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 Isopoda, 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 Isopoda, 

 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 



