352 p:ssentials of zoology 



stome is located in the posterior part of the segment and leads into 

 the tubule of the segment just posterior to it. The nephridium coils 

 two or three times before reaching the nephridiopore. 



The Nervous System 



The brain of the earthworm consists of the suprapharyngeal 

 (janglion, two cirmimpharyngeal connectives, and the suhpharyngeal 

 ganglion. The ventral nerve cord extends posteriorly the length of 

 the bod}^ with a ganglion and three pairs of nerves in each segment. 

 Each ganglion is really the fusion of two, a deviation from the con- 

 dition found in many annelids and arthropods where there are two 

 ganglia in each segment and the nerve cord is double. The supra- 

 pharyngeal ganglion lies dorsal to the pharynx in the third segment 

 and the subpharj^ngeal ganglion lies ventral to the pharynx in the 

 fourth segment. Nerves from these two ganglia innervate the first 

 three segments and the prostomium. 



Stimuli are received by sensory cells and are passed into the 

 ventral nerve ganglia by the afferent nerves. The stimulus is modi- 

 fied in the ventral ganglia and sent to the responding organs by 

 efferent neurons. Nerve impulses then have the nature of a simple 

 reflex except that the ventral ganglia are connected by association 

 neurons which conduct stimuli from one to the other. Because of 

 this arrangement a stimulus applied to any part of the body will 

 cause responses to occur in a wavelike manner in both directions 

 from the point of stimulation. Located in the dorsal part of the nerve 

 cord are three giant fibers which serve as the sole means of conducting 

 an impulse directly from one end of the body to the other. By this 

 means the worm can contract its entire body at one time. 



Reproduction 



As has already been described, the earthworm is hermaphroditic. 

 Self-fertilization does not occur, however, each egg being fertilized 

 by a sperm from another individual. In reproduction two animals 

 come together with their anterior ends pointing in opposite direc- 

 tions and the ventral surfaces of their bodies in close contact from 

 the anterior end to the clitellum. With their bodies in close contact a 

 closed passage is formed between the genital openings of the two in- 

 dividuals. Sperms pass from the testes out through the seminal 



