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 

 chiefly due to a variety in the amount 

 of growth 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 



