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 Ghaetopods, 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 hke 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 pm. 98.— Ovaoriginatiugfrom 

 leave the body through the nephridia. tiie i™ng epithelium of a 



_,, -I T ^ ^ • 7 • J parapodium of Tomopteris. 



The head 01 Arenicola is not After Gegenbaur. 

 provided with any special appendage, 



but in those worms which live permanently in fixed 

 tubes, the anterior end of the body often bears the 

 branchiae, and is usually provided with tentacles. Terebella 

 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- 

 lidcte 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 



