VII EYES OF ONCHIDIUM AND CHITON I 8/ 



acicula, which is never seen above the surface, is altogether 

 destitute of eyes. A species of Zospeum, a Helix, and a Bithynella 

 from darlv caves in Carniola have suffered a similar loss. On the 

 other hand, a small Hycdinia 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 fiQh.{Pcrio])Mlialmus)\\h.iQ\\ skips along 

 the beach by the aid of its large ventral fins, and feeds principally 

 on insects and Onchidium. Karl Semper suggests -^ that the eyes 

 are of service to Onchidium as enabling it to apprehend the 

 shadow of the approaching Feriophthalmus, 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 Pcriojohthalmus away. This theory — for it is no more than 

 theory — may or may not be true, but it is remarkalile that 

 Onchidium with dorsal eyes have precisely the same geographical 

 distribution as Periophthalmiis, and that where no Periojphthcdmus 

 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.- 



Dorsal Eyes in the Chitonidae. — 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 sm-face 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 sm-face 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 



' Animal Life, p. 372 f. - Bergli, Morph. Jahrh. x. p. ll'l. 



■ ^ Ann. Mag, Nat. Hist. (5) xiv. ]). 141. 



