CRUSTACEA 331 



are adapted for respiration by the presence upon them of gills, which 

 are usually protected by a carapace of moderate size. Thus this region 

 of the body of the Malacostraca is, in its own ways, as many-functioned 

 as the corresponding part of the trunk of Chirocephalus . The Malacos- 

 traca maintain in typical cases (Figs. 269, 282) the swimming function 

 of the limbs on the hinder portion (abdomen) of the trunk, and some 

 of the class have found other uses (ovigerous, copulatory, etc.) for 

 these appendages. Accordingly there is seldom any reduction in the 

 fixed number of fourteen (or fifteen) trunk somites which, arranged 

 always in a thorax of eight and an abdomen of six (or seven), cha- 

 racterizes the class. Nevertheless in all but one of the orders the 

 abdomen has lost a somite, in the crabs (Fig. 284) and some others 

 of the highest order {Decapoda) it is reduced, and in a few members 

 of the class it is a limbless and unsegmented stump. 



The name Entoviostraca was formerly used in the classification of 

 the subphylum, to distinguish from the Malacostraca a division con- 

 taining all the other classes. Since, however, these differ from one 

 another as widely as each of them does from the Malacostraca, the 

 name is no longer used in classification but is only a convenient 

 designation for the lower crustacean classes as a whole. 



When feeding is restricted to a few limbs it is often, though not 

 always, accomplished in some other way than by the original habit 

 of gathering food in small particles. Continuous and automatic 

 straining-out of such particles, which is practised (though in different 

 modes) by the most primitive members of all classes except the 

 Branchiura, is superseded in various members of different classes 

 by the intermittent seizure, by particular limbs, of particles of some 

 size, and this by the grasping of larger objects, which may lead to a 

 predatory habit. Finally, either of these modes of feeding may be 

 replaced in parasites by suction or absorption, through organs which 

 do not always represent appendages at all. (Parasites, however, are not 

 known among the Branchiopoda or Ostracoda.) Needless to say, each 

 change in the mode of obtaining nutriment has entrained numerous 

 alterations in organs other than those by which the food is actually 

 taken, as in the means of locomotion, sense organs, weapons of offence, 

 etc. On the other hand, adaptations to mere differences of habitat, 

 in the Crustacea, as in other arthropods, are, as a rule, strikingly small. 

 There is, for instance, remarkably little difference between a land 

 crustacean and its nearest marine^relatives. Pelagic genera, however, 

 are sometimes considerably modified. 



We must now proceed to review in more detail the common organi- 

 zation of the Crustacea and the variation which it presents throughout 

 the group. 



The cuticle of a crustacean is, save for the joints, usually stout 



