, but others have been the ancestors of not only recent reptiles but also birds and mammals.
There are more than a dozen orders of reptiles of which only four include living species. During the Golden Mesozoic Age of Reptiles, which lasted according to some geologists, from 125 to 150 million years, these ruling animals attained a great diversity of form and adaptation, enabling them to live in a variety of habitats, such as forests, water, swamps, dry land, and air. The imagination is thrilled by a picture of the Mesozoic landscape with its weird reptilian population. Some of these strange creatures of past precamera days are suggested by the sketches in Figure 35.
The cotylosaurs, or stem-reptiles, were the earliest members of the group (Fig. 35A). Structurally they closely resembled the most primitive of the Amphibia. In external appearance they probably looked like the larger of our lizards.
The therapsids, another early group, possessed certain features which indicate they were ancestral to the mammals (Fig. 35). Their teeth were differentiated into the three major types: cutting incisors; large, pointed canines; and grinding molariform teeth. Their jaws also showed a trend toward the mammalian plan.
The dinosaurs (Fig. 36B, D, and H), including many species which varied widely in appearance, ranged in size from tiny forms about the size of a hen to the well-known enormous species which were the largest animals ever to walk on the face of the earth. Many of these reptiles were probably bipeds, capable of raising themselves up on their hind legs for more rapid running. To counterbalance the front parts of the body there was a long, rather heavy tail. The front legs, shorter than the hind legs, were presumably used when the animal was resting or walking slowly. These bipedal forms may have been close cousins of the earliest birds, both groups coming from the same immediate ancestral stock. The largest of the dinosaurs were undoubtedly quadrupeds (four-footed). Apparently many of them developed amphibious habits, spending much of their time in lagoons and swamps where the water reduced the weight which it was necessary for their legs to bear.
Of the reptiles which returned to the water to live, the ichthyosaurs (ichthyo, fish; saur, lizard, reptile) were the best adapted to aquatic life (Fig. 36c). Their paired appendages were flippers, modified from typical tetrapod legs. In general appearance they were fish-like. Without doubt they breathed by means of lungs. As amniotes, each of their embryos presumably developed an amnion and an allantois and was incapable of surviving if the egg was laid in the water. A number of specimens have been found with small complete skeletons inside the body of the adult. It is believed, therefore, that these animals were viviparous.
Some of the bipedal early reptiles apparently evolved into flying species (Fig. 37). These were pterosaurs (ptero, wing). The fourth digit of each front appendage became strong and considerably elongated to support the wing, a fold of the skin. Just how much of their activity was true flying and how much was gliding is still problematical.
Modern surviving reptiles may be grouped into the following orders: Rhynchocephalia, Chelonia, Squamata, and Crocodilia.
The rhynchocephals, which include many fossil kinds, are represented today by a single surviving genus, Sphenodon, or the “tuatara” of New Zealand. This is a long-tailed, lizard-like animal usually somewhat less than two feet in length as an adult. In common with the many extinct species of this Order, Sphenodon shows many primitive reptilian features. It probably owes its survival to its isolation in an area where it did not have to compete with mammals. Reference has already been made to the median eye of this interesting “old curiosity shop” of ancestral peculiarities, and there will be future occasion to rummage further in this anatomical-attic for sidelights of the vertebrate past.
The chelonians, or turtles and tortoises, modified “reptiles in a box,” are unusual in many respects. Their internal organs, in both shape and arrangement, are adapted to fit into a short, broad box formed by the ventral, relatively flat plastron and the dorsal, more or less curved carapace. This shell consists of bony plates covered over with large, thin, homy scales. The trunk vertebrae are attached to a row of these plates running down the middle of the carapace. Consequently the only flexible portions of the vertebral column are the neck and tail regions. These two regions, together with the four legs, ’may be withdrawn beneath the shell into a place of protection. Turtles possess toothless jaws encased in horny beaks.
A few of the genera of turtles are: the “leatherback,” Dermochelys (Fig. 38); the “loggerhead,” Thala’ssochelys, which cruises about in salt water and may attain a weight of several hundred pounds; the “green turtle,” Chelonia mydas, also a seagoing animal, prized as food; the “snappers,” Chelydra (Fig. 38) and Macrochelys, the latter of which has a bite powerful enough to amputate a foot; the “box turtle,” Terrapene, that is able to withdraw its head entirely within its shell and to close the door with a hinged lid; and finally, the small beautifully decorated “painted turtle,” Chrysemys. America of all the continents is particularly rich in chelonial inhabitants.
The squamates are reptiles clothed with a great number of regularly placed scales which cannot be separately detached like the scales of bony fishes but are connected together into a continuous armor. They comprise two suborders: the LACERTILIA, or lizards, and the OPHIDIA, or snakes, distinguished from each other by the fact that the former have movable eyelids, visible earpits and usually legs, while the latter do not.
The lizards are typically sun worshippers, dwelling in regions of much sunshine, and for the most part avoiding water. A notable exception is the water lizard, Amblyrhynchus, which is an inhabitant of the rocky shores of the Galapagos Islands.
The “geckos,” of the Malay region and the Mediterranean countries, have adhesive toes that enable them to clamber about with great agility after flies and other insects in trees and upon the walls inside of houses (Fig. 39). The “flying dragon” of India, Draco (Fig. 40), is able to volplane from branch to branch of the trees that it inhabits, by means of a capelike expansion of the skin down the sides.
Iguana is a large arboreal Mexican lizard of fierce aspect but harmless habit which is regarded as good to eat. The giant of them all is the rare Vor anus komodoensis, the “dragon” of the East Indies, which may attain a length of twelve feet.
The “chameleon,” Chameleon (Fig. 41), has a prehensile tail and grasping feet and flaunts a Joseph’s coat of many colors. It is a native of Africa, although its name is sometimes erroneously applied to the little American Anolis (Fig. 42) of changeable colors that inhabits the cane fields of the South and preys upon the insects which are attracted by the sweet juice that oozes from the cane.
The “glass snake,” Ophisaurus of the Old World, and the “slow worm,” Amphisbaena, are legless lizards.
In the desert region of the southwestern United States are found the grotesque “horned toad,” Phrynosoma (Fig. 43), that is called a toad only by the undiscriminating, and the “Gila monster,” Heloderma (Fig. 44), an ugly black and orange beast, with a large round stubby tail, which is the only lizard whose bite is venomous.
The serpents are the legless snakes, described by Ruskin as “a wave but without wind, a current but with no fall.” They walk upon their numerous ribs, or “jerk themselves forward by a rapid straightening of their sinuous curves” (Thomson). The curious arrangement of the internal organs of these creatures has a direct connection with their external architecture.
Most snakes have few human friends, in spite of the fact that most of them are beneficial animals, feeding largely upon injurious insects and small rodents. They have suffered vicariously from an unsavory reputation ever since one of their number was reported to have taken part in the original eternal triangle play in the Garden of Eden.
Of approximately 110 species in the United States, less than 20 are venomous. The most dangerous of these, so far as man is concerned, are the several species of rattlesnakes in the genus Crotalus (Fig. 45); the “copperhead,” Agkistrodon mokasen, and the “water moccasin,” Agkistrodon piscivorus, of the South; and the “coral snakes,” Micrurus, also of the South.
The “black snake,” Coluber; the “puff adder,” Heterodon; the “milk snake,” Lampropeltis; and the “garter snake,” Thamnophis, are among the common harmless varieties.
The crocodiles are, in a number of respects, the most advanced of the reptiles. Their lungs are very efficient organs. Their heart has two completely separate ventricles, a condition found also in birds and mammals. The brain has large cerebral hemispheres. The Crocodilia include the crocodiles proper of India, China, Africa, the Malay Archipelago, Central America, Mexico, and the southeastern United States; the broad-snouted alligators of the Mississippi Basin, Florida, and China (Fig. 46); the caimans of Central and South America; and the narrow-snouted gavials of the Ganges in India. All are inhabitants of tropical or semi-tropical countries and, though clumsy and stiff-necked on land, are quite at home in shallow water, where their powerful laterally compressed tails enable them both to swim forward and to strike powerful side blows.