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.
Eyes of Onchidium.—Many species of Onchidium, a naked land pulmonate which creeps on rocks near high-water mark, are provided with dorsal eyes of various degrees of organisation, and in numbers varying up to nearly one hundred. The tropical Onchidium are the prey of a fish (Periophthalmus) which skips along the beach by the aid of its large ventral fins, and feeds principally on insects and Onchidium. Karl Semper suggests[290] that the eyes are of service to Onchidium as enabling it to apprehend the shadow of the approaching Periophthalmus, and defend itself by suddenly contracting certain glands on the skin and expressing a liquid secretion which flies into the air like shot and frightens the Periophthalmus away. This theory for it is no more than theory—may or may not be true, but it is remarkable that Onchidium with dorsal eyes have precisely the same geographical distribution as Periophthalmus, and that where no Periophthalmus exists, e.g. on our own S.W. coasts, the Onchidium are entirely destitute of dorsal eyes. In those species of Onchidium which have no dorsal eyes, the latter are on the tips of the tentacles, as in Helix. The eyes are developed on the head, and afterwards ascend with the growth of the ommatophores, while in Helix the ommatophores are formed first, and the eyes developed upon them.[291]
Dorsal Eyes in the Chitonidae.—The remarkable discoveries of Moseley with regard to the dorsal eyes of Chiton were first published in 1884.[292] He happened to notice, while examining a specimen of Schizochiton incisus, a number of minute black dots on the outer surface of the shell, which appeared to refract light as if composed of glass or crystal. These ‘eyes,’ in all the species of Chiton yet examined, are restricted to the outer surface of the exposed area of the shell, never being on the laminae of insertion or on the girdle. In certain sub-genera of Chiton the eyes are scattered irregularly over the surface, in others they are arranged symmetrically in rows diverging from the apex of each plate, but in old specimens the eyes towards the apices are generally rubbed off by erosion or abrasion. Moseley regarded the occurrence of scattered eyes as indicating an original stage of development, when the eyes were at first disposed irregularly all over the surface of the shell; the gathering into regular rows showing a later stage.
Fig. 92.—Dorsal eyes of Chitonidae, showing the various forms of arrangement in the first and fourth valves of 1, 1a, Acanthopleura spinigera Sowb., E. Indies, × 2; 2, 2a, Tonicia suezensis Reeve, Suez, × 3; 3, 3a, Acanthopleura granulata Gmel., W. Indies, × 2; 4, 4a, Tonicia lineolata Fremb., Chili, × 2. From specimens in the Museum of Zoology, Cambridge.
The eyes appear to be invariably more numerous on the anterior plate. Thus in Corephium aculeatum there are about 12,000 in all, of which more than 3000 are on the anterior plate. In Schizochiton they are arranged in very symmetrical rows, six of which are situated on the anterior, and only two, sometimes only one, on the central plates. In Tonicia marmorata the eyes are sunk in little cup-shaped depressions of the shell, possibly to escape abrasion. As regards shape and size, in Ch. incisus they are circular, and about 1/35 inch in diameter, this being the largest size known; in Ch. spiniger and Ch. aculeatus they are oval, measuring about 1/100 x 1/600 inch. There are no eyes in Chiton proper, nor in Mopalia, Maugeria, Lorica, and Ischnochiton.[293] None of our English species appear to possess them.[294]
Eyes in Bivalve Mollusca.—Some, possibly most, of the Pelecypoda possess, in the larval state, true paired eyes at the oral end of the body. These become aborted as the animal develops, since that part of the body becomes entirely screened from the light by the growth of the shell. To compensate for their loss, numerous ocelli, or pigmented spots sensitive to the action of light, are in many cases developed on different parts of the mantle, functionally corresponding to the ‘eyes’ of Chiton described above. As in Chiton, too, we have here an interesting series of instances in which true eyes have suffered total obliteration, through disuse, and, as if to restore to the animal in some measure its lost sense, visual organs of a low power have subsequently been developed and are now observed in various stages of specialisation.
Concentration of Eyes in Special Parts of the Mantle.—Sharp has shown[295] that in several species of Ostrea, Cardium, Anomia, Lima, Avicula, Arca, and Tellina pigmented cells, with a highly refractive cuticle, are scattered over a considerable portion of the mantle. Experiment has proved the powers of ‘vision,’ i.e. of sensitiveness to different degrees of light, possessed by these organs. In Dreissena polymorpha, Tapes decussatus, and two species of Venus these cells are concentrated on that particular part of the mantle which is not always covered by the shell, i.e. the siphon, but since the siphon can be completely retracted within the shell, there is no special provision for their protection. A further step is shown in the case of Mya arenaria, where the siphon is scarcely capable of complete retraction. Here, while some of the pigment cells are scattered about over the surface of the siphon, the majority are placed in grooves at the base of the siphonal tentacles, forming an intensely black band round them. A higher stage still is shown in Solen vagina, S. ensis, and Mactra solidissima, where the cells are situated only in the siphonal grooves, which are more or less specialised in numbers and complexity.
Arca Noae, according to Patten, is very sensitive to any sudden change in the amount of light falling upon its mantle-edge. A faint shadow cast upon it by the hand is sufficient to cause it to close its valves quickly, but always one or two seconds afterwards, the promptitude in all cases depending upon the depth of the shadow. Sensitiveness in this direction was found to depend greatly upon the vitality of the animals themselves, since it always became less in those specimens which had been kept for long in confinement. A shadow was not always necessary to make them close. An ordinary black pencil, if approached within two or three inches with extreme caution, produced the same result, while a glass rod brought within the same distance, and even moved rapidly to and fro, appeared to cause no alarm. Sensitiveness to change in intensity of light was experimentally noticed by the same author in the case of Ostrea, Mactra, Avicula (to a special extent), and Cardium. It is very remarkable to find that increased elaboration in the structure of the eyes does not necessarily carry with it increased sensitiveness, i.e. higher visual powers. Avicula, which is only provided with a few scattered ommatidia, which would entirely escape the notice of any one who had not seen them better developed elsewhere, was considerably more sensitive to light and shade than Arca, with its eyes of conspicuous size and much more perfect organisation, instantly contracting the mantle upon the impact of a shadow so faint as to be invisible to the experimenter.[296]
Visual Faculties of Solen and Ostrea.—The visual power of Solen may be exemplified by any one who is walking along almost any of our sandy bays at extreme low-water mark. If the day be warm and sunny, numbers of Solen will be seen raising themselves an inch or two out of their holes; but if you wish to catch them you must approach very cautiously, and on no account allow your shadow to fall upon them, or they will pop down into their burrows in an instant, and it is vain to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson Smith, with reference to oysters,[297] ‘the creatures are to the light above them; the shadow [of the boat] as it passes overhead is instantaneously noted, and, snap! the lips are firmly closed.’