ISOPODA. 99 
Order—ISOPODA. 
Turis order of Crustacea was first defined by Latreille, 
and was named Isopoda, in contradistinction to the Am- 
phipoda, from the Greek words toc, equal, and wédee, 
feet, in allusion to the general conformity in size and 
function of the seven pairs of legs, the two anterior 
pairs of which, as in the Amphipods, are the homologues 
of the two outer pairs of foot-jaws of the Decapod Crus- 
taceans. If we were to adopt the ideas of some writers, 
that large size constituted the especial character of a 
typical group, we should be compelled to regard the last- 
named animals as the types of the class Crustacea, and 
should be, consequently, obliged to consider the Isopods 
as further removed from the type than the Amphipods, 
in consequence of the former being destitute of branchial 
appendages, in the shape of vesicles, affixed to the base 
of the legs (a character which constitutes a marked dis- 
tinction between the two orders which form the subjects 
of this work), and by the possession of which the Am- 
phipoda are more nearly approximated to the Decapoda. 
If, on the other hand, the type of the articulated class of 
animals is to be sought for in such members as present a 
more regularly articulated body, each articulation being 
furnished with a pair of equal-sized articulated limbs, 
we must consider the animals of the present order as 
more typical of the Crustacean class than the Amphipoda. 
In immediate connection with the character derived from 
the want of branchial appendages attached to the base 
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