AN INTRODUCTION TO THE VERTEBRATES 239 



(7) The nervous system in higher animals consists of two 

 parts: (a) central and (b) peripheral. The brain and spinal 

 cord constitute the central nervous system. The organs of 

 special sense, such as sight, smell, taste, hearing, touch, tempera- 

 ture, and equilibrium, and the nerves connected with them, and 

 all other nerves connecting the central nervous system with 

 various parts of the body, constitute the peripheral nervous 

 system. Efferent (motor) nerve fibers conduct impulses from 

 the brain and nerve cord to an active organ like a muscle or 

 gland. 



(8) The reproductive system consists of the germ cells, and 

 the organs necessary for furnishing yolk and protective envelopes, 

 and for insuring the union of the eggs and spermatozoa. The 

 essential reproductive organs in complex animals are usually 

 the ovaries, which contain the eggs, and the testes, in which the 

 spermatozoa ripen. The accessory organs are generally ducts 



. leading to the exterior, glands connected with these ducts, and 

 organs for transferring the spermatozoa from the male to the 

 female. 



Structure of Organs. — We cannot understand how an organ 

 performs its duty unless we have a knowledge of the structure 

 of the organ. We shall not attempt to learn all there is known 

 about organs, but just enough to understand their activities. In 

 the first place the entire body, as previously stated (p. 202), is 

 either a single cell (Protozoa, p. 218), a colony of cells (colonial 

 Protozoa, p. 231), or a many-celled organism with the cells 

 closely bound together (Metazoa). We have seen how the vari- 

 ous physiological processes are performed by a single cell (Fig. 

 139), as in Paramecium (p. 218), and also how in many-celled 

 animals like Hydra (p. 202) groups of cells are set aside for 

 carrying on different functions, that is, division of labor has 

 taken place. This is true of all many-celled animals. 



Protoplasm. — The substance of which every cell is com- 

 posed is called protoplasm. As in Paramecium (p. 219) every 

 cell contains a central body, the nucleus. The nucleus is a 



