21^ 



THE ANATOMY OF IXVERTEBKATED ANIMALS. 



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

 to the prgestomium where it exists ; but, in the remarkable 

 genus Folyophthalmus, De Quatrefages discovered, besides 



Fig. 55.— J., anterior end of the nervous system of Polynoe sqtiamata (after De Qua- 

 trefages) : a, cerebral sanglia ; 6, oesophageal commissures ; o, longitudinal com- 

 missures of the ventral ganglia. 



B, anterior end of the nervous system of Sabella fiahellata (after De Quatrefages) : a, 

 cerebral ganglia ; h^ oesophageal commissures ; c, longitudinal commissures of the 

 ventral ganglia. Those of opposite sides are united by long transverse commis- 

 sures. 



the ordinary cephalic eyes, a double series of additional visual 

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

 chiomma, eyes are situated at the ends of the branchial 

 plumes. Ehrenberg has described two caudal eyes in Amphi- 

 cora, and De Quatrefages has shown that similarly placed 

 eyes exist in three other species of Polychceta, two of which 

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

 form, related to Lnmhrinereis. These curious worms are said 

 to swim about with the caudal extremity forward. 



Auditory sacs, containing many otoliths, have been ob- 

 served upon each side of the oesophageal ring in Are?ucolcf, 

 and similar organs have been noticed in other Tuhicola ; but 

 hitherto their existence has not been certainly determined in 

 the Errantia. 



The genitalia of the polychagtous Annelida are excessively 

 simple in their structure ; indeed, special reproductive organs 

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



