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 nenxpomatia (after Simroth).* ct. Cuticle ; a, epithelium ; b, cornea ; 

 c, envelope of the eye ; d, cellular laj^er ; 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, f 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 uus. einh. Weiclitbiere," 

 Zeit. fur Wiss. Zool, 1876. 



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

 scopical Science, 1875. 



