THE ANATOMY OF A NEW SPECIES OF BATHYDOEIS. 285 



Bathydons possesses all except the lateral anus. Further, it retains oral tentacles in 

 a condition more strongly reminiscent of those of Pleurobranchxa than that of the oral 

 veil of Tritonia, while the separate ganglia of the brain and the separate gonad of 

 Baihydoris can certainly not be regarded as new and derived features in that genus. 

 Thus, excluding the case of the lateral anus, which will be considered separately, Batliy- 

 doris, which is essentially Dorid in construction, exhibits all the primitive features of 

 Tritonia, some indeed being more primitive than the corresponding ones in Tritonia, 

 the supposed ancestor. At this reductio ad dbsurdwn we arrive by considering only 

 those primitive features selected by PKLSENEEH, without calling in the evidence of the 

 blood and respiratory systems wherein Tritonia, with its symmetrical auricle receiving 

 blood from symmetrical lateral sinuses, appears very modern indeed. It is in the complete 

 avoidance of any comparison between vascular and respiratory systems in Tritonia and 

 Pleurobranchids that the weakness of the Tritonid theory of Nudibranch descent lies, 

 and it is significant that on the characters of these very systems is primarily based any 

 discussion of gastropod and even molluscan affinities. Previous application of this 

 criterion in the Opisthobranchs has resulted in their cleavage into Tectibranchs with 

 a ctenidial gill and Nudibranchs with pallial outgrowths of varied form and distribution 

 replacing the lost ctenidium. Of these neomorphic gills the lateral tufts of Tritonia 

 have been regarded as an early type, but it is not clear whether the Dorid circlet was 

 derived from them by concentration or by local specialisation round a posterior anus or 

 was evolved independently. Nor is it clear why modern writers on the Opisthobranchs 

 have always accepted the neomorphic nature of the Dorid circlet. It is true that a 

 comparison of the highly specialised, multipimiate plumes placed in a pit in the 

 tuberculate dorsum of some Dorids provides no suggestion of homology with the 

 ctenidium of a Tectibranch ; but it is not such a comparison of extremes that evinces 

 homologies. In Batliydoris, however, the gill is in two portions only, joined by a 

 crinkled ridge, it shows but the beginnings of pinnation, its lobes have the broad laminae 

 running from the afferent to the efferent side seen in the ctenidium of the Tectibranch, 

 and there is no suggestion of the circumanal ring in either the -gill or the underlying 

 vessels. From this point of view the extreme similarity of the condition of the 

 auricle, the efferent branchial vessel, and the circular sinus in Batliydoris and the 

 Pleurobranchids acquires a special significance. Evidence derived from the nature of 

 the innervation is perhaps of doubtful value ; but, so far as it goes, it is favourable 

 to the present contention, since the Dorid gill is jointly innervated from pleural and 

 visceral centres, while other Nudibranch gills receive no visceral nerves unless invaded 

 by ramifications of the liver. The dorsal position of the Dorid gill should present little 

 difficulty, since the pallial edge of the Dorids is undoubtedly a new formation of 

 mechanical value which progressively increases in width within the group and is absent 

 in many genera. In any case, the same difficulty would apply to the anus and renal 

 pore, and there is no proposal to class them as new formations in the Dorids. The separa- 

 tion into two or more parts also forms no objection to the ctenidial nature of the Dorid 



(ROY. soc. EDIX. TRANS., VOL. L., 207.) 



