AN INTRODUCTION TO THE METAZOA 79 



example of an exoskeleton ; the bones of man form an endoskele- 

 ton. In either case the skeleton not only supports and protects 

 the soft parts of the body but also provides places for the attach- 

 ment of muscles. 



(7) The nervous system in higher Metazoa 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, temperature, 

 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. Afferent (sensory) nerve fibers conduct impulses from 

 end organs of sense, like the eye, to the brain or spinal cord. 

 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 enve- 

 lopes, 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 

 copulatory organs. 



4. Reproduction 



(1) Methods of Reproduction. — In the Protozoa reproduc- 

 tion is usually by binary fission, budding, or sporulation (see 

 pp. 32 and 49); these processes may be preceded by conjuga- 

 tion, which is a temporary or permanent union of two cells (see 

 pp. 59-62). In the Metazoa reproduction is usually sexual, 

 although asexual processes are normal in some species. 



Sexual Reproduction. — Reproduction is said to be sexual 

 when the individual develops from a mature egg which usually 

 fuses with a spermatozoon (pp. 84-85). In many cases the egg 

 does not unite with a spermatozoon^ before development; when 



