140 CUTTLE-FISH. 



to be a very general arrangement in the Mollusca. 

 The power of sight given by such an eye can be but 

 small. Indeed, it is probable that it does little more 



Fig. 95.— Eye of aeiix pomatia (after Simroth).* ct. Cuticle ; a, epithelium ; b, cornea ; 

 c, envelope of the eye ; d, cellular layer ; e, fibrils of the optic nerve ; /, feeler 

 cell ; na, nerve of the tentacle ; no, optic nerve. 



than distinguish degrees of light. According to Lespes, 

 a Cyclostoma only perceives the shadow of a hand at a 

 distance of five inches, and a Paludina of eight. 



It is interesting that, as Lankester first showed,t the 

 eye of Mollusca, in its gradual development, passes 

 through the stages which we find are the permanent 

 conditions in Patella and Haliotis, commencing as a 

 depression, which grows deeper and deeper, and 

 gradually closes over. 



Even in the Nautilus the cornea leaves an opening, 



* Simroth, "Ueber die Sinneswerkzeuge uns, einli. Weiclithiere," 

 Zeit. fur Wiss. Zool, 1876. 



t "Obs. on the Dev. of Cephalopoda," Quarterly Journal of Micro- 

 scopical Science, 1875. 



