7 
108 STRUCTURE AND METAMORPHOSIS OF CRUSTACEA. 
hopper, the rings or segments are almost as similar to each 
other as they are in the centipede tribe. There is no class in 
which we find the same parts exhibiting so great a variety of 
forms, and rendered subservient to so many uses. Thus in 
the Crab and Lobster the members of the first pair are not 
used for walking, but form the claws or arms by which the 
food is seized ; in the Cray-fish, these members may be used 
either as- legs or claws; whilst in the Sand-hopper, they 
closely resemble the other legs. And the jaws of the higher 
Crustacea, of which there are several pairs, are really meta- 
morphosed legs ; as may be seen by comparing them with the 
corresponding appendages of the Limulus or king-crab, the 
first joints of which act as jaws, whilst the remaining portions 
of these members serve either as legs for locomotion, or as 
claws for prehension. 
101. Most of the Crustacea, like insects, come forth from 
the egg in a state very different from 
their adult form ; and afterwards undergo 
a series of changes, which are in some 
instances so remarkable as to approach 
the complete metamorphosis of insects, 
and which end in the production of the 
complete form. An early form of the 
common crab, at a time when it is of 
the minute size indicated on the scroll, 
is shown in fig. 48. The immature 
Crustacea of different tribes bear much 
more resemblance to each other, than do 
the forms into which they are ulti- 
mately to be developed; and the dif- 
ferences they afterwards present are 
ie chiefly due to a variety in the amount 
Fig. 48. 208M ORLARVAL of orowth which the different parts 
undergo. 
102. It is one of the most remarkable results of modern 
zoological research, that in immediate connexion with the 
class of Crustacea, if not as actual members of it, we have 
to place a group of animals which were for some time asso- 
ciated with the Mollusca; their bodies being inclosed: in 
shells, which do not fit closely around them, nor give more 
than a general protection to their members. This group is 
