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 1/400 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.