A DEFINITION OF EVOLUTION 



xiv xyi XK. 



/'' '• 



^ 5 EGG 



MATING CARRIAGE SWIMMING 



Figure 14. Types of Appendage of the Chayfish. Roman niinierals indicate the 

 body segment from which each was taken, pr, protopodite; ex, exopodite; en, endopo- 

 dite; ep, epipodite; g, gill. (From Storcr and Usinger, "General Zoology," 3rd Ed., 

 McGraw-Hill Book Co., Inc., 1957. ) 



is more striking than the appendages of the Arthropoda, especially those 

 of the Crustacea. In the typical crustacean, there is one pair of appendages 

 borne by each segment of the body. These are all referable to a single 

 structural plan, and so they are considered to be serially homologous. 

 Serial homology is typical of all animals whose body structure is based 

 upon a series of essentially similar segments. The major examples are in 

 the phyla Annelida, Arthropoda, and Chordata. In the crustacean append- 

 age, there is typically a basal protopodite of two segments. Distal to this 

 are two parallel structures, a medial cndopoditc and a lateral exopodite, 

 each consisting of a series of segments. The appendages of xarious body 

 segments have been modified in the most diverse ways, adaptive to widely 

 different uses (Figure 14). Their condition in the crayfish is a familiar 

 example. The first two pairs of appendages have been greatly modified 

 to form antennae. Whether the eyestalk also represents a greatly reduced 

 appendage is still subject to debate among specialists. The heavy, biting 

 mandibles are formed h\ the shortened protopodite and endopoditc of 

 the third segment, while the cxopoditt^ has bc>en eliminated altogether. 

 The fourth pair of appendages forms the first maxilla, in which the pro- 

 topodite and the endopoditc form a flattened plate which is used for 

 handling food. The fifth pair are the second maxillae, but iIk> exojiodite 

 is ]")r(\sent here, and together with a dorsal outgrowth called the ejiipodite, 

 it forms a vane to pass a current of water over the gills. This is the last of 

 the appendages of the head region. Tlu> first three pairs of thoracic ap- 

 pendages are maxillipeds, the protopoditc^s of which avc (lattened and 

 serve for tlu> handling of food, just as do (lie maxillae*. Tlu> cMidopodite 

 and exopodite are present on these appendages, but (\\c\ are not very 

 large, and {\\c\ mav be sensorv in ftinction. The first maxilliped bcnu's an 

 epipodite, while liie second and third bear gills which extend dorsal!) 

 under the carapace. The remaining five pairs of llioracic appendages are 

 specialized as walking legs, with the exopodite absent and the endopoditc 



36 



