126 



THE CRAYFISH 



a. The tergnm is the convex dorsal plate of the 



exoskeleton. 



b. The sternum is the ventral transverse bar between 



the bases of the limbs. 



c. The pleura are a pair of plates projecting down at 



the sides. 



d. The epimera are the portions of hardened cuticle 



between the pleura and the bases of the limbs. 

 They are very small in this segment. 



Fig. 25. — Astaeus flnviatilis, var. nobilis. Transverse section 

 through the abdomen, (o. H. h.) 



DA, dorsaj abdominal artery. "EUL, extensor muscles of the abdomen. 

 EP, eplmeron. FM, flexor muscles of tlie abdomen, M, muscles of tbe 

 appendage. U, endopodlte. M"G, nerve-ganglion. P, protopodite : the 

 proximal joint, which is very short, has been almost entirely cut away ; its 

 length is indicated by the notch on the outer side of the protopodite. 

 PL, pleuron. PK, proctodoeum. S, sternum. T, tergum. V, ventral 

 abdominal artery. X, exopodite. 



e. Soft uncalcified portions connect the segment with 



those in front of and behind it, and allow move- 

 ment of one segment upon another. 



f. The joints between the successive segments are 



provided vsdth a rounded peg or pivot at each side 

 on the anterior margin of the tergum, near its 



