NILS HJ. ODHNER, PTISANULA LIMNiEOIDES. 13 



of the specialized laraelliferous projection occurring in most 

 of the Nudibranchia. 



From the above consideration the deduction may be 

 made that the organs named »rhinophores», though derived 

 from a primarily indifferent cephalic lobe (Actceon, Ptisanula 

 etc.), are indeed not absolutely homologous in all the groups, 

 because they are developed from differently specialized parts 

 of the cephalic lobe. Habitual similarity thus often depends 

 on convergence, e. g. Ercolania and many Aeolididce. The 

 open rhinophores of Åscoglossa cannot be derived from the 

 cylindric foliated or smooth ones of Aeolididce. That must, 

 however, be the deduction from the opinion expressed by 

 Pelseneer (1894) and Eltot (1910), that Hermceidce, Elysiidce 

 and allies might be modified Aeolids. 



As to the sensory nerves, the optic and the olfactory 

 ones are always separated in the higher forms (Nudibranchia). 1 

 The optic nerve is either long (in forms with less rednced 

 rhinophores) or verv short in Aeolididce and others, where 

 the olfactory sense thus seems to compensate the sigbt. The 

 tentacular nerve has no proximal (tentacular or rhinophorial) 

 ganglion in Lophocercidce, Pleurobr,anchidce, Dendronotus, T ri- 

 tonia (Pelseneer 1894), Janidce (Pelseneer 1891)andafew 

 others, but is furnished with one in all the Dorids. A distal 

 ganglion occurs in the long tentacular nerve of the Pleuro- 

 branchidce, Tritonia, Dorididce and others, evidently originating 

 from a concentration of the diffuse olfactory nerve cells; to 

 this centre the name olfactory ganglion must therefore be 

 restricted. In Aeolididce, lastly, only one ganglion is present 

 in the short rhinophorial nerve, possibly formed by a fusion 

 of the tentacular and the olfactory ganglia. (Confer the fi- 

 gures of nervous systems given by Bergh, Vayssiére and 

 Pelseneer.) 



Relations of Ptisanula. 



In the structure of the radula, in the absence of parapo- 

 dia, operculura, tentacles, jaws and gizzard-plates, and al so in 

 the nervous and geni tal systems the closest agreement prevails 

 between Ptisanula and Diaphana. In most respects the first- 



1 In Tritonia plebeia, Bergh describes and figures a common root of 

 optic and tentacular nerves, but Pelseneer (1894) has found them separated. 



