vii PRACTICAL DIRECTIONS 387 



A. External Characters. 



1. Note again the cephalothorax and abdomen, and that 

 the abdomen consists of seven movable segments or meta- 

 meres. Examine the ventral side of the cephalothorax, 

 and note that it also is composed of a number of segments 

 all fused except the ventral part of the last. 



2. Examine the third or fourth abdominal segment 

 closely, and note that it is connected with the segments in 

 front and behind by a sort of peg-and-socket joint on either 

 side, and that the chitinous exoskeleton at the joints is soft 

 and pliable, forming an articular membrane, while elsewhere 

 it is calcined. Distinguish between the dorsal convex 

 tergum, the ventral sternum, and the pleuron projecting 

 downwards on either side from the tergum. 



3. Examine the appendages of the same segment : they 

 are attached to the sterna, near the pleura, by articular 

 membranes. Each consists of a basal or proximal portion 

 the protopodite, to which two distal, many -jointed parts 

 are attached aji inner endopodite, and an outer exopodite. 

 The cuticle covering the segments of the limb, or podomeres, 

 is more or less calcined, and the distal segments are covered 

 with feathery setcz. 



4. The second to the sixth abdominal segments are 

 essentially similar to one another except as regards the 

 appendages of the second in the male and of the sixth in 

 both sexes (p. 364). The first abdominal segment is smaller 

 than the others, and its pleura are reduced. The sixth 

 abdominal appendages are very large, and, together with 

 the anal segment or telson, form the tail-fin. 



5. The cephalothorax is reckoned as consisting of a 

 prostomium and 12 metameres, which are completely fused 

 dorsally and laterally, forming a large calcified shield the 

 carapace. Thus the entire number of segments is 20 (pros- 

 tomium -f 1 8 metameres + telson). The sternal region of 

 the head is bent upwards. 



6. Note : a, the transverse cervical groove on the dorsal 

 surface of the carapace, extending forwards laterally and 

 forming the boundary between the head (prostomium -j- 4 

 metameres) and the thorax (8 metameres) ; b, the two 

 longitudinal branchio-pericardial grooves on the tergal 

 region of the thoracic portion of the carapace, about ^ inch 

 apart : the part of the exoskeleton between these covers 

 the heart, and the part below each groove forms a large plate, 

 the gill-cover, at the side of the thorax ; c, the rostrum, 

 movable eye-stalks, and epistoma (p. 363). 



