Encyclopædia Britannica, Ninth Edition/Boa
BOA, a name formerly applied to all large Serpents, which, devoid of poison fangs, killed their prey by constriction; but now confined to that section of them occurring in America, the Old World forms being known as Pythons. The true boas are widely distributed throughout tropical America, occurring most abundantly in Guiana and Brazil, where they are found in dry sandy localities, amid forests, on the banks of rivers and lakes, and in the water itself, according to the habits of the various species. They feed chiefly on the smaller quadrupeds, in search of which they often ascend trees, suspending, themselves from the branches by the tail, and thus awaiting motionless the approach of their victim. While so hanging they are partly supported by two spine-like hooks, situated one on each side of the vent, which are connected with several small bones concealed beneath the skin and attached to the main skeleton. These bones, terminating thus in an external claw, are characteristic of the family Boidæ, and are recognized by anatomists as the rudiments of those which form the hind limbs in all quadrupeds. The size of the boa's prey often seems enormously beyond its apparent capacity for swallowing, a difficulty which disappears on acquaintance with the peculiar structure of the creature's jaws. The bones composing these are not knit together as in Mammals, but are merely connected by ligaments, which can be distended at pleasure. The mouth of the boa can thus be made to open transversely as well as vertically; and in addition to this the two jaws are not connected directly as in other animals, but by the intervention of a distinct bone, which adds greatly to the extent of its gape. It has also the power of moving one half of the jaw independently of the other, and can thus keep a firm hold of its victim while gradually swallowing it. The boa possesses a double row of solid sharp teeth in the upper jaw, and a single row beneath, all pointing inwards, so that, its prey once caught, it would be well-nigh impossible even for the boa itself to release it. After feeding, boas, like all other reptiles, become inactive, and remain so while the process of digestion is going on, which, in the case of a full meal, may extend over a few weeks, and during this period they are readily killed. All the species are ovoviviparous. The Jiboya or Boa constrictor — the latter name having been loosely given to all the species — is an inhabitant of the dry and sandy districts of tropical America, and rarely exceeds 20 feet in length. Its food consists chiefly of the agoutis, capybaras, and ant-bears, which abound in those districts. It seeks to avoid man, and is not feared by the inhabitants, who kill it readily with a sharp blow from a stick. The Water-Boa or Anaconda (Eunectens murinus) is a much more formidable creature, attaining, it is said, a length of 40 feet, and being thus probably the largest of living serpents. It inhabits the lakes, rivers, and marshes of Brazil and Guiana, and passes a considerable portion of its existence in the water. It is exceedingly voracious, feeding on fishes and on such animals as may come to the banks of the stream to drink, for which it lies in wait with only a small part of its head above the surface of the water. It also occasionally visits the farmyards, carrying off poultry and young cattle, and it has been known to attack man.