356 ON THE STUDY OF ZOOLOGY 



I may say, therefore, that the sixth segment Is like the 

 others in plan, but that it is modified in its details. 



The first segment is like the others, so far as its ring Is 

 concerned, and though its appendages differ from any of 

 those yet examined in the simplicity of their structure, parts 

 corresponding with the stem and one of the divisions of the 

 appendages of the other segments can be readily discerned 

 in them. 



Thus it appears that the lobster's tail is composed of a series 

 of segments which are fundamentally similar, though each 

 presents peculiar modifications of the plan common to all. 

 But when I turn to the fore part of the body I see, at first, 

 nothing but a great shield-like shell, called technically the 

 " carapace," ending in front in a sharp spine, on either side 

 of which are the curious compound eyes, set upon the ends of 

 stout movable stalks. Behind these, on the under side of the 

 body, are two pairs of long feelers, or antennae, followed 

 by six pairs of jaws folded against one another over the 

 mouth, and five pairs of legs, the foremost of these being 

 the great pinchers, or claws, of the lobster. 



It looks, at first, a little hopeless to attempt to find in this 

 complex mass a series of rings, each with its pair of append- 

 ages, such as I have shown you in the abdomen, and yet 

 it is not difficult to demonstrate their existence. Strip off 

 the legs, and you will find that each pair is attached to a very 

 definite segment of the under wall of the body ; but these 

 segments, instead of being the lower parts of free rings, as 

 in the tail, are such parts of rings which are all solidly united 

 and bound together ; and the like is true of the jaws, the 

 feelers, and the eye-stalks, every pair of which is borne upon 

 its own special segment. Thus the conclusion is gradually 

 forced upon us, that the body of the lobster is composed 

 of as many rings as there are pairs of appendages, namely, 

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

 and movable, 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 instruc- 

 tion 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 



