PHYSIOLOGY 167 



shell in others seem to be the particular developments related to 

 these special functions in this group. 



The skeleton and integumentary structures serve the Metazoa 

 primarily for a support and protection. The corals of the phylum 

 Coelenterata secrete a calcareous or horny skeleton arjound the ex- 

 ternal surface of the body proper. The sponges, as a rule, each 

 have a calcareous, siliceous (glassy), or horny skeleton extending 

 throughout the body. Such forms as snails, crayfishes, beetles and 

 representatives of their respective phyla secrete a well-developed 

 exoskeleton as an external cover over most of the other tissues of 

 the body. The muscles and other tissues are attached within. 

 There are special cells of the epidermis which function primarily 

 in production of this skeletal material. The echinoderms, including 

 animals like the starfish, possess calcareous skeletal plates which are 

 essentially similar to exoskeleton except that they are principally 

 beneath the skin. 



There is no well-developed endoskeletal structure known in non- 

 chordate animals but the endophragmal structures extending into the 

 thorax of some Crustacea are thought to be the forerunner of the 

 endoskeleton. A number of exoskeletal modifications are used for 

 protection and temperature regulation in most of the groups of 

 vertebrates. Such structures as scales, shells, feathers, hair, nails, 

 horns, and even enamel of teeth are of this type. 



Primitively the notochord is the original endoskeletal structure of 

 the chordate group. Around it are developed the basic structures of 

 the vertebral column which functions as the principal axial support 

 of all vertebrates. The sternum, girdles, and paired limbs have de- 

 veloped with the terrestrial life of vertebrates and the necessity for 

 locomotion on land. 



Bone is a firm, hard tissue consisting of abundant matrix composed 

 of inorganic salts, and the bone cells which are held in pocketlike 

 lacunae in the matrix. The outer membranous covering of bone is 

 called periosteum. The mineral part of the bone consists chiefly of 

 calcium phosphate and calcium carbonate. They give it firmness and 

 rigidity. The animal matter is composed of the bone cells and 

 cartilage which serve to give the bone life and resilience. A weak 

 acid, such as the acetic acid in vinegar, will dissolve the mineral matter 

 of the bone if allowed sufficient time, in which ease the bone will 

 lose its rigidity. Caustic solutions will destroy the animal matter and 

 make the bone brittle. 



