xii 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 freqnent 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 a true organ. After the third joint, we are not aware 

 that any secondary appendage is ever produced, though 

 in some genera, as in Palamon, the primary flagellum of 

 the anterior antennae 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 antennae, 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 Bopyrida, Hyperiida, 

 and Cyamus, a circumstance that induces us to believe 

 that the second pair of antennae 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 antennae are well 

 developed, are terrestrial. 



The oral apparatus in most Crustacea is a somewhat 



