186 HOMOLOGY 



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 assist- 

 ing in procuring and manipulating food. So too, the crab, or 

 shrimp, and many allied forms have the same distribution of the 

 thoracic 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 rudimentary, 

 none being developed for walking (Order Cumacea), while in 

 another, seven of the eight pairs are developed for walking, and 

 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 similarities, 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 inheritance 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 more than 200,000 are known. In 

 these myriads of forms, the adaptations of wings and mouth 

 parts are particularly striking. 



Superficially, the insects are so similar to Crustacea that form- 

 erly they were all classed together in the common phylum 



