200 THE STUDY OF ZOOLOGY vil 



commoner the better, and let us see how the appli- 

 cation of common sense and common logic to the 

 obvious facts it presents, inevitably leads us into 

 all these branches of zoological science. 



I have before me a lobster. When I examine 

 it, what appears to be the most striking character it 

 presents ? Why, I observe that this part which we 

 call the tail of the lobster, is made up of six distinct 

 hard rings and a seventh terminal piece. If I 

 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 re- 

 present a transverse section of the ring and its 

 appendages 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, "homologous parts." The ring of the 

 third division is the " homologue " of the ring of 

 the fifth, the appendage of the former is the homo- 

 logue of the appendage of the latter. And, as 

 each division exhibits corresponding 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, 



