ANNELIDA . THE EARTHWORM, NEREIS 223 



and prolonged at their inner ends into fibres, which run in 



the nervous system (Fig. 140). These are sense 



rgans, CQ ^ an( j j n ^ f ore p ar ); f the body some 



of them are collected into groups, which are rudimentary 

 sense organs. Experiment shows that the worms are 

 sensitive to light and 

 to vibrations of the 

 ground and can 

 smell, but gives no 

 evidence of a sense 

 of hearing. 



The excretory or- 

 gans of 



Excretion. ° ,, 



an earth- 

 worm, like those of 

 the frog, consist of 

 tubes whose walls 

 are excretory and 

 richly supplied with 

 blood vessels, but the 

 tubes, instead of be- 

 ing collected into 

 compact kidneys, are 

 distributed along the 

 body, one pair to 

 each segment, except 

 the first three and 

 the last. Each tube 

 or nephridium is 

 thrown into loops, 

 bound together by 

 connective tissue 

 containing blood 

 vessels. The nephri- 

 dium begins as a 

 kidney-shaped fun- 

 nel or nephrostome 

 hanging from the 

 front side of a 

 septum near the 

 nerve cord. The 



Fig. 140. — A diagram showing the mode of 

 ending of the sensory nerve fibres of the 

 earthworm and the relation of this type 

 to that which is found in most of the 

 sensory fibres of the frog. See also 

 Fig. 48. 



A , The arrangement found in the earthworm ; B, that 

 of the worm Nereis ; C, that of a fish ; D, that of 

 a frog or man. 



c.n.s., Ending of the neuron in the central nervous 

 system ; ef., ending in the epidermis. 



