84 LAY SERMONS, ESSAYS, AND REVIEWS. [VL 



separate one of the middle rings, say the third, I find it carries 

 upon its under surface a pair of limbs or appendages, each of 

 which consists of a stalk and two terminal pieces. So that 

 I can represent a transverse section of the ring and its appen 

 dages upon the diagram board in this way. 



If I now take the fourth ring I find it has the same structure, 

 and so have the fifth and the second ; so that, in each of these 

 divisions of the tail, I find parts which correspond with one 

 another, a ring and two appendages ; and in each appendage a 

 stalk and two end pieces. These corresponding parts are called, 

 in the technical language of anatomy, &quot; homologous parts.&quot; The 

 ring of the third division is the &quot; homologue &quot; of the ring of the 

 fifth, the appendage of the former is the homologue of the 

 appendage of the latter. And, as each division exhibits corre 

 sponding parts in corresponding places, we say that all the 

 divisions are constructed upon the same plan. But now let us 

 consider the sixth division. It is similar to, and yet different 

 from, the others. The ring is essentially the same as in the 

 other divisions ; but the appendages look at first as if they were 

 very different ; and yet when we regard them closely, what do 

 we find ? A stalk and two terminal divisions, exactly as in the 

 others, but the stalk is very short and very thick, the terminal 

 divisions are very broad and flat, and one of them is divided into 

 two pieces. 



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 con 

 cerned, 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 &quot; carapace,&quot; 



