Armor That Can See

The remarkable discoveries of Moseley with regard to the dorsal eyes of Chiton were first published in 1884. 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.

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. None of our English species appear to possess them.