Fig. 90.—Eye in A, Loligo; B, Helix or Limax; C, Nautilus: a.o.c, anterior optic chamber; c, cornea; int, integument; ir, iris; l, lens; l´, external portion of lens; op.n, optic nerve; op.g, optic ganglion; p.o.c, posterior optic chamber; r, retina. (After Grenacher.)
The developed eye in the dibranchiate Cephalopods consists of a transparent cornea, which may or may not be closed over the front of the lens. Behind the cornea is a narrow chamber (the anterior optic chamber) which is continued for three parts round the whole circle of the eye, and into which project the front portion of the lens and the folds of the iris. Throughout its whole extent, the anterior optic chamber is lined by the integument, the portion of which on the inner side is the choroid. The lens is divided into an outer and inner segment by a thin membrane, and is supported by the ciliary body which forms a continuation of the retina. The main portion of the lens lies within the posterior optic chamber, at the back and sides of which is found the retina (Grenacher).
There can be no doubt that the Cephalopoda use their eyes to observe, but there is nothing to show that any other Mollusca use their eyes for this purpose, the sense of smell in their case largely taking the place of visual observation. Madame Jeannette Power once saw[286] the Octopus in her aquarium holding a fragment of rock in one of its arms, and watching a Pinna which was opening its valves. As soon as they were perfectly open, the Poulpe, with incredible address and promptitude, placed the stone between the valves, preventing the Pinna from closing again, upon which it set about devouring its victim. The next day the Poulpe was seen, after crushing some Tellina, to stretch himself down close by a Triton nodiferus, and watch it attentively. After four hours the Triton emerged from its shell, when the Octopus sprang upon it, and surrounded it with its arms.
Powers of Vision in Land Mollusca.—The Helicidae are undoubtedly very short-sighted. Seldom emerging from their retreats except in twilight and darkness, they are naturally myopic, and see better in a subdued than in a bright light. Experiment has shown that a Helix can perceive an object better at 6 centimetres distance in a weak light than at 4 or 5 millimetres in a strong one. Cyclostoma elegans and Paludina vivipara are comparatively long-sighted, perceiving objects at a distance of 20 to 30 centimetres.[287] The increased power of vision is due, in these two cases, to increased elaboration in the construction of the eye, Paludina possessing a large and almost spherical lens, to which the vitreous humour closely adheres, while in Cyclostoma the lens is remarkably hard, and the aqueous humour very abundant. According to V. Willem,[288] the Pulmonata are very sensitive to the slightest movement of the air or jarring of the surface on which they crawl, but are so short-sighted as only to perceive a confused image of a large object at about 1 cm., and to distinguish the form of objects at not more than 1 or 2 mm. The senses of touch and smell are far more active than that of sight. A bean-pod enclosed in a narrow glass case and placed before a hungry snail was not noticed, but when taken out of the case and placed 8 cm. behind the snail, the latter at once turned towards it to devour it.
Some interesting experiments were conducted by the same author with the view of ascertaining whether snails avoid or court the light. He placed a number of species in different wooden boxes, which were divided into a light and a dark compartment, having previously well soaked the boxes in water to secure a humid atmosphere and surface, and so induce the snails to move about. The result showed that nearly all species have a marked predilection one way or the other, but not all in the same way. Helix aspersa, Arion empiricorum, six species of Limax, and three of Planorbis, are lovers of darkness, while H. nemoralis, Succinea putris, and two species of Limnaea are lovers of light. Physa fontinalis stands alone in being quite indifferent.
M. Willem endeavoured further to discover whether any of the Mollusca possessed ‘dermatoptic perception,’ or the faculty of perceiving variation of light by means of the skin alone. He accordingly repeated the above-mentioned experiments, having previously extirpated the eyes in all cases. The result was remarkable. In a few instances the experiment was not conclusive, but H. aspersa, A. empiricorum, several species of Limax, and one Limnaea shunned or sought the light just as they had done when their eyes were present. A few marine Mollusca (Littorina littorea, Trochus cinerarius, T. umbilicatus, Patella vulgata) were also shown to be exceedingly sensitive to the impact of a shadow, whether with or without their eyes.
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.
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.