ISOPODA. 99 



Order— ISOPODA. 



This order of Crustacea was first defined by Latreille, 

 and was named Isopoda, in contradistinction to the Am- 

 phipoda, from the Greek words 'iaog, equal, and iroSig, 

 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, aflixed 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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