CRUSTACEA 291 



corresponding concentrations of function and differentiation of the 

 contents of somites. 



Five radically different lines of evolution have established the classes 

 of the subphylum. (i) In the phyllopod orders of the Branchiopoda, it 

 is only on the head that differentiation among the appendages has pro- 

 ceeded to any considerable extent. Here each is specialized for some 

 particular function, as for the service of the senses or the manducation 

 of food. In the trunk the limbs are still, as has been said above, similar, 

 and all subserve the functions of swimming, feeding, and respiration. 

 In two of the three orders, however (not in the Conchostraca), there 

 is a group of limbless segments at the hinder end of the body, and 

 this perhaps indicates that the efficiency of the limbs has already 

 reached a pitch which has permitted a reduction in their number. 

 The resemblance between the limbs of the series is closest in the order 

 AnostracUy to which Chirocephalus belongs. In the orders Conchostraca 

 and Notostraca there is a certain progressive enfeeblement of the 

 limbs from before backwards on the trunk, and those of the 

 hinder pairs probably serve respiration almost exclusively, while one 

 or two pairs at the anterior end are a good deal modified for new 

 functions, though all are essentially alike. (The most conspicuous 

 differences between the phyllopod orders lie, however, not in any 

 features of their trunk limbs, but in peculiarities of the antennae, and 

 in the fact that the Anostraca have no carapace, the Notostraca (Fig. 

 231) possess one as a broad dorsal shield, and in the Conchostraca 

 (Fig. 232) it is present as a bivalve shell.) In the order Cladocera (Fig. 

 233) an advance in differentiation appears. In these animals a high 

 degree of specialization of the trunk limbs for feeding enables that 

 function to be performed by some half-dozen pairs, and accordingly 

 the body is much shortened. Here the carapace forms (except in some 

 aberrant genera) a part of the food-collecting apparatus, and the 

 function of swimming, abandoned by the trunk limbs, is performed 

 by the antennae. (2) A similar trend of evolution is even more strongly 

 manifested by the class Ostracoda (Fig. 237), which are very short- 

 bodied and completely enclosed in a bivalve shell formed by the 

 carapace. Whereas, however, in the Branchiopoda it is always by 

 trunk limbs that food is gathered, in the Ostracoda that function 

 is performed by limbs of the head. The trunk limbs, which have lost 

 the functions of swimming and respiration as well as that of feeding, 

 serve relatively unimportant subsidiary purposes, and are reduced, at 

 most, to two pairs. Some members of the class carry shortening to an 

 extreme pitch by contriving to dispense with one or both of these 

 pairs. (3) The members of the class Copepoda (Fig. 238) also feed 

 by means of appendages on the head, though they use these differently 

 from the Ostracoda. In contrast to that group they have no carapace, 



19-2 



