14 J. D. Dana on the Classification of Crustacea. 



Art. IV. — A Review of the Classification of Crustacea with refer' 

 ence to certain principles of Classification ; by James D. Dana.* 



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, exclusive of the Eadiata ; the length varying from 

 nearly two feet to a small fraction of a line, the largest exceed- 

 ing 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 antenna? 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 abdo- 

 men 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 ; — the inner antennas may be very small, 

 and retractile into fissures fitted to receive them, or they may be 

 very long organs, constantly thrown forward of the head ; and 

 descending but a single step, we come to species of Decapoda 

 without proper branchiae, some having the abdominal legs fur- 

 nished with branchial appendages, and others with no abdominal 

 members at all. 



When we consider, that these diversities occur in a class that 

 may not embrace in all over ten thousand species (not half of 

 which are now known), we then comprehend the wide diversity 

 in the distinctions that exist. The series of species followed 

 through, gives us an enlarged view of those distinctive charac- 

 teristics upon which the limits and relations of groups depend. 

 The network of affiliations, it is true, is like that in other de- 

 partments ; but it is more magnified to the view. 



Moreover, the distinctions are obviously distinctions of rank. 

 There is no ambiguity as to which is the higher or superior 

 group, as among Insects. The variations are manifestly varia- 

 tions in grade, and we may readily trace out the several steps 



* From the author's Expl. Exped. Report on Crustacea, Vol. II, pp. 1395 — 1437. 



