STRUCTURE OF ARENICOLA. 



203 



the next nineteen have rudiments. The dorsal part con- 

 sists of a tuft of bristles, whose bases are enclosed in a 

 sac ; — the ventral part, separated by a short interval, bears 

 several hooks. 



Nervous System. 



The nervous system is in its general features like that of 

 the earthworm, but ganglia are not developed. In the 

 ventral nerve cord, the ring round the 

 gullet, and the slight cerebral enlarge- 

 ment which represents a brain, nerve 

 cells occur diffusely scattered among 

 the nerve fibres. Along the dorsal sur- 

 face of the nerve cord run two " giant 

 fibres " like those in the earthworm. 



Fig. 65. — Anterior 

 part of nervous system 

 in Arenicola. (After 

 VoGT and Yung.) 



c. , Cerebral part on 

 dorsal surface ; (t-.r., 

 oesophageal ring; g., 

 gullet : 7J.H.C., ventral 

 nerve cord ; /.«., lateral 

 nerves ; d/., otocyst. 



particles ; in A. Grubii 

 tain intrinsic otoliths of 



In some species at least, the head lobe is 

 distinctly sensory and there are two ciliated 

 "neck organs." Otherwise sense organs are 

 represented only by a pair of otocyst sacs, one 

 on each side of the oesophageal nerve ring. 

 These sacs, like those which occur in many 

 otlier Invertebrates, seem to have to do rather 

 with the direction of the animal's movements 

 than with hearing. Prof. Ehlers notes an in- 

 teresting series : — In A. Claparedii, there are 

 simply two open grooves ; in A. inarina, the 

 sacs have open necks and contain foreign 

 and A. aiiti/lensis, the sacs are closed and con- 

 lime. 



I^ond Canal. 



The mouth is at the end of a protrusible cup-like 

 proboscis ; the gullet has smooth walls, and bears an an- 

 terior and a larger posterior pair of glands which secrete 

 a yellowish fluid perhaps digestive ; the succeeding part 

 of the gut is covered with yellow cells and many blood 

 vessels, and is divided into rings ; the terminal portion is 

 full of sand from which the nutritive matter has been 

 absorbed ; the anus is at the very end. 



T/ie Body Cavity. 



The body cavity is spacious, except in the tail region, 

 and contains a fluid. Anteriorly there are three transverse, 



