CLASS OF CRUSTACEA. 



475 



dentition ; but that which especially merits attention is, that the 

 original type of the generic being is of a character so ample as to 

 embrace all possible forms which the dentition can assume in 

 any species of that natural family. Nothing is wanting ; nothing 

 new appears ; nothing lias to be supplied ; all is foreseen ; all 

 provided for. To institute a species, all that is required is to 

 omit, or cause to disappear, or cease to grow, some parts of the 

 organ or apparatus already existing in the generic being. In 

 every natural family there is a species which bears, to the generic 

 animal, that is, to the young, a stronger resemblance than any 

 other. In the Salnioniflge it is the common trout of fresh-water 

 rivers, but there may be others. In the Solipeda it seems to be 

 the quagga of Southern Africa." E. K.] 



pm p* p" fp n 



Fig. 507. Palsemon (Fab.) ; Prawn.* 



563. The lateral appendages of the various rings con- 

 stituting the body are in general very numerous, and present 

 also considerable differences in their conformation and their 

 uses, whether we consider them in the various parts of the 

 same individual, or compare them in distinct species. Those 

 of the first rings, in general, have relations to the functions 

 of animal life, and carry eyes or form antennae ; the follow- 

 ing surround the mouth, and serve for the prehension or the 



* a, antennae of the first pair ; ai, antennae of the second pair, or inferior 

 antennae; I, lamellated appendix covering the base; r, rostrum, or frontal 

 prolongation of the carapace; y, eyes ; pm, external limb-jaw : p', thoracic 

 limb of the first pair ; p", thoracic limb of the second pair; fp, false swim- 

 ming limbs of the abdomen ; , caudal fin. 



