CHAPTER IX 

 THE PERPETUATION OF ADAPTATIONS 



WE have seen that the appendages of the Crustacea are built 

 primarily on the same type of structure, and that, in different 

 orders, the several appendages have become modified in one 

 way or another for the performance of different functions. In 

 the Malacostraca the appendages are reduced to twenty pairs in 

 all, but in the more primitive forms included in trie group 

 Entomostraca there are large numbers of pairs made up of 

 primitive biramous appendages all performing practically the 

 same functions. The twenty pairs (in one order, Phyllocarida, 

 there are twenty-one pairs) are distributed in the same way in 

 all of the Malacostraca (Fig. 68) ; five belong to the head, eight 

 to the thorax, and seven to the abdomen. The abdominal 

 appendages retain the original biramous condition, but the other 

 thirteen pairs are variously modified for the performance of 

 different functions. Examining only the thoracic appendages, 

 we find one order (Schizopoda) in which all eight pairs are 

 biramous and of the generalized type, all serving for swimming 

 (Fig. 87). In the other orders they are adapted for feeding 

 and walking in a variety of ways. In the order Stomatopoda, 

 five pairs are modified for food getting and only three pairs for 

 walking; in the order Decapoda three pairs are modified for 

 food getting and five are devoted to walking; in the order Cum- 

 acea only two pairs are for food getting while six are devoted to 

 walking; and in the order Arthrostraca only one pair is for food 

 getting 3 the other seven for walking. In all of these cases the 

 morphology of these organs indicates that the most highly 

 differentiated appendages and the most generalized ones are 

 built on the same plan of structure. The study of the develop- 

 ment of the individual crustacean indicates that the most highly 

 differentiated appendages appear first as generalized biramous 



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