54 



EVENINGS AT THE MICROSCOPE. 



ralists have doubted, and even denied, that the tentacle 

 was anything more than a very delicate organ of touch, 

 yet it has been abundantly proved by dissection, and is 

 now incontrovertibly established, that its tip carries an 

 eye, even more completely developed than those of the 

 Pecten which we have just been looking at. The eye is 

 situated, not indeed on the very summit of the tentacle, 

 but on one side of a movable bulb there placed. It is 

 very minute, almost spherical, but slightly flattened in 

 front. It is protected by a very thin transparent layer of 

 the common skin, and is surrounded at the side and 

 behind by a perfectly black membrane, called the choroid, 

 or pigment-membrane. This black globule contains a 

 transparent and semi-fluid substance, with which it is 

 completely filled; towards the bottom it is of thinner 

 consistence, and appears to contain many brilliant particles 

 when the eye is dissected under the microscope ; this may 

 be considered as the vitreous humour. In the front part 

 of the eye there is a crystalline lens, a small, circular, 

 flattish body, shaped like a lens, perfectly clear and trans- 

 lucent, but a little more solid than the vitreous humour. 



Now protection for these so delicate organs is provided 

 in a way quite different from, yet equally effective with, 

 that which we just now admired in the case of the Pecten. 

 You know that if you touch, though ever so tenderly, the 

 eye of the Snail, it is instantly drawn into the horn by a 



STRUCTURE OP EYE OP SNAIL. 



most curious process of inversion. This action is per- 

 formed by means of a long muscular ribbon, which origi- 



