THE CRAYFISH 131 



of the parts of the appendages in a series ; the other, a diver- 

 gence in the forms of appendages because of adaptations to 

 different uses. Perhaps no fact in nature is better known 

 than the fact that organs are adapted in general form and in 

 special parts to perform particular functions. It is only when 

 we come to examine a series of organs like the appendages of 

 the crayfish that we obtain some insight into the very great 

 changes which must often take place before organs become 

 adapted to the performance of even slightly different func- 

 tions. Morphologists, realizing the extent of the principle of 

 adaptation, have been able to show that in a series of organs, 

 like the appendages of the crayfish, any part of an appendage, 

 as, for example, the endopodite, can be shown to correspond to 

 the endopodite in all the other appendages, no matter what 

 the superficial differences may be. 



In referring to these corresponding parts we use the tech- 

 nical term " homologous parts." The protopodites of all the 

 appendages are structurally similar ; that is to say, they are 

 situated in the same relation to adjacent parts all through 

 the series. Likewise the legs and the antennse, being inner 

 branches (endopodites), and being jointed to the second seg- 

 ment of the appendages, are homologous parts. If such unlike 

 parts as the antenna; and the legs are homologous, it is, of 

 course, very clear that the legs and the chelipeds are homolo- 

 gous as whole endopodites, and also segment by segment from 

 the base to the tip. The crayfish appendages are examples of 

 serial homology ; in a broader sense, homology deals with 

 organs of all sorts that have a corresponding position and origin 

 in different animals. In connection with the study of homol- 

 ogy, the science of structurally similar parts, we find the word 

 " analogy " in frequent use. The difference between homolo- 

 gous parts and analogous parts is one easily made clear by 

 applying the terms to the present study. The legs of cray- 

 fishes are analogous parts, because their use is the same ; they 



