23 



The First Antenna or Antennule (PI. II, fig. 4, PI. 

 Ill, fig. 20) is situated in a deep depression on the ventral 

 side of the cephalic sternum (s.a. 1 ). This depression or 

 socket is bounded in front by the rostrum (rost.}, and 

 behind by the lateral expansion of the first sternum (S l ). 

 The outer boundary is formed by the inner edge of the 

 second antenna (ant.}, and the inner boundary by the 

 median portion of the first sternum. The appendage 

 consists of a broad basal joint, from which is given off on 

 its inner side a two-jointed portion. These three pieces 

 together form the protopodite (2>rot.}. From the end of 

 the distal segment of the protopodite arise two many- 

 jointed flagella an inner endopodite (end.} and an outer 

 exopodite (ex.}. The exopodite is the larger of the two, 

 and bears on its inner side a tuft of long setae. The 

 " olfactory " setae are small setae on the ventral side of 

 the exopodite (see section on Sense Organs). 



On the dorsal side of the basal segment of the 

 protopodite is a longitudinal groove covered with long 

 setae. This groove marks the place where the auditory 

 sac opens to the exterior in the young animal. In the 

 adult crab this groove is completely closed, although it 

 remains open a short time after ecdysis. 



In their natural position the three parts of the 

 protopodite are folded on one another. The .second 

 segment is closely a'pplied to the inner side of the basal 

 segment. The third segment is bent back along the 

 dorsal side of the second, and its distal end lies in an 

 excavation made for its reception in the dorsal wall of the 

 basal segment. 



Second Antenna (PI. II, fig. 5, PI. Ill, fig. 20). This 

 consists of a large basal portion (prot.} which is fused to 

 the carapace, and a distal nagellurn, which consists of 

 two long basal segments and a number of short rings, 



