NEURONS 151 



about concerted action of the entire chain of somites, (4) giant, 

 fibers the functions of which are somewhat problematical, 

 but which may have both supporting and coordinating functions, 

 and (5) glia cells which form the matrix or main body of the 

 chain (Fig. 62). 



Each young nerve cell in development first forms an axial 

 process called the axon, which carries impulses away from the 

 cell. Other processes of the cell are termed dendrites, which 

 are shorter and more branched than the axon, and they carry 

 impulses to the cell. The entire system of cell body, axon, 

 and dendrites constitute a nerve unit called the neuron. 



Every neuron is separate and distinct, and adjacent neu- 

 rons, while in contact, do not fuse; impulses, however, are 

 transmitted from one to another by contact. 



The muscular and nervous systems are sometimes grouped to- 

 gether as organs of relation, since it is through them that the 

 organism is in touch with its environment. These systems, 

 together with those of nutrition, circulation and excretion are 

 organs of the individual and have only to do with one animal. 

 One other system of organs the reproductive organs has 

 little to do with the individual since it consists of organs having 

 nothing to do with metabolism, secretion, excretion or nervous 

 response, but is, primarily, a system of organs of the race, with 

 the one common function of perpetuating the species by re- 

 production of the same kind of worm. Hence we consider it 

 separately. 



/. THE REPRODUCTIVE SYSTEM. Like Hydra and the fern, 

 the earthworm is hermaphrodite having both male and female 

 organs of reproduction. These are somewhat complicated and 

 involve two sets of structures, one set for the receiving and 

 storing of spermatozoa received from another worm during 

 copulation, the other set for the manufacture, development and 

 emission of the mature spermatozoa and eggs. 



The receptive organs termed seminal receptacles (Fig. 63) 

 consist of two pairs of globular sacs in the pth and loth 

 somites, with openings to the outside on the ventral surface. 

 They are small spherical sacs close to the dissepiments and one 



