THE SKELETAL SYSTEM
Animals possess two kinds of skeleton. The exoskeleton, found among the Arthropods, covers the outside of the body, protecting the delicate internal organs, and assisting in locomotion by providing surfaces for the attachment of muscles. The disadvantage of an exoskeleton, like that of a crayfish, is that growth must be accompanied by periodic molts to permit of increase in body size. Vertebrates possess an endoskeleton, which is completely inclosed by the soft parts of the body. The endoskeleton supports the body and thereby preserves its general form. It facilitates locomotion by providing levers which are moved by the contraction of the muscles. The skull protects the delicate structures of the brain, while the ribs prevent injury to the organs of the thorax. One important advantage of the endoskeleton is that it does not interfere with growth, for the bones of the vertebrate body increase in length, width, and thickness as the soft parts grow.
The axial skeleton of a vertebrate comprises the skull, backbone, ribs, and sternum. The appendicular skeleton consists of the pectoral and pelvic girdles, the fore legs, hind legs, and feet.
THE SKULL
The skull of the higher vertebrates is a bony case completely surrounding and protecting the delicate tissues of the brain. It incloses and protects the olfactory organ,
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