THE METAZOA 



67 



the account of the Protozoa. The special study of these 

 constitutes, as already pointed out in the Introduction, the 

 science of Physiology, which forms accordingly an important 

 part of the study of Zoology, and a part to which frequent 

 reference will be made in dealing with the structure of the 

 various groups of animals. 



The various internal parts of an animal are supported 

 and protected by the skin and the skeleton. The skin or in- 

 tegument consists of a layer of cells — the epidermis — with, 



Fig. 30. — Bones of the human arm and fore-arm with the biceps muscle, showing 

 the shortening and thickening of the muscle during contraction and the conse- 

 quent change in the relative position of the bones — viz., flexion of the fore-arm 

 on the upper arm. (From Huxley's Physiology,') 



superficial to it, in many animals, a non-cellular layer known 

 as the cuticle, and below it usually a fibrous layer, the 

 dermis. The skeleton is, as already explained in the 

 section on the Protozoa, a system of hard parts, external or 

 internal, serving for the protection and support of the softer 

 substance of the body. When these hard parts are external 

 they form an exoskeleton, when internal an endoskeleton. 

 An exoskeleton is formed by the thickening and hardening 

 of portions of one or other of the layers of the integument, 

 — cuticle, epidermis, or dermis. An endoskeleton usually 



