30 THE VOYAGE OF H.M.S. CHALLENGER. 



character of an entire group of some extent, but is merely the result of a special adapta- 

 tion of some restricted division. 



I ought to mention at the outset that among the Gastropod Molluscs which have 

 been cited as devoid of eyes, there are several which have not been subjected to sufficiently 

 complete investigation, and which have been credited with blindness without special 

 research, when the fact was that the eyes were only in some degree rudimentary. The 

 conclusion was simply based on the observation that the head did not exhibit pigmented 

 spots visible externally. Such are, for example, — 



On the one hand, (A) a certain number of BuUoidea' {Scaphander, Philine, 

 Doridium, &c.) and also Naticidae, in which the eye really exists and is pigmented, 

 but is covered by a tegumentary and muscular layer of some thickness. The latter 

 is the result of adaptation to the burrowing habits of the animals. Through the 

 tegumentary layer the above animals can still appreciate different degrees of light and 

 darkness. It is possible that in certain forms of Natica the eye does not persist, but 

 is altogether absent. (B) Phyllirhoe, reputed to be blind by von Siebold,^ possesses 

 pigmented eyes f but these are again covered by integument, and situated directly 

 upon the nervous system, as is also the case (C) in certain Nudibranclis, such as Dons, 

 Eolis, Scyllaia, Tethys, &c. In all these animals the rudimentary character of the 

 eye consists solely in its being concealed below the integument, and in a moderate 

 reduction of its dimensions. 



On the other hand, (D) in Guivillea, and in certain other Gastropods afterwards 

 mentioned, the eye still exists, but has been subjected to important modifications of 

 structure, and has lost its pigment, as we have noted above. 



I have unfortunately been unable, for lack of time, to corroborate, by personal 

 examination, all the cited cases of Anisopleural Gastropods said to have no eyes, or to 

 have visual organs atrophied in some way or other. It is therefore possible that 

 in regard to some of the forms reputed to be " blind," some reservations may have to 

 be made, as in the above cases. 



Among these forms, perhaps the most astonishing are two large species of Auricula 

 [Auricula auris judw and Auricula auris viidm), for in these animals there does not 

 seem to be any apparent reason for the atrophy of eyes ; while in the cases to be 

 mentioned below, the rudimentary nature or total disappearance of these organs is the 

 result of a special and perfectly definite adaptation. It is evident that the primitive 

 Anisopleura all possessed eyes, but that these organs may become rudimentary under the 

 influence of special external conditions. 



' Krohn, Fernere Beitriige zur Kenntniss des Schneckenauges, Arch./. Naturgesch., 1839, p. 335. 

 ^ Lehrbuch der vergleichenden Anatomie der Wirbellosen Thiere, p. 310. 

 ' Souleyet, Voyage de la Bonite, Zoologie, t. ii. p. 410. 



