The skeleton of a dog; a typical vertebrate.
VIII. The Vertebrates.—All of the animals we have studied thus far agree in having whatever skeleton or hard parts they possess on the outside of the body. Collectively, they are called Invertebrates. This exoskeleton differs from the main or axial skeleton of the higher animals, the latter being inside of the body. The exoskeleton is dead, being secreted by the cells lining the body, while the endoskeleton is, in part at least, alive and is capable of growth, e.g. a broken arm or leg bone will grow together. But a man has certain parts of the skeleton, as nails or hair, formed by the skin and in addition possesses inside bones to which the muscles are attached. Some of the bones are arranged in a flexible column in the dorsal (the back) side of the body. This vertebral column, as it is called, is distinctive of all vertebrates. Within its bony protection lies the delicate central nervous system, and to this column are attached the big bones of the legs and arms. The vertebrate animals deserve more of our attention than other forms of life because man himself is a vertebrate.
The sand shark, an elasmobranch. Note the slits leading from the gills. (From a photograph loaned by the American Museum of Natural History.)
Five groups or classes of vertebrates exist. Fishes, Amphibians, Reptiles, Birds, and Mammals. Let us see how to distinguish one class from another.
The sturgeon, a ganoid fish.
Fishes.—Fishes are familiar animals to most of us. We know that they live in the water, have a backbone, and that they have fins. They breathe by means of gills, delicate organs fitted for taking oxygen out of the water. The heart has two chambers, an auricle and a ventricle. They have a skin in which are glands secreting mucus, a slimy substance which helps them go through the water easily. They usually lay very many eggs.
Classification of Fishes
Order I. The Elasmobranchs. Fishes which have a soft skeleton made of cartilage and exposed gill slits. Examples: sharks, skates, and rays.