CRUSTACEA 327 



the development, from the hinder part of the head, of a skin fold — 

 the carapace — by which the important anterior region of the body is 

 overhung and protected, and the setting up in the surrounding water 

 of currents for purposes of respiration and feeding is facilitated. Not 

 all crustaceans possess the carapace: in some it has perhaps never 

 existed, others have discarded it. In those which have it, its extent 

 varies : in extreme cases it encloses the whole body. 



The transformation of the external make-up of the body is of 

 course reflected in the internal organization, which shows corre- 

 sponding concentrations of function and differentiation of the contents 

 of somites. 



On these general lines evolution has given rise to six Classes of 

 Crustacea. We must now briefly survey them, (i) In the Bran- 

 chiopoda feeding is performed by the limbs of the trunk. In the 

 "phyllopod" groups of this class, mentioned above, it is only on the 

 head that difi^erentiation among the appendages has proceeded to any 

 considerable extent. Of the head limbs each, as we have seen, is 

 specialized for some particular function, such as the service of the 

 senses or the manducation of food. On the trunk the limbs, which 

 are numerous, are still similar and all subserve at least the functions 

 of feeding and respiration. In the order Anostraca, to which Chiro- 

 cephalus belongs, there is no carapace, and the trunk limbs, whose 

 similarity is very strong, retain the function of swimming. In the 

 order Notostraca (Fig. 242), also phyllopodous, there is a carapace 

 but it is wide and shallow and does not enclose the trunk limbs, and 

 they are still used for swimming. A certain degree of diflFerentiation 

 exists between these limbs, the anterior pairs for instance being 

 capable of clasping objects. In both the foregoing orders limbs have 

 been dispensed with on some of the hinder somites. The remaining 

 phyllopod group, the Conchostraca (Fig. 243), are united with the 

 non-phyllopod group Cladocera as the order Diplostraca. In the 

 members of that order (except a few aberrant cladocera) the carapace 

 encloses the trunk limbs, which are not used for swimming, that 

 function being taken over by the antennae. The Conchostraca alone 

 among branchiopods retain limbs on all their trunk somites like the 

 trilobites, but as in the Notostraca there is a certain degree of diff'er- 

 entiation between the members of the series. In the Cladocera (Fig 

 244), the highest group of the Branchiopoda, a compact and very 

 efficient feeding apparatus is formed by some half-dozen pairs of 

 limbs, the trunk is correspondingly shortened, and even so some of the 

 hinder somites are limbless. In certain members of this group, such 

 as the water-flea Daphnia there is a high degree of differentiation be- 

 tween the trunk limbs (Fig. 245). (2) A similar habit of body is even 

 more strongly developed in the class Ostracoda (Fig. 248), which are 



