XX INTRODUCTION. 
generally fused closely with the dorsal surface of the seg- 
ment; but in the genus Apseudes, as we have shown in 
fig. p, page 148, vol. ii., they are free. Here we have a 
distinct exposition of the relation which the squamiform 
side-piece holds both to the segment and the movable 
bifurcate appendage. The segment is distinctly separated 
from the squamiform side-piece, which, articulating with 
it, forms the first joint of the pleopoda or swimming-leg, 
and is developed into a large scale-like process, to the 
base of which the second joint is articulated, from whence 
is suspended freely a third, which in its turn supports the 
two free plates which form the terminal appendage of the 
anterior pleopoda. In the Isopoda, as well as the Amphi-~ 
poda, this interpretation illustrates the relation of the 
parts of the pleopoda to the segments of the pleon. The 
forms of the pleopoda may and do change, according to 
the law of modification of parts, to suit their require- 
ments; but under whatever condition they may exist, they 
consist of three normal joints, more or less fused together, 
and with the segments of the pleon and a depreciation of 
the four terminal joints into one or a pair of movable 
plates, as in the /sopoda, or articulated flagella, as in the 
Amphipoda. 
The three anterior pairs in the Amphipoda are deve- 
loped upon this type; the two succeeding have the double 
appendages stiff and unyielding, and the posterior is 
generally variable in the different genera. In the Jsopoda, 
the four anterior pleopoda are developed upon one type, 
while the fifth is converted into an operculum. Some 
variation of the anterior pairs also takes place in relation to 
the sex of the animal. The last, or twenty-first segment, 
differs from the rest in most Crustacea by not carrying 
any appendage. To this we know of but one exception 
among the Crustacea, and that is in a genus in the family 
