x INTRODUCTION. 
Orchestia and Talitrus, and their rudimentary condition in 
the terrestrial Jsopoda. 
The inferior or second pair of antenne are formed on 
the simplest character of the Macrural type, and consist 
of a peduncle with five joints, of which the first two, (the 
homotypes of the coxa and basis joints of the true 
leg,) are very closely associated, and carry the olfactory 
denticle. In the higher groups, the two basal joints 
are fused together, and often with the nearest part of the 
segment to which they belong. Sometimes, so perfect is 
the union, that not the slightest trace of the relation of 
one part to another is capable of being detected. This 
complete association of the appendage with the body of 
the animal lessens with the degradation of the creature, 
until we find the five separate joints distinguishable from 
each other and from the body of the animal. 
The denticle at the base of the second pair of antenne 
in the Amphipoda (Fig. 1), homologizes with a perforated 

Fic. 1, 
tubercle situated on the ventral surface of the cephalon 
in the Brachyura, laterally anterior to the oral apparatus, 
and indeed covered by some of the appendages, in the 
higher groups of the class. The denticle in the Amphipoda, 
upon close examination, appears to have an open extremity, 
through which a cylindrical tube, retained in its place by 
membranous ligatures, protrudes. This tube closes at the 
