IL-COMPARATIVE PART. 



I. GASTROPODA. 



We have seen that in the majority of the Gastropoda from great depths, the organs 

 of visions are very rudimentary or even completely absent. 



But, on the other hand, adjacent species (as for Pleurotoma brychia, Pleurotoma 

 lepta, Fossarus cereus, and Puncturella brychia) and allied genera (as in the case of 

 Guivillea) may possess well-developed and normal eyes. The state of these organs 

 in the Gastropods from the depths cannot therefore be regarded as a zoological 

 characteristic of their group, which has been retained in a new habitat. It is rather 

 a modification impressed upon these organs by the conditions of abyssal life. 



In fact, there are other Gastropods, which are, as regards their eyes, in the same 

 condition as those from the deep sea. 



The Gastropods in which the eyes are wholly absent are not numerous. The 

 instance of Vermetus, quoted by Gegenbaur,' is wholly erroneous : very evident eyes 

 are to be seen in Vermetus triqueter, where they have been observed by Lacaze-Duthiers, 

 and in Vermetus gigas, as I have myself noted, &c. 



In the group formed by Neomenia and Chsetoderma, designated Aplacophora, the 

 organs of vision are absent. As to the Polyplacophora (or Chitons), though they have 

 numerous dorsal eyes, as H. N. Moseley has shown, they do not possess in their adult 

 life organs homologous to the two cephalic eyes of the Anisopleural Gastropods. It is 

 therefore a distinct character of the entire group of Isopleura, that they have no cephalic 

 eyes. 



It is inaccurate, however, to say, as Glaus ^ does, that " cephalic eyes are absent 

 only in the Chitons," for some other Gastropoda are reputed to be without visual organs. 

 But in these cases the absence of visual organs is not, as in the Isopleura, a general 



' Grundriss der vergleichenden Anatomie (1878), p. 373. 

 2 Grundziige der Zoologie (1882), Bd. ii. p. 31. 



