xviii INTRODUCTION. 



a feature that, with the exception of the Isopoda, is 

 common to most Crustacea, even including the aberrant 

 Isopods. We have thought it convenient to describe 

 them under a name distinguishing them from the true 

 ambulatory legs, although by doing so we must include 

 some genera of Isopoda, where they assimilate to and 

 fulfil the conditions of true walking-legs. In the 

 Brachyura, the gnathopoda are developed, so as to 

 serve chiefly as protecting the oral apparatus. In the 

 Macrura, they assume a pediform appearance, and are 

 used in seizing and holding food. In the Stomapodctj 

 the Squillidce have them developed into formidable pre- 

 hensile organs. This change takes place gradually from 

 the highest Crustaceans to the Amphipoda. The cha- 

 racter is still increased in some of the aberrant genera, 

 until it becomes a perfectly didactyle chela. In the 

 Isopoda, the prehensile character may be said to be 

 lost, presenting itself only occasionally in the anterior 

 pair, in the male animals. 



The five remaining pairs of walking-legs (the pere- 

 iopoda) homologize with the five pairs of legs in the 

 Stalk- eyed Crustacea, that give the name of Decapoda 

 to the order. These are produced on a somewhat 

 different plan from the walking-legs of the Stalk-eyed 

 Crustacea, the modification, as it appears to us, taking 

 place in accordance with certain necessities that have 

 arisen from the depreciation of their general develop- 

 ment. The two anterior pairs of legs, or gnathopoda, are 

 developed upon one type; the two succeeding pairs, or 

 first and second pairs of pereiopoda, on a second ; and 

 the last three on a third. 



The normally developed appendage of every kind in 

 Crustacea consists of seven joints. In the Brachyura, 

 the first, or coxa, is anchylosed with, and forms part of, 



