THE ANATOMY OF A NEW SPECIES OF BATHYDORIS. 271 



of the back are smallest ; while round the dorsal tentacles and along the nottual 

 margin they are much larger. They seem to be more numerous in brownii than in 

 any of the other three species possessing them. They are undoubtedly caducous, and, 

 as recorded by ELIOT, the points of attachment of detached papilla? are marked on bare 

 places by small circles with a spot in the centre. The only other conspicuous features 

 of the dorsal surface are the dorsal tentacles and the anal complex, consisting of the 

 anus, the renal pore, and the branchise. The rhinophores are club-shaped and perfoliated 

 all round. There is no trace of a pocket. The branchiui are two in number, and are 

 placed symmetrically in front of the prominent anal papilla. Both numerically 

 and structurally the branchue appear to differ very considerably from those of all the 

 other species of Bathydoris. Thus, B. abyssorum has live and /ni/n/finna ten arranged 

 in a circle in front of the anus, while ELIOT'S species have eight and five or six 

 respectively similarly arranged. The two tufts in 11. l>/-<>/n/ii are united by a ridge 

 which, on dissection, is found to contain the afferent and efferent vessels of the 

 branchiae. As the structure of the gills will be described later, it is sufficient here to 

 mention that they are not fine dendritic structures like the gill-plumes represented by 

 BERGH. The anal papilla is much shorter than in the other species, but this is probably 

 due to the relatively greater local distension of the hinder end of the specimen. The 

 renal papilla is inconspicuous, and lies in the median line between the anus and the 

 base of the gills. 



The extruded genitalia stand out conspicuously on the right side between the 

 notgeal margin and the foot, and are thrown further back than in the Dorids by the 

 great size of the buccal region. 



ANATOMICAL DESCRIPTION. 



The importance of Bathydoris in any discussion of the interrelationships and 

 classification of the Nudibranchs was recognised by both BKRGH and ELIOT, and, during 

 the examination of the anatomical structure of the present species, this importance 

 became increasingly impressive as system after system was considered. This was 

 deemed to justify a more complete and detailed account of the anatomy than has 

 hitherto been given, especially as facilities for such an account were amply provided in 

 virtue of the excellent preservation of the specimen. In the course of this memoir it 

 will be noted that the description disagrees with those of BERGH and ELIOT on matters 

 the bearings of which are of great theoretical interest. These contradictions are some- 

 times so striking that the inclusion of the species in the genus Bathydons seemed 

 jeopardised. The points of agreement, however, form such convincing evidence of 

 o-eneric identity that the serious divergencies here given must be regarded as corrections, 

 based on examination of a more favourable specimen, of observations partially frustrated 

 by the poor condition of the material observed by previous workers. 



The various systems will now be considered in order. 



(ROY. sou. KDIN. TRANS., VOL. L., 193.) 



