MOLLUSCA : THEIK EYES. 61 



don, and is now incontrovertibly established, tliat its 

 tip carries an eye even more completely developed than 

 those of the Pecten, which we have just been looking 

 at. The eye is sitnated, not indeed on the very sum- 

 mit 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 shin and is sur- 

 rounded 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 fllled ; towards 

 the bottom it is of thinner consistence, and aj^pears to 

 contain many brilliant particles when the eye is dis- 

 sected 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, or 

 rather lenticular body, perfectly clear and translucent, 

 but a little more solid than the vitreous humour. 



Kow protection for these so delicate organs is pro- 

 vided in a way quite diff'erent from, yet equally effec- 

 tive with, that which we just now admired in the 

 ease of the Pecten. You know that if you touch, 

 though ever so tenderly, the eye of the Snail, it is in- 

 stantly drawn into the horn by a most curious process 

 of inversion. Tlie action is performed by means of a 

 long muscular ribbon, which originates from the gi'eal 

 muscle that retracts the head within the shell, and 

 which is inserted into the extremity of the hollow 

 tentacle. "When this ribbon contracts at the will of the 

 animal, and still more forcibly, when it is aided by the 

 contraction of the great head-muscle, the tip of the 

 tentacle with its eye is drawn within the surrounding 



