182 HOMOLOGY 



origin. Thus in this one group of Crustacea to which the lobster 

 belongs we find that eight pairs of thoracic appendages is the 

 rule. In the lobster we see that five of these pairs are adapted 

 for walking or locomotion and three of them for assisting in pro- 

 curing and manipulating food. So too, the crab, or shrimp, 

 and many allied forms have the same distribution of the tho- 

 racic appendages and zoologists group them together as an order 

 of Crustacea called Decapoda. In other groups, however, we 

 find different distributions of the eight pairs of thoracic ap- 

 pendages. One such group has only three pairs of walking 

 legs, the remaining five are adapted for food manipulation 

 (Order Stomatopoda). In another group all eight are rudi- 

 mentary, none being developed for walking (Order Cumacca) 

 while in another seven of the eight pairs are developed for 

 walking while only one pair serves for food manipulation (Order 

 Arthrostraca) . The assumption is made that all of these 

 different types of Crustacea, because of their striking simi- 

 larities, must be closely related and must have had a descent 

 from common ancestors. Such ancestors could not have been 

 more specialized than are these types today; they must have 

 been more generalized forms from which different lines of 

 adaptation could come. 



Such generalized ancestral forms of the Crustacea are repre- 

 sented among existing types which form a sub-class (Entomos- 

 traca) of the Crustacea. Their appendages and somites 

 are more numerous than twenty pairs and the appendages are 

 of the primitive biramous type. How the more specialized 

 forms of Crustacea were derived from these more generalized 

 types is a matter of speculation involving the factors of inherit- 

 ance and evolution which will be considered in a subsequent 

 chapter. 



III. INSECTS 



Another series of illustrations of homology may be found in 

 the group of insects of which from 200,000 to 400,000 are known. 

 In these myriads of forms the adaptations of wings and mouth 

 parts are particularly striking. 



