EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 



39 



typical swimmeret of a crayfish (XVI, Fig. 3.11). The basal portion of 

 the appendage, protopodite, is unpaired but may consist of more than one 

 segment. Attached to the protopodite are the two "fingers," each com- 

 posed of several or many segments. The "finger" nearest the midline of 

 the body is called the endopodite, the lateral one the exopodite. The la- 

 beling of Fig. 3.11 indicates clearly how, starting from this primitive ar- 

 rangement, appendages adapted for the wide variety of functions have 

 been derived by modification, and in some cases loss, of one or another of 

 the original parts. 



XIY xyi XDC 



^ '$ EGG 



MATING CARRIAGE SWIMMING 



FIG. 3.11. Crayfish appendages; pr, protopodite; en, endopodite; ex, exopodite; ep, 

 epipodite; g, gill. (By permission from Genera/ Zoology, by Storer, p. 442. Copyright, 

 1943. McGraw-Hill Book Company, Inc.) 



What are the implications of serial homology for evolution? It will be 

 noted that arthropods, such as the crayfish, have bodies composed of a 

 series of metameres or somites (this segmentation is particularly clear in 

 the abdomen of the crayfish. Fig. 3.10). Each metamere is provided with 

 a pair of jointed appendages, modifications of the biramous pattern. It 

 would seem that the common ancestor from which these arthropods in- 

 herited the arrangement described must have had a body composed of a 

 series of metameres, somewhat hke the earthworm's, and had each meta- 

 mere equipped with a pair of biramous appendages in typical form. In 

 descendants from this ancestor some of the metameres became fused to- 

 gether, to form a cephalothorax as in the crayfish (Fig. 3.10), or to form 

 separate head and thorax as in an insect. At the same time appendages 

 attached to difi'erent metameres of the body became modified to serve a 

 variety of functions. Thus, like homology in general, serial homology finds 

 its most reasonable explanation in a theory of descent with modification, 

 i.e., of evolution. 



