vi.] OJV THE STUDY OF ZOOLOGY. 99 



segment. Thus the conclusion is gradually forced upon 

 us, that the body of the lobster is composed of as many 

 rings as there arc pairs of appendages, namely, twenty 

 in all, but that the six hindmost rings remain free and 

 moveable, while the fourteen front rings become firmly 

 soldered together, their backs forming one continuous 

 shield — the carapace. 



Unity of plan, diversity in execution, is the lesson 

 taught by the study of the rings of the body, and the 

 same instruction is given still more emphatically by the 

 appendages. If I examine the outermost jaw I find it 

 consists of three distinct portions, an inner, a middle, 

 and an outer, mounted 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 remark- 

 able conclusion that a unity of plan, of the same kind as 

 that discoverable in the tail or abdomen of the lobster, 

 pervades the whole organization 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 sent of scheme or plan of any ring of the body. 1 

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



