ON HAPLOBRANCHUS. 173 



intestine. Their large size probably necessitates their passing 

 to the exterior by rupture of the body wall. 



Nervous system. — I have been unable to determine the 

 structure of the nervous system. The supra-oesophageal gan- 

 glion nearly fills the prostomium. 



There are no caudal eyes. 



There are no auditory capsules. 



Affinities. — Haplobranchus comes into the family Ser- 

 pulidse on account of its capitobranchiate nature, but differs 

 from all hitherto known genera of the family in that the ten- 

 tacles, while they remain free, are devoid of any secondary fila- 

 ments and of any trace of cartilaginous support. It agrees with 

 the sub-family Sabellidae in the absence of any thoracic mem- 

 brane and operculum. 



There are certain genera of the Sabellidbe which present 

 some approach to its simplicity of structure. These are : Am- 

 phiglena. Clap.; Fabricia, Blainville; and Amphicorina, 

 De Quatr. These forms present certain characters in common 

 which are absent in Haplobranchus. The first pair of Ne- 

 phridia, belonging to the second somite, which are modified 

 as tubiparous glands, and which in all true Sabellids open on 

 each side at the base of the parapodium, are united in these 

 three genera in the dorsal region, and open by a single median 

 dorsal pore at the base of the branchiae. In Haplobranchus, 

 although I have not been able to make out their exact rela- 

 tions, there is no doubt they are not thus specially modified. 

 These three genera present auditory capsules in the peristomial 

 segment and caudal eyes, of neither of which is there any trace 

 in Haplobranchus. 



On the other hand, these three genera agree with one another 

 and with Haplobranchus in the comparatively simple struc- 

 ture of the head ; the prostomium not being completely fused 

 with the peristomium is still recognisable, and presents prosto- 

 mial tentacles and palps. The peristomial collar, completely ab- 

 sent in Amphiglena, is only slightly developed in the other 

 forms. There is little differentiation of the regions of the body 



