REPRODUCTIVE SYSTEM OF THE EARTHWORM 155 



and carry impulses to the cell. The entire system of cell body, 

 axon, and dendrites constitutes 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 to do only 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 Qth and loth somites, 

 with openings to the outside on the ventral surface. They 

 are small spherical sacs close to the dissepiments, and one 

 pair, at least, may be hidden by the overlying seminal vesicles 

 or organs for the manufacture of spermatozoa. At the period 

 of maturity, these receptacles are usually filled with mature 

 spermatozoa which have been formed in another worm. When 

 the eggs are mature these spermatozoa are squeezed out, and 

 fertilization takes place on the outside of the body. 



The spermatozoa-forming organs are more complicated, con- 

 sisting essentially of three pairs of closed sacs, called the 



seminal vesicles, united in the median line and enclosing the 

 11 



