10 ARKIV FÖR ZOOLOGI. BAND 8. NIO 25. 



In Diaphana the eye has sunk deeper in the cephalic 

 lobe and has become embedded in the connective tissue 

 (Fig. 13). It appears to be somewhat reduced in size if 

 compared with that of Ptisanula, the specimen of D. glacialis 

 being twice as large as the Ptisanula one; the eye, however, 

 is of the same size as in the latter. The structure is chiefly 

 uniform but here a homogeneous sphseroid lens of a solid 

 consistency replaces the vitreous corpuscule. The hind half 

 of the lens is covered by pigment, the front half of it pro- 

 jects semisphseroidally with a strongly convex surface. 



If the tentacular ner ve is followed up in its clistal rami- 

 fications, a stratum of diffuse nervous cells will be found, as 

 already mentioned, located just in the angle between the 

 under-side of the cephalic disc and the sides of the head. 

 Here, the olfactory or rhinophorial organ is to be found, as 

 stated above. The ectoderm is folded irregularly and con- 

 sists of cylindrical ciliated cells with a large basal nucleus; 

 they are in contact with the ganglious cells. The composition 

 displays the same scheme as the rhinophorial lamellse of 

 Haminea hydatis, as described by Pelseneer 1894. Conse- 

 quently the epithelium mentioned is to be considered as 

 representing a rudiment of rhinophores, not yet differentiated 

 to the same degree as in Bulla. In Ptisanula and Diaphana 

 we meet with the first stage of development of the rhino- 

 phores, which are so characteristic for the Opistobranchia. 

 They assume the olfactory function by the gradual reduction 

 of the osphradium. Starting on the assumption that the 

 forms described here represent this first stage, I have made 

 an attempt at analysing the further development of the 

 rhinophores. Though it lies, strictly speaking, outside my 

 present subject, this survey may be given here, as nothing 

 of the kind has hitherto been published, the origin of the 

 rhinophores having been very uncertain. Further, this pro- 

 cess, on the assumption here given, seems to throw a new 

 light upon the mutual relationship existing among the Opistho- 

 branchia. 



Phylogeny of the Rhinophores of Opistobranchia. 



In Actozon I found, on examining sections, that the optic 

 and the tentacular nerves start from a common, though short, 



