212 THE ANATOMY OF IXVERTEBRATED AXIMALS. 



are usually confined to the anterior extremity of the body, and 

 to the prc^stomium where it exists ; but, iiQ the remarkable 

 genus Polyophthalmus, De Quatrefages discovered, besides 



—c 



Fig. 55. — A, anterior end of the nerrous system of Tclynde sq-uamcfa (aftr-r De Qna- 

 trefagest : o, cerebral yanfrlia ; b, oesi)phai;eal cominissures ; e. !oi,<riiuaiual com- 

 missures of the venrral ganglia. 



B, anterior end of the nervous system of SaMla fiabellcta (after De Quatrefages) : o, 

 cerebral gan^rlia; b, oesophageal commissures ; c.loniritr.dinal commissures of the 

 venrral giinglia. Those of opposite sides are united by long transverse commie- 

 Eures. 



the ordinary cephalic eyes, a double series of additional visual 

 organs, one pair being allotted to each somite. In 13ran- 

 chiotnma, eyes are situated at the ends of the branchial 

 plumes. Ehrenberg has described two caudal eyes in Ay/iphi- 

 cora, and De Quatrefages has shown that similarly placed 

 eyes exist in three other species of PolychcpAa^ two of which 

 are closely allied to Amphicora, while the other is an errant 

 form, related to Lvmhrinereis. These curious worms are said 

 to swim about with the caudal extremity forward. 



Auditory sacs, containing m,any otoliths, have been ob- 

 served upon each side of the oesophageal ring in Arenicola^ 

 and similar organs have been noticed in other Tiibicola ; but 

 hitherto their existence has not been certain]}'" determined in 

 the Kr rant la. 



The crenitalia of the polychaetous Annelida are excessively 

 simple in their structure ; indeed, special reproductive organs 

 can hardly be said to exist in most, the generative products 



