The Encyclopedia Americana (1920)/Serpents
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|Edition of 1920. See also Snake on Wikipedia, and the disclaimer.|
SERPENTS, or SNAKES, reptiles of the saurian class Ophidia, characterized by an elongated, cylindrical, limbless, scaly form, and distinguished from lizards (q.v.) by the fact that the halves (rami) of the lower jaws are not solidly united at the chin, but movably connected by an elastic ligament. The vertebræ are very numerous, gastrocentrous and procœlous
, Each vertebra is connected with its neighbors that is, hollow in front and convex posteriorly, by free ball-and-socket joints, horizontal connecting projections preventing twisting, while admitting of considerable vertical and horizontal play; and bears ribs, which may exceed 300 pairs in number. No front limbs, sternum or sacrum are ever developed, but vestiges of ancestral hind limbs remain in the boas, pythons and closely related forms, and in some lowly burrowing groups. The ventral tips of each pair of ribs are fastened to opposite ends of one of the abdominal scutes (gastrosteges), forming a mechanism for locomotion in the absence of legs. Progression is by three methods. “The animal,” says Stejneger, “may glide, perhaps in a perfectly straight line, by use of its ventral scutes, each, on finding some resistance, forcibly pushing the animal forward. It may walk by allowing each scute to act as a pair of feet, the lateral portions being alternately carried forward and pushed back; an undulatory movement, like that of myriapods, would result from this mode. The third method is by pushing, as the underground snakes do almost exclusively. Ordinarily ophidians combine the three methods. The sea-snakes progress by an undulatory movement and by the sculling action of the paddle-like tail. . . . It is impossible for any ophidian to jump, and it is with extreme difficulty that more than the anterior half of the body can be raised, unassisted, from the ground.”
Structure.— The skull of serpents consists of a series of bones homologous with the cranial bones of other vertebrates, but slight and, outside of the brain-box, joined by elastic ligaments, permitting great distention of the mouth and throat, so that serpents are able to swallow objects of large bulk. This arrangement is further facilitated by the quadrate bone, which articulates the lower jaw with the skull, being movable. The jaws are provided with hooked teeth of conical shape, ossified to the jaws, but not lodged in distinct sockets, which are useless for mastication, and are of service only in holding the prey in the mouth. The teeth are never permanent, but are capable of being renewed, like those of fishes, whenever the old ones become worthless. Valuable characters in classification are derived from the conformation and disposition of the teeth. Thus in the typical, non-poisonous serpents, both jaws and the palate bear continuous rows of solid conical teeth and the upper maxillæ are immobile. In the viperine snakes simple conical teeth are absent on the upper maxillary or jaw bones, which are of small size and can be moved upward or downward at will. The upper maxillæ in these latter snakes further bear each a so-called “poison-fang,” an elongated canaliculated tooth perforated by a canal which communicates internally with the duct of the poison-gland. These fangs are capable of being elevated or depressed. (See Rattlesnake; Vipers). As regards their internal structure serpents present few points requiring notice. The digestive system comprises large salivary glands, a distensible gullet, stomach and intestine, which terminates in a cloaca — the external opening of the cloaca being transverse in conformation. There is no urinary bladder, yet serpents drink a great deal of water and must have it in abundance when kept in captivity; at the same time they have remarkable powers of long fasting. The heart (see Reptiles) consists of three chambers only — two auricles and a ventricle, the circulation being of the mixed character characteristic of reptiles and amphibians. The lungs, ovaries and other paired or symmetrical organs exhibit an abortive condition of one of these structures, the left lung, for example, far outgrowing the right one, and doing all the work. “The trachea is long and may be provided with air-cells and the larynx can be projected during the tedious process of swallowing, when, too, the air-cells and the posterior reservoir come into play.” All these modifications are in adaptation to the slender, elongated form of the animal and to its methods of life. The sexes are perfectly distinguished and the penis of the male consists of a pair of organs (hemipenes), grooved on their inner sides, which when erected are pressed together and so form a tubular intromittent instrument. The special modifications of this organ constitute one of the most stable “characters” or criteria by which to classify the subdivisions of the Ophidia. The eggs of the viperine, and some other snakes, are retained within the oviduct of the female until the young escape, so that these species are ovo-viviparous. In the majority of serpents, however, the eggs, which are oblong and have a leathery integument, are deposited in warm soil, among rotting wood, in heaps of decaying vegetation or in some similar warm and concealed place, where they are left to mature; but the pythons coil about their eggs and incubate them by the feeble heat of their bodies; and all kinds wait for and protect their young when born. It is true, to a limited degree, that at times of danger, the little snakes take refuge in the capacious mouth and throat of the mother.
Sense-organs.— The senses of serpents vary in respect to acuteness. The skin is highly sensitive to touch, and a few snakes have tentacle-like outgrowths upon the muzzle supposed to be “feelers”; this sense is principally developed, however, in the tongue, which is thread-like, forked at the end, usually black, very long and far-protrusible through a notch in the upper lip when the mouth is closed. With this tongue — mistaken by the ignorant for a “sting” — the animal tests the quality of all objects as it moves about and gains most of its information. There are no external ears, but the hearing in some, if not in all, is good. The nostrils are at the tip of the snout, and the sense of smell is keen, — serpents find and follow their prey mainly by its aid. The eyes of serpents have no eyelids, yet they sleep; a watch-glass-like layer of the skin overlies and protects the ball and peels off in the annual exuviation of the outer skin (see Molting). The pupil is usually round, but in the boas and some others is a mere vertical slit.
Phylogeny.— Serpents are the latest and highest development of the reptilian type and seem to be the only representatives of the class which are now flourishing rather than declining. They are distributed principally in the warmer regions, only the smaller forms extending into the northern temperate zone. Where the climate is even moderately cold in winter snakes creep into animals' burrows, holes among loose rocks and other underground places and undergo hibernation, — often many entwine together, especially toward spring, when they are seeking mates. A similar retirement (æstivation) occurs in countries exposed to annual periods of heated drought. By far the greater number of ophidians are terrestrial, although some are arboreal, others amphibious and a few exclusively marine. About 400 recent genera and nearly 1,800 species are known, but only about 35 fossil forms, according to Eastman, who reminds us that many fragmentary remains of the Cretaceous Age, at first regarded as ophidian, are now known to be dolichosaurian. Tertiary snakes can hardly be distinguished from modern species and occur in all parts of the world, including the Eocene of New Jersey and the Eocene and Miocene of the Rocky Mountain region. These remains and other considerations make it plain that the serpents are an outgrowth from the same ancestral stock (Sauria) as the lizards.
Classification.— The following is a progressive arrangement of families, from lowest to highest in organization, some of them including several groups formerly regarded as separate families:
1. Typhlopidæ. Burrowing snakes of southern Asia and the islands of the Indian Ocean. Have vestiges of the pelvis and the eyes covered by skin.
2. Glauconiidæ. Replace the Typhlopidæ in Africa and tropical America.
3. Ilysiidæ. Burrowing snakes of the tropics with vestigial hind limbs. Tail short and blunt (blind snakes).
4. Uropeltidæ. Burrowing snakes of Ceylon and southern India. Tail truncate, ending in a flat shield (shield-tails).
5. Boidæ. Large, typical snakes, with rudiments of hind legs and pelvis and ventral scales transversely enlarged. The family comprises about 70 species, found all over the world in tropical and subtropical countries and noted for their rapacious habits and ability to crush animals, large as compared with themselves. Typical genera are Python (q.v.), with about 20 Palæo-tropical and Australian species, one in Mexico, and the Boa (q.v.), which has many South American species and a few in Madagascar.
6. Xenopeltidæ, one Malayan species.
7. Colubridæ. Includes 90 per cent of all snakes; the pterygoid bone reaches the quadrate. It is divisible into three series: (1) No teeth grooved; (2) the posterior maxillary teeth grooved (poison-fangs); (3) the anterior maxillary teeth grooved (poison-fangs). The first includes the non-venomous colubrine snakes of more than 1,000 species, such as the blacksnake, garter-snake, water-snake, hog-nose and the ordinary harmless serpents of all parts of the world. The second group (2) comprises opisthoglyph, partly poisonous snakes, found in the tropics of both continents, terrestrial, aquatic, even marine species. The third group (3) contains many violently poisonous, proteroglyph snakes. Here fall the cobras, coral snakes and other elapine serpents, whose bite is to be dreaded.
8. Amblycephalidæ. Tropical snakes, in which the ends of the pterygoids are free, not reaching the quadrates. Harmless, but resembling poisonous snakes.
9. Viperidæ. Maxillaries, short and movable, so as to erect the poison-fangs, which are the only maxillary teeth. Two subfamilies: (1) the Old World vipers; (2) the rattlesnakes (q.v.) and related pit-vipers.
Bibliography.— Books mentioned under Reptiles; also Hopley, ‘Snakes’ (London 1882); Stejneger, ‘Standard Natural History,’ Vol. III (Boston 1885); Gadow, ‘Amphibia and Reptiles’ (New York 1901); Fayrer, ‘Thanatophidia of India’ (London 1874); Ewart, ‘Poisonous Snakes of India’ (London 1872); Kreft, ‘Snakes of Australia’ (Sydney 1869); Holbrook, ‘North American Herptology’ (Philadelphia 1842); Ditmars, ‘The Reptile Book’ (New York 1907); Cope, ‘Crocodilians, Lizards and Snakes of North America’ (Washington 1900). The last named is the most complete work on American serpents and is published by the Smithsonian Institution, in whose annual reports and other publications will be found much additional valuable information, especially the contributions by Yarrow, Stejneger, Mitchell and Hay.
|1. Milk Snake||4. Mountain Blacksnake||6. Water-snake|
|2. Copperhead||5. Eastern Rattlesnake||7. Ribbon-snake|
|3. Blacksnake||8. Garter-snake|