External Form and General Considerations

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. External Form and General Considerations

External Form and General Considerations

Section 1. It is unnecessary to enter upon a description of the appearance of this familiar type, but it is not perhaps superfluous, as we proceed to consider its anatomy, to call attention to one or two points in its external, or externally apparent structure. Most of our readers know that it belongs to that one of two primary animal divisions which is called the vertebrata, and that the distinctive feature which place it in this division is the possession of a spinal column or backbone, really a series of small ring-like bones, the vertebrae () strung together, as it were, on the main nerve axis, the spinal cord (). This spinal column can be felt along the neck and back to the tail. This tail is small, tilted up, and conspicuously white beneath, and it serves as a "recognition mark" to guide the young when, during feeding, an alarm is given and a bolt is made for the burrows. In those more primitive (older and simpler-fashioned) vertebrata, the fishes, the tail is much large and far more important, as compared with the rest of the body, than it is in most of the air-inhabiting vertebrates. In the former it is invariably a great muscular mass to propel the body forward; in the latter it may disappear, as in the frog, be simply a feather-bearing stump, as in the pigeon, a fly flicker, as in the cow or horse, a fur cape in squirrel, or be otherwise reduced and modified to meet special requirements.). Between the head and what is called "the body," in the more restricted sense of the word, is the neck. The neck gives freedom of movement to the head, enables the animal to look this way and that, to turn its ears about to determine the direction of a sound, and to perform endless motions in connexion with biting and so forth easily. We may note that in types which swim through the water, the neck dose not appear-- in the fish and frog, for instance-- and the head simply widens out as one passes back to the body. The high resistance offered by water necessitates this tendency to a cigar or ship outline, just as it has determined the cigar shape of the ordinary fish torpedo.), and the backbone behind, and called the chest or thorax; and posteriorly a part called the abdomen, which has no bony protection over its belly, or ventral surface. These parts together with the neck constitute the trunk. As a consequence of these things, in the backbone of the rabbit there are four regions: the neck, or cervical part, consisting of seven vertebrae, the thoracic part of twelve joined to ribs, the abdominal (also called the lumbar) region of seven without ribs, and the tail or caudal of about fifteen. Between the lumbar and caudal come four vertebrae, the sacral, which tend to run together into a bony mass as the animal grows old, and which form a firm attachment for the base of the hind limb.). This structure is distinctive of that class of the vertebrata called mammals, and which includes man, most of the larger and commoner land animals, and whales and manatee. We shall find later that it is essentially connected with the perfection of the air breathing to which this group has attained. Another characteristic shared by all mammals, and by no other creature, is the presence of hair. In birds we have an equally characteristic cover in the feathers, the frog is naked, and the fishes we find either naked skins or scales.

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