Xll INTRODUCTION. 
the fifth; and in every case the secondary appendage, 
whether in the form of a scale attached to the lower, or a 
filamentary appendage, or several, invariably in upper and 
lower alike arises from the distal extremity of the third. 
This appears to be a very constant condition with 
all the appendages of the cephalon, pereion, and pleon. 
The most frequent exception exists in the first joint 
or coxa, as exemplified in the branchial appendages and 
the ovigerous plates of the female Amphipoda and Iso- 
poda. According to our experience, whenever any 
secondary appendage is developed from the second 
joint or basis, it exists more as a rudimentary effort than 
as atrue organ. After the third joint, we are not aware 
that any secondary appendage is ever produced, though 
in some genera, as in Palemon, the primary flagellum of 
the anterior antennz occasionally divides or sends off a 
smaller one. 
The flagellum in the Sessile-eyed Crustacea is generally 
multi-articulate. It attains its most filamentary character 
in the sub-family Gammarides ; but in some genera many, 
and sometimes all, the numerous articuli coalesce into one 
or more joints, as in Podocerus, Corophium, Chelura, the 
terrestrial Isopods, &c., in all which cases they become 
organs assisting in climbing and grasping. Unlike the 
superior antennz, the inferior pair appears to be always 
present, and we only know of their being reduced to an 
immature condition in those Crustacea that pass their 
lives as parasites upon others, as the Bopyride, Hyperiide, 
and Cyamus, a circumstance that induces us to believe 
that the second pair of antenne is the seat of a sense 
which undergoes but slight modifications to enable it to 
be equally distinguishable whether in air or water, since 
the Isopoda and Orchestia, in which the antennz are well 
developed, are terrestrial. 
The oral apparatus in most Crustacea is a somewhat 
