MOLLUSCA : THEIR EARS. 55 



nates 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 

 tentacle 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 

 tentacle, like so many rings throughout its whole 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, 

 perhaps, 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 colour that we can readily 

 look through its tissues, — we shall easily find its ears, 

 though they are not quite so prominent as those of 

 an ass. 



I subject the animal to a gentle pressure by means of 

 the compressoriurn,* just sufficient to flatten its soft body 

 a little, without injuring it. And now, with this low power, 

 you may see what Siebold, a learned zoologist and com- 

 parative anatomist, familiar with the curious phenomena 

 of life, truly calls " a wonderful spectacle." In the neck 

 of the little animal you discern, deep-seated in the soft 

 flesh, a pair of perfectly transparent globules, or capsules, 

 without any opening, but filled with a clear fluid, in which 

 there are some minute bodies performing the most extra- 



* An instrument for compressing or flattening an object under the 

 microscope. 



