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REVIEW, &c. 
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TE 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. 'lhis 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 Radiata; 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 4 
number of pairs of legs may vary from fifty to one, or none. . 
The antennze 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 branehial and the 
internal systems of structure, the variety is equally remarkable ; 
for there may be large branchis, 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 1t may be far the larger 
part of the body, and furnished with several pairs of large 
natatory appendages;—the inner antenn: 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 branchis, 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. 'lhe 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, itis true, is hke that in other de- 
partments; but itis 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. 'lhe variations are manifestly varia- 
tions in grade, and we may readily trace out the several steps 
SECOND SERIES, VOL. XXII, NO. 64.—JULY, 1856, 1 
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