XXH 



INTRODUCTION. 



of three distinct parts. The Stalk, which constitutes the 

 essential part, and which is usually multiarticulate ; the 

 Palp, which is an appendage to the stalk, and ordinarily 

 arises from its basal segment; and the Lash. It is not in 

 all cases that these three portions exist, and in the Bra- 

 chyura, for example, the foot-jaws are the only ones in 

 which they are all present. The ambulatory thoracic legs 

 in these are obviously composed only of the stalk, without 

 either of the other members, and consist of six distinct 

 joints. In the Macroura, however, the ambulatory feet, in 

 some genera, have all the three elements ; in others, one 

 of them is wanting. Their modifications are almost 

 innumerable, and often it would be impossible to distin- 

 guish their homologues, without extensive comparative 

 examination. 



It is impossible, in a mere sketch, introductory to a 

 local Fauna, to enter, at any detail, into the various modi- 

 fications now merely alluded to, but perhaps there is 

 scarcely any group of animals in which the homologies 

 are more recondite, the variations more interesting, and 

 the relations between those variations and the habits 

 and requirements of the animals more beautiful and 

 instructive. 



In order to give a general idea of the extent of these 

 modifications, it may be stated that the ocular peduncles 

 are the only appendages which are never devoted to 

 any but their normal objects. The antenna are, as has 

 been before observed, sometimes modified into locomotive 

 organs. The cephalic appendages about the mouth, the 

 mandibles and maxillae, are sometimes rudimentary, at 

 other times they are modified into mere organs of appre- 

 hension. The thoracic members are sometimes locomo- 



