THE CRAYFISH AND ARTHROPODS IN GENERAL 195 



STRUCTURE OF A SEGMENT. A typical segment is shown in 

 cross section in Figure 100. It consists of the following parts: (i) 

 a convex dorsal plate, the tergum (T), (2) a ventral transverse bar, 

 the sternum (S), (3) plates projecting down at the sides, the 

 pleura (PL), and (4) smaller plates between the pleura and the 

 basis of the limb, the epimera (EP). 



CEPHALOTHORAX. The cephalothorax consists of segments 

 I-XIII, which are inclosed dorsally and laterally by a cuticular 

 shield, the carapace. An indentation, termed the cervical groove, 

 runs across the mid-dorsal region of the carapace, and obliquely 

 forward on either side, separating the cephalic or head region 

 from the posterior thoracic portion. The Anterior pointed exten- 

 sion of the carapace is known as the rostrum. Beneath this on 

 either side is an eye at the end of a movable peduncle. The mouth 

 is situated on the ventral surface near the posterior end of the 

 head region. It is partly obscured by the neighboring append- 

 ages. The carapace of the thorax is separated by branchio- 

 cardiac grooves into three parts, a median dorsal longitudinal 

 strip, the areola, and two large convex flaps, one on either side, 

 the branchiostegites, which protect the gills beneath them. 



ABDOMEN. In the abdomen there are six segments, and a ter- 

 minal extension, the telson, bearing on its ventral surface the longi- 

 tudinal anal opening. Whether or not the telson is a true seg- 

 ment is still in dispute; we shall adopt the view that it is not. 

 The first abdominal segment (XIV) is smaller than the others and 

 lacks the pleurae. Segments XV-XIX are like the type described 

 above. 



APPENDAGES. With the possible exception of the first ab- 

 dominal segment in the female, every segment of the body bears 

 a pair of jointed appendages. These are all variations of a com- 

 mon type (Fig. 100), consisting of a basal segment, the protopo- 

 dite (P), which bears two branches, an inner endopodite (N), 

 and an outer exopodite (X). Beginning at the anterior end, the 

 appendages are arranged as follows (Fig. 101). In front of the 

 mouth are (I) the antennules, and (II) the antenna; the mouth 



