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 w r ork^ 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, Decapoda and Stomapoda. 
Order I. DECAPODA, 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 Sijphonostoma and the Lernceadae. 
