STRUCTURE OF A RE NICOLA. 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. 



In some species at least, the head lobe is 

 distinctly sensory and there are two ciliated 



FIG. 65. Anterior "neck organs." Otherwise sense organs are 



part of nervous system represented only by a pair of otocyst sacs, one 

 in Arenicola. (After on each side of the oesophageal nerve ring. 

 VOGT and YUNG.) These sacs, like those which occur in many 



other Invertebrates, seem to have to do rather 



c., Cerebral part on -, , ,. i . r ,_, . ,, 



dorsal surface ; ee.r. with tne direction of the animals movements 



oesophageal ring; g.\ than with hearing. Prof. Ehlers notes an in- 



nerve t; rd'*'f ' 7^! terestin g series : In A. Claparedii, there are 



nervesT^.', otocyst!* simply two open grooves ; in A. marina, the 



sacs have open necks and contain foreign 



particles ; in A. Grubii and A. antillensis, the sacs are closed and con- 

 tain intrinsic otoliths of lime. 



Food 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. 



The Body Cavity. 



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

 and contains a fluid. Anteriorly there are three transverse, 



