246 ORDER DECAPODA ; GENERAL CHARACTERS. 



respiration of many species, the nearest approach to the class 

 MYRIAPODA; and in some members of the Entomostracous 

 section, we find a considerable approximation to the higher 

 ROTIFERA. 



SECTION A. PODOPHTHALMA. 



ORDER I. DECAPODA. 



784. In this Order we find the highest general organisation, 

 the largest size, and the most varied habits, which we anywhere 

 meet with among Crustacea ; it is the one most useful to Man ; 

 and also most interesting to the Naturalist. The Lobsters, 

 Crabs, Cray-fish, Prawns, Shrimps, in fact, nearly all the 

 species that are ever used as food, belong to it. Their growth 

 is slow ; but they ordinarily live a long time. Their habits are 

 mostly aquatic ; but, in consequence of the manner in which 

 their gills are inclosed, none of them are killed at once by being 

 withdrawn from the water ; and some of them pass the greatest 

 part of their lives in air. They are naturally voracious and 

 carnivorous : and the first pair of legs is transformed into a pair 

 of powerful claws, by which they seize their food and convey it 

 to the mouth, the claw of one side usually having a sharp edge 

 for cutting, whilst that of the other is provided with a blunt 

 rounded edge for bruising. The form and size of the claws, 

 relatively to the rest of the extremities, varies greatly in the 

 different species; thus in the Cray -fish (Fig. 471), they only 

 seem like legs somewhat enlarged ; whilst in the Crabs 

 (Fig. 475), they commonly seem to be distinct organs. It is 

 in this group, that we find the mouth furnished with the most 

 complicated set of appendages ; and we may trace in these a 

 gradual transition from the form of jaws to that of legs. This 

 is shown in the accompanying figures, which represent the 

 under surface of the Cray-fish, and the series of feet-jaws sepa- 

 rately displayed. This Order is divided into three sub-orders, 



