PHYLUM ANNELIDA 



209 



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 



Apertures of 

 5eminal receptacles 



Aperture of 

 Vas deferens 



C /it el lam 



5eminal droove Pore of oviduct 



Dorsal 

 blood vessel 



body wall 



Intestine 



5eminal 

 groove 



Band of 

 mucus 

 secreted 

 elite II am 



Fig. 109.— Reproduction in earthworm showing copulation and the cocoon. A, 

 two worms enclosed in bands of mucus ; B, transverse section showing the seminal 

 grooves ; O, cocoon. 



