CRUSTACEA 295 



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 desig- 

 nation for the lower crustacean classes as a whole. 



The restriction of feeding to a few limbs is often, though not 

 always, accompanied by the replacement of the original habit of 

 gathering food in small particles by other modes of feeding. 

 Continuous and automatic straining-out of such particles, which is 

 practised (though in different modes) by the most primitive members 

 of various classes, is superseded in some cases by the intermittent 

 seizure, by particular limbs, of particles of some size, and this by the 

 graspingof 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 Crus- 

 tacea, 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. 



In the various ways which have been outlined above, the common 

 organization of the subphylum exhibits modifications which, as will 

 be seen from what follows, are as many and as far-reaching as those 

 which are to be found in any division of the Animal Kingdom. 



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

 relative to the size of the animal, but is thinner and flexible in many 

 parasitic genera. It is often strengthened by calcification, and in 

 certain ostracods, barnacles, and crabs this gives it a stony hardness. 

 In each somite there may or may not be distinguishable a dorsal 

 plate or tergite (tergum) and a ventral sternite {sternum). The tergite 

 may project at each side as a pleuron. 



There are embryological indications that the body should be re- 

 garded as containing, besides the somites^ an anterior presegmental 

 region, to which the eyes belong, corresponding to the prostomium of 

 a worm, and a postsegmental region or telson, on which the anus opens. 

 Each somite, except the first, which is purely embryonic, may bear 

 a pair of appendages, though it is rarely that the appendages of all the 

 somites are present at the same time. The somites never all remain 

 distinct in the adult. Always some of them are fused together and 



