SCHARFF ; ORGANS OF SENSE IN MOLLUSCA. 307 



pointed out by Leydig, there are two parts to be made out in the 

 interior of the eye of Paludina, viz. : a vitreous humour and a 

 lens. A similar condition holds good for Neritina and Planorbis. 

 A good deal of doubt has been cast on the existence of a 

 vitreous humour in Helix, and there has been much controversy 

 about its presence. Simroth, however, who recently made very 

 elaborate researches, was unable to discover it in Helix pomatia, 

 and in all probability it is also absent in other land-shells, 

 excluding perhaps Cyclostoma. According to Lespes, the lens is 

 absent in Neritina fluviatilis. 



Behind the lens we have the cup-shaped retina, forming the 

 posterior wall of the eye-ball. The following three parts may be 

 distinguished in the retina of Helix : — 



1. A layer of rods containing pigment. (This inner- 



most layer constitutes about one-half of the whole 

 thickness of the retina). 



2. A cellular layer. 



3. A nervous layer. 



This arrangement, generally speaking, appears to be 

 appUcable to all gasteropods. In Limax and Limnaea, which 

 have been studied more minutely, the three layers have been 

 shown to exist, but the internal one is devoid of pigment. 



With regard to the physiological functions of the snail's eye, 

 several authors have attempted experiments in order to ascertain 

 to what degree the visionary power may be utilized. Moquin- 

 Tandon draws an important distinction between the sight of 

 Paludina and that of the Helices. That of the latter merely 

 suffices to give the animal an impression of an object 5-6 mills, 

 distant. In twilight the power of vision is increased. In 

 Paludina, sight appears to be far more powerful. It quickly 

 withdraws into its shell, when a large object comes within 30 

 centims. of the eye. These results agree pretty well with what 

 we should expect, judging by the anatomical structure of the 

 organ, and they have been confirmed by a number of 

 conchologists. 



