x INTRODUCTION. 



Orchestia and Talitrus, and their rudimentary condition in 

 the terrestrial Isopoda. 



The inferior or second pair of antennae are formed on 

 the simplest character of the Mac-rural 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 antennae 

 in the Amphipoda (Fig. 1), homologizes with a perforated 



FIG. 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 



