CHAETOPODA 155 
The nervous system consists of two small cerebral ganglia, 
which are connected by circum-oesophageal commissures with 
a ventral cord which is embedded amongst the longitudinal 
muscles. This gives off a number of lateral nerves, and is 
supported by two giant fibres, but does not exhibit any divi- 
sion into ganglia and inter-ganglionic connectives. 
Arenicola has no eyes, but it possesses what are not com- 
mon in Chaetopods, namely otocysts. On each of the cerebral 
ganglia a small hollow vesicle is found. The walls of this 
consist of connective tissue with a lining of very columnar 
cells, probably ciliated. The vesicle contains a fluid in which a 
number of concretions—otoliths—float ; a special nerve passes 
to its walls. The whole structure is strikingly like the otocyst 
of many Lamellibranchs. 
Arenicola is dioecious, and the ovaries and testes occupy 
similar positions in the male and 
female. The ova and spermatozoa are 
formed from certain of the peritoneal 
epithelial cells, which become in the 
breeding season heaped up round the 
bases of the nephridia. They break off 
and float in the coelomic fluid, and jy4 98, ova originating from 
leave the body through the nephridia. the lining epithelium of a 
1 1 ee : parapodium of Tomopteris. 
The head of Avenicola is not After Gegenbaur. 
provided with any special appendage, 
but in those worms which lve permanently in fixed 
tubes, the anterior end of the body often bears the 
branchiae, and is usually provided with tentacles. TZerebella 
is provided with numerous tentacles, into which the coelom is 
prolonged ; they are exceedingly extensile, and stretch out in the 
form of a network all round the worm. Behind the tentacles 
are situated the branchiae. The appendages of the prostomium 
are sometimes distinguished by the name antennae from those 
of the peristomium, on which the mouth opens, which are 
termed the tentacles. Ventrally-situated palps, probably tactile 
organs, are also common on the head. In some of the Serpu- 
lidae a modified tentacle on the head forms an operculum, which 
closes the tube when the worm is retracted. 
The division into different regions, which is well marked 
