(4) _Fresh-Water Snakes_ are especially adapted for an aquatic existence, and have their nostrils on the top of the snout, to enable them to breathe easily when in the water. Some of them can hold on to weeds or other things by their tails. They swim and dive, and are as active as eels. None are very large, and all are harmless. But a good many of the second group that are poisonous, spend so much of their time in the water that they are known as ‘water vipers,’ ‘water moccasins,’ etc., though not truly water snakes. (5) _Sea Snakes._—All highly venomous. These, as also the fresh-water snakes, are treated fully in chapters xiii. and xiv. The five divisions assist the student towards grasping an idea of the principal groups, but the whole five pass into each other by intermediate forms and imperceptible degrees. Some other general characteristics of the Ophidia are that all are carnivorous, catching their prey alive; all are oviparous; and in organization and intelligence they rank between birds and fishes,—higher than fishes in having lungs, and lower than birds, which are warm-blooded animals. Their heart is so formed as to send only a portion of blood to the lungs on each contraction of it; their temperature, therefore, is that of the surrounding atmosphere (see p. 142). Their normal condition, particularly that of the venomous species, is one of lethargic repose and indolence, with a disposition to retreat and hide, rather than to obtrude themselves. On this account, and also because so many of them are nocturnal in their habits, less has been truly known of serpents than of most other creatures, prejudice having added to a prevailing indifference regarding them. The duration of their lives is uncertain, or whether they have a stated period of growth. Some naturalists think they grow all their lives; but this must not be taken literally, or that if a small snake happened to escape dangers, and live a very long while, it would acquire the dimensions of a python. Some think that formerly the constrictors did attain more formidable proportions than those of the present day. Snakes have small brains, slight intelligence, and slow sensations, amounting almost to insensibility to pain. They can live a long while without their brains and without their heart; while the latter, if taken from the body, will continue its pulsations for a considerable time. Also if the head be severed, the body will for a certain time continue to move, coil, and even spring, and the head will try to bite, and the tongue dart out as in life.