MOLLUSC A: THEIR EYES 71 



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 

 most curious process of inversion. The action is performed 

 by means of a long muscular ribbon, which originates from 

 the great 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 ten- 



STRUCTURE OF EYE IN SNAIL. 



tacle with its eye is drawn within the surrounding parts 

 just like the finger of a glove. When the animal would 

 again protrude its eye, the fibres which surround the ten- 

 tacle, like so many rings throughout its whble length, 

 successively contract, and thus gradually squeeze out, as 

 it were, the inverted part, until it is turned back to its 

 original position. 



But the ears of this homely " creeping thing" are, per- 

 haps, even more curious than its eyes; though far less 

 elaborate in their structure. You will imagine, now, that 

 I refer to the other pair of tentacles, as you are accus- 

 tomed to associate the idea of ears with projecting organs 

 situated on the head. No, you must not look there for 

 them. Here, in this young Garden Slug, which is so 

 small as to be conveniently examined on the stage of the 

 microscope, and so devoid of color that we can readily 



