VII THE STUDY OF ZOOLOGY 203 



upon a common stem ; and if I compare this jaw 

 with the legs behind it, or the jaws in front of it, 

 I find it quite easy to see, that, in the legs, it is 

 the part of the appendage which corresponds with 

 the inner division, which becomes modified into 

 what we know familiarly as the " leg," while the 

 middle division disappears, and the outer division 

 is hidden under the carapace. Nor is it more 

 difficult to discern that, in the appendages of the 

 tail, the middle division appears again and the 

 outer vanishes ; while, on the other hand, in the 

 foremost jaw, the so-called mandible, the inner 

 division only is left ; and, in the same way, the 

 parts of the feelers and of the eye-stalks can be 

 identified with those of the legs and jaws. 



But whither does all this tend ? To the very 

 remarkable conclusion that a unity of plan, of the 

 same kind as that discoverable in the tail or 

 abdomen of the lobster, pervades the whole organ- 

 isation of its skeleton, so that I can return to the 

 diagram representing any one of the rings of the 

 tail, which I drew upon the board, and by adding a 

 third division to each appendage, I can use it as a 

 sort of scheme or plan of any ring of the body. I 

 can give names to all the parts of that figure, and 

 then if I take any segment of the body of the 

 lobster, I can point out to you exactly, what modi- 

 fication the general plan has undergone in that 

 particular segment ; what part has remained 

 movable, and what has become fixed to another ; 



