THE ORGANS OF
CIRCULATION
The heart is four-chambered, as in mammals and birds, and not three-chambered, as in other reptiles and amphibia. The pure and impure blood do not, therefore, mix inside the heart; but as such blending takes place outside, owing to imperfect separation of the great vessels, the net result is much the same as in the lizards.
HOW SNAKES SECRETE
POISONS
In venomous serpents some of the glands opening into the mouth secrete a poisonous fluid, which is introduced into the blood of a bitten victim. The largest amount of specialization is found among the vipers, where the teeth are reduced to a pair of hollow “fangs” in the front of the upper jaw, and there are two large poison-glands, one on either side of the head, giving it a characteristic resemblance to the ace of spades. In a state of rest, when the mouth is shut, the poison-fangs are pressed against the roof of the mouth, with their tips directed backwards. But when the snake opens its mouth and “strikes,” the fangs are rotated forward so that their sharp tips can be brought into action. The poison flows into the upper end of the tooth-canal and, in vipers, enters the wound by a small hole on the side of the tip. Were it at the end a blockage might result. We have, in fact, an anticipation of the device used in the construction of the needles employed with hypodermic syringe.
WHY SNAKES ARE
COLORED
Snakes, like lizards, are very commonly colored in such a way that they may harmonize with their surroundings. A good many poisonous forms, on the other hand, advertise their dangerous properties by brilliant hues and striking patterns. Such “warning coloration” is seen, for example, in the coral snakes of tropical America, which are marked with broad red rings, alternating with others of whitish tint, shading into black at the front and back of each ring. These coral snakes serve as models which certain harmless forms unconsciously mimic, thus securing a certain amount of immunity from attack by sailing under false colors.
SOME SNAKES THAT
WARN
In the American rattlesnakes, at each periodical casting of the skin or slough, a little knob remains at the end of the tail. A series of these loosely united together make up the “rattle,” used for the production of warning sounds. The “hissing” of a snake has the same purpose. Venomous snakes also commonly assume a warning attitude, raising the front part of the body from the ground and, in some cases, as illustrated by the cobra, inflating a kind of hood—in this particular instance bringing a black, spectacle-shaped mark into prominence.
But in these and other animals it must not be supposed that the “warning” is for the benefit of the prey, but may be taken as a hint to aggressive birds and mammals that discretion is the better part of valor. The success of this device is shown by the terror with which all monkeys regard serpents.