1 86 



BLIND AND EYELESS MOLLUSCA 



CHAP. 



Fig. 91. — Sigaretus lae- 

 vigatus Lam., a species 

 frequenting wet sand, 

 and destitute of ex- 

 ternal eyes: F, ante- 

 rior portion of foot. 

 (After Souleyet.) 



Mollusca which habitually creep about half buried in wet sand 

 QBullia^ Sigaretus^ Scaphander^ Philine)^ eyes are altogether 

 absent. In some species of Natica and Sig- 

 aretus^ and in Doris, eyes are developed, 

 but are enclosed in a thick layer of skin, 

 through which they can probably do little 

 more than faintly appreciate different de- 

 grees of light and darkness. Chiton has 

 cephalic eyes in the embryo, but loses them 

 in the adult stage. The two great Auricula, 

 A. auris Judae and A. auris Midae, which 

 habitually creep about in the liquid mud 

 of mangrove swamps, have entirely lost 

 their eyes. Certain pelagic Mollusca seem 

 to have a tendency, which is not easily 

 explained, to lose their eyes or the power 

 of seeing with them. Thus lanthina has 

 no eyes at all. Pteropoda as a rule have no 

 eyes, and the few that have (^Creseis, Cavo- 

 llnia') possess only certain pigmented spots placed near to the 

 nervous centres. In the Heteropoda, however, and the Cephalo- 

 poda, many of which are pelagic, the eyes are unusually large. 



Eyes in Deep-sea and Underground Mollusca. — Deep-sea 

 Mollusca, as a rule, possess no visual organs, or possess them 

 only in a rudimentary state, but this rule has its exceptions. 

 Dr. Pelseneer found ^ no trace of eyes in two species of Pleuro- 

 toma from 1850 and 1950 fath., none in a Fossarus from 1400 f., 

 none in a Puncturella from 1340 f. A remarkable form of 

 Voluta (^Guivillea} from 1600 f. possessed eyes which could 

 hardly be functional, as they were destitute of pigment, and 

 exhibited other changes of structure. On the other hand, it is 

 remarkable to notice that in three different species of Trochus 

 from 450 f., 565 f., and 1375 f., the eyes were pigmented and 

 well developed. 



In land Mollusca which live beneath the surface of the ground 

 or in absolute darkness, the eyes are generally more or less 

 modified. Thus in Testacella, which usually burrows deeply in 

 the soil, but occasionally emerges into the open air, the eyes are 

 very small, but distinct and pigmented. Our little Caecilianella 

 ^ ' Challenger ' Reports^ Zoology, vol. xxvii. part Ixxiv. p. 3. 



