A REVIEW 



OF THE CLASSIFICATION OF CRUSTACEA, WITH REFERENCE TO 

 CERTAIN PRINCIPLES OF CLASSIFICATION. 



The class Crustacea exhibits a clearness of outline in its types, and 

 a display of relations, transitions, and distinctions, among its several 

 groups, exceeding any other department of the animal kingdom. 

 This fact arises from the very great range in structure occupied by 

 the species. The limits in size exceed those of any other class, exclu- 

 sive of the Kadiata; the length varying from nearly two feet to a small 

 fraction of a line, the largest exceeding the smallest lineally more 

 than a thousand-fold. In the structure of the limbs, the diversity is 

 most surprising, for even the jaws of one division may be the only 

 legs of another; the number of pairs of legs may vary from fifty to 

 one, or none. The antennae may be either simple organs of sense or 

 organs of locomotion and prehension ; and the joints of the body may 

 be widely various in number and form. In- the branchial and the 

 internal systems of structure, the variety is equally remarkable ; for 

 there may be large branchiae, or none ; a heart, or none ; a system of 

 distinct arterial vessels, or none ; a pair of large liver glands, or but 

 rudiments of them; a series of ganglions in the nervous cord, or but 

 one ganglion for the whole body. 



Taking even a single natural group, the Decapods ; — the abdomen 

 may be very small, without appendages, and flexed beneath the broad 

 cephalothorax out of view, or it may be far the larger part of the 

 body, and furnished with several pairs of large natatory appendages ; 



