INTRODUCTION. Xxiii 



tive organs, at others they subserve the nutritive function : 

 the remaining thoracic members are, in some cases, pre- 

 hensile, in others ambulatory, in others natatory, in others 

 partially branchiophorous, and so on. The abdominal 

 sometimes serve the purpose of swimming, at others of 

 bearing and protecting the eggs, at others they are 

 partially converted into branchisB. Besides these modi- 

 fications, some or other of them are, in many forms, 

 either wholly wanting or rudimentary. 



The digestive system appears under very various phases 

 in the different groups of the Crustacea. The extremes 

 of this diversity are found in those two primary divisions, 

 the food of which is most opposite in its kind. In the 

 one group, the whole of which are parasitic upon other 

 animals, and which I have in this Introduction considei'ed 

 as belonging to a distinct class, the aliment consists of 

 the juices of the creatures to which they are attached, and 

 is obtained by suction. In these the normal elements of 

 the organs for procuring or preparing the food for diges- 

 tion are either rudimentary or wanting. In the higher 

 forms of the true Crustacea, on the contrary, which sub- 

 sist upon solid and often hard substances, and in many 

 cases on living prey, the organs for pursuing, seizing, 

 tearing, and comminuting the food, are carried to a high 

 degree of development, and a corresponding difference is 

 also found in the digestive organs themselves. The most 

 elaborate condition of these organs is exhibited in the 

 Decapoda, and especially in the Brachyura. It has been 

 already stated that the appendages belonging to certain 

 of the cephalo- thoracic segments are variously modified to 

 serve their several offices; and in the latter order they have 



