206 MR T. J. EVANS ON 



and by Hecht. The absence of ramifications is, doubtless, a primitive 

 character. 



(d) The blood system is built essentially on Dorid lines, but presents a greater 



number of primitive features than any other. Chief among them are the 

 possession of but one auricular efferent opening, the union of the circular 

 collecting canal of the integumental system with the efferent branchial, and 

 the position of the blood glands. It is noteworthy that these primitive 

 features are points of agreement with the Tectibranchs, especially the 

 Pleurobranchids. 



(e) As to the reproductive system, its diaulic condition makes it more primitive 



than that of any other known Dorid ; but, apart from that very important 

 divergence, it closely resembles that of Doris, since the separation of the 

 gonad from the liver is found in a typical Dorid like Alloiodoris. 



(f) The nervous system, in spite of a close similarity to the Dorid type in most 



respects, differs from it in several important points. Of these, the length 



of the nerve collar and the position of the brain on the top of the buccal 



mass are paralleled in Tritonia and the Pleurobranchids, and should probably 



be regarded as primitive, while the distinctness of the ganglia of the brain 



and the absence of separate gastro- oesophageal, if primitive features, take 



us back to a condition earlier than that found in the Pleurobranchids and 



Tritonia. The fusion of the ganglia of the visceral loop with the pleurals 



is, on the other hand, a modern feature, as is the loss of eyes consequent 



on the adoption of a deep-water habitat. 



We conclude, therefore, that Bathydoris is a highly primitive Dorid possessing 



some characters that adapt it to a specialised habitat and mode of life, while those that 



are primitive connect it with the Tectibranchs, particularly the Pleurobranchids among 



existing forms. The derivation of the Dorids from Pleurobranchid ancestors is, 



however, no new proposition. Guiart, for example, has recently advocated their union 



into one group, and Pelseneer has derived all Nudibranchs from the Pleurobranchids 



with Tritonia as an intermediate link. 



Bergh's advocacy of a special relationship between Bathydoris and Tritonia on the 

 evidence of the buccal apparatus has already been criticised. Pelseneer's position, 

 however, takes a wider outlook, but takes no cognisance of Bathydoris at all. He 

 bases his contention of the Tritonid origin of all Nudibranchs on the possession by 

 Tritonia, in common with the Pleurobranchids, of a large number of primitive Nudibranch 

 characters which are not found together in any other Nudibranchs. These are : — a 

 frontal veil, formed by the fusion of the oral tentacles of the Pleurobranchid, a wide 

 foot, a ventricle turned to the right, a broad raclula, a nervous system placed on the 

 buccal bulb, an oesophageal crop, extensive salivary glands, a saccular, unramified 

 kidney, a long reno-pericardial tube, pericardial glands on the auricle, male and female 

 openings in a common vestibule, and a lateral anus. Of these, it will be noticed that 



