38 



INTRODUCTION TO EVOLUTION 



V, Fig. 3.11), aid in grasping food and conveying it to the mouth. One 

 pair, the mandibles (III, Fig. 3.11), crush the food. Two other pairs, the 

 antennae and antennules (II and I, Fig. 3.11), form sensory "feelers." 

 The great variety of functions served by the appendages of the crayfish 

 will be evident from the foregoing summary. Careful study reveals that 



Antenna 



-Anfennule 



CephalofhoraK 



Uropool 



Tclson 



FIG. 3.10. Crayfish, dorsal view. (From Hagen, A^emo/rs f^^- 

 seum of Comparafive Zoology, Harvard University.) 



they are all modifications of a single pattern. We have spoken of a five- 

 fingered (pentadactyl) pattern underlying vertebrate forelimb structure; 

 similarly we might say that a two-fingered pattern underlies the structure 

 of crayfish appendages, as well as of those of all other members of Phylum 

 Arthropoda: crustaceans, spiders, insects, and their kin. This two-fingered 

 structure is called a biramous appendage and is well illustrated by the 



