10 HISTORY OF BRITISH CRUSTACEA. 



curious transformations, so that in their early stages there 

 is but little, if any, resemblance to the mature form. 



The Crustacea are primarily divided into two natural 

 groups, the smallest and the last treated in this work* hav- 

 ing the mouth prolonged into a sucker, so that they are 

 nourished from the animal fluids which they obtain from 

 the creatures on which they are parasitic ; the largest group 

 contains the Crustacea whose mouths are furnished with 

 jaws with which they masticate their food. 



Of the latter group, the most important and largest series 

 is that in which the eyes are borne on pedicels and are 

 movable : hence their name, Podophthalma. In these 

 the thorax is covered by a great shield called the carapace, 

 while the legs are partly adapted for walking and partly for 

 seizing. They form two orders, Decafoda and Stomapoda. 



Order I. DECAPOD A, Latr. 



In the species of this Order the branchiae are fixed on the 

 sides of the carapace, and enclosed in special respiratory ca- 

 vities. The mouth apparatus consists of six pairs of mem- 

 bers, so that the number of thoracic legs is reduced to five 



* The Siphonostoma and the Lernaadce. 



