306 SCHARFF : ORGANS OF SENSE IN MOLLUSCA. 



Cyclostoma). In all those possessing only two tentacles the 

 organs of sight lie at the base. This rule holds good for all 

 land and freshwater shells, save the Vertigo group, in which 

 there are only two tentacles, which bear the eyes on their apex. 

 The eyes do not, strictly speaking, occupy the apex of the 

 oculiferous tentacles, but are placed somewhat obliquely at their 

 outer edge. 



The nomenclature used in a description of the eye has to 

 be borrowed from that of the Vertebrates, but it must be clearly 

 understood that the only thing the two have in common is their 

 physiological function. The anatomical structure of the verte- 

 brate eye and that of a mollusc presents very wide differences, 

 still it is convenient to retain the same terms in both cases. 



To judge by its structure the Gasteropod eye appears to 

 be a modification of the unicorneal eye of the Annelids and 

 Arthropods. We may look upon it as having arisen from a 

 ball-like structure, to the posterior part of which a nerve passes 

 This ball assumes an elliptic shape in Paludina, or is drawn out 

 conically behind as in Neritina. In Planorbis the conic or 

 pear-shaped form is found to be the most constant. At its 

 anterior portion the ball touches the skin, the cells of which are 

 transparent so as to allow the light to fall through into the eye. 

 We may compare this (together with the anterior part of the 

 eye-bulb) to the cornea of the vertebrate eye, with which it 

 corresponds in position. 



As regards the optic nerve, it is sometimes united with the 

 nerve supplying the tentacle ; but it has been shown by Johannes 

 Miiller that in the genus Helix at any rate the two nerves are 

 completely separated. The whole eye is surrounded by a 

 structureless membrane called ' sclera,' with the exception of the 

 point, where the nerve enters. Moquin-Tandon, using the 

 nomenclature of the vertebrate eye, makes mention of an aqueous 

 humour : a lens and a vitreous humour as existing in the eye of 

 the snail. For several of the land and freshwater shells this view 

 has subsequently been proved to be erroneous. As has been 



J.C, iv., April, 1885, 



