HORNS OF THE SNAIL. 129 



of the foot, and the expanse by which the snail draws 

 itself into its shell, accompanies the optic nerve, (in the 

 larger horns,) and runs up the tube to its termination. 

 By its contraction, it inverts the horn, drawing down 

 first the apex, and thus, as it were, sheaths the organ 

 in itself. Such is the simple but admirable mode in 

 which the retraction takes place ; but the organ thus 

 retracted has to be protruded, and this is effected by 

 means of a system of circular muscles composing the 

 walls of each hollowed horn, which, by their contraction, 

 evert or squeeze out the indrawn tentacle, and, having 

 accomplished this, they give it firmness and motion. 



The nerve which runs to the eye, or that which gives 

 the sense of touch to the tentacle, must, it would at first 

 appear, be stretched when the horn is protruded ; but 

 such is not the case : nerves do not admit of such rude 

 treatment without pain and loss of their peculiar powers. 

 Here, then, is a simple yet admirable provision indica- 

 tive of design, which, indeed, every part of the structure 

 of these animals clearly exhibits. The nerve is as long 

 as the horn when stretched to its uttermost, and so far 

 its safety is provided for ; but when the horn is in- 

 verted, the nerve is then thrown into a beautiful series 

 of coils, lodged within the cavity into which the horn 

 is retracted. The annexed diagram will explain the 



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