=Blind and Eyeless Mollusca.=--In a large number of marine Mollusca which habitually creep about half buried in wet sand (_Bullia_, _Sigaretus_, _Scaphander_, _Philine_), eyes are altogether absent. In some species of _Natica_ and _Sigaretus_, 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 degrees 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 _Ianthina_ has no eyes at all. Pteropoda as a rule have no eyes, and the few that have (_Creseis_, _Cavolinia_) possess only certain pigmented spots placed near to the nervous centres. In the Heteropoda, however, and the Cephalopoda, many of which are pelagic, the eyes are unusually large. [Illustration: FIG. 91.--_Sigaretus laevigatus_ Lam., a species frequenting wet sand, and destitute of external eyes: =F=, anterior portion of foot. (After Souleyet.)] =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[289] no trace of eyes in two species of _Pleurotoma_ 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 acicula_, which is never seen above the surface, is altogether destitute of eyes. A species of _Zospeum_, a _Helix_, and a _Bithynella_ from dark caves in Carniola have suffered a similar loss. On the other hand, a small _Hyalinia_ from a dark cave in Utah (probably a recent addition to the cave fauna) has the eyes normally developed.