VII EYES OF ONCHIDIUM AND CHITON 1 8/ 



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

 destitute of eyes. A species of Zospeum^ a Helix^ and a BitJiy- 

 nella 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 ^ 

 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 ommato- 

 phores, while in Helix the ommatophores are formed first, and 

 the eyes developed upon them.^ 



Dorsal Eyes in the Chitonidae. — The remarkable dis- 

 coveries of Moseley with regard to the dorsal eyes of Chiton were 

 first published in 1884.^ He happened to notice, while examin- 

 ing a specimen of Schizochiton iricisus^ 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 sur- 

 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 



1 Animal Life, p. 372 f. 2 Bergh, Morph. Jahrb. x. p. 172. 



^ Ann. Mag. Nat. Hist. (5) xiv. p. 141. 



