PHYLUM ARTHROPODA 



203 



ous, the second maxilla, (2) biramous, the 

 swimmerets, and (3) uniramous, the walk- 

 ing legs. All these appendages have probably 

 been derived from a single type, the modifi- 

 cations being correlated with the functions 

 performed by them. The biramous type may 

 represent the condition from which the 

 other types developed as shown in Fig. 115. 

 The uniramous walking legs, for example, 

 pass through a biramous stage during their 

 embr)'ologic development. Again, the bira- 

 mous embryonic maxillipeds are converted 

 into the foliaceous type by expansion of 



their basal segments. Other types of appen- 

 dages undergo similar changes. 



Structures that have a similar fundamental 

 structure, regardless of function, due to des- 

 cent from a common ancestor, are said to be 

 homologous. The highly specialized cheli- 

 peds, walking legs, jaws, and other structures 

 of the crayfish have evidently developed 

 from a fundamental type and have become 

 different in function. When homologous 

 structures are repeated in a series the condi- 

 tion is known as serial homology. This is a 

 most striking example of serial homology 



Generalized Biramous Appendage 

 1. Antenna (touching, tasting) 



18. Uropod (swimming) 



[ Protopodife 



M^i EndoDodite 



13. First abdominal 

 oppendage of male 

 (copulating) 



First abdominal 



appendage of 



female 



(rudimentary) 



11. Fourth walking leg (walking) 



8. First wolking leg (pinching) 

 Exopodite 



Figure 115. Homology and evolution of crayfish appendages. All are believed to have been 

 derived from a generalized two-branched (biramous) appendage consisting of protopodite, en- 

 dopodite, and exopodite. This basic plan of structure has been modified (specialized) for the 

 various uses noted. The appendages demonstrate in a striking way the changes that occur in 

 the evolution of structures. 



