Xvill 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 Stomapoda, 
the Squillide 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, 
