GENERAL LAWS OF ANIMAL STRUCTURE. 



271 



doubt whether these belong to the same category as the 

 other appendages, but they are usually so regarded. In 

 Fig. 171 we give the whole series of appendages in the 

 crawfish : 



The crab is much more modified, and the consolida- 

 tion more complete. The tail seems to be absent, but 

 is really only diminished in size and bent under and con- 

 cealed beneath the body ; but in the embryo the tail is 

 similar to that of the crawfish (Fig. 172). The maxilli- 



Fig. 172.— Development of Carcinus mcenas : A, zosa stage ; B, megalopa 

 stage ; C, final state. (After Couch.) 



peds and sense appendages are also much reduced in size, 

 but they are all present. 



Going down the Scale.— We have taken a case 

 from about the middle of the articulate scale, because 

 there the essential identity and adaptive modification 

 are evenly balanced and both conspicuous. But the evi- 

 dence is completed by going down and up the scale. In 

 the lower crustaceans the rings are all separate and the 

 appendages less and less modified (Fig. 173). A series 

 19 



