vii PRACTICAL DIRECTIONS 3^7 



with chloroform (p. 31), and at the end of the day's work, preserve your 

 dissection in 70 per cent, spirit or 3 per cent, formaline. 



A. External characters. 



1. Note again the cephalothorax and abdomen, and that the abdomen 

 consists of seven movable segments or metameres. 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 an inner endopoditc, and an outer exopodite. 

 The cuticle covering the segments of the limb, or podonieres, is more or 

 less calcified, and the distal segments are covered with feathery seta. 



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-Jin. 



5. The cephalothorax is reckoned as consisting of a prostomium and 

 12 metameres, which are completely fused dorsally and laterally, form- 

 ing a large calcified shield the carapace. Thus the entire number 

 of segments is 20 (prostomium + 18 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 + 4 metameres) and the thorax (8 metameres); 

 by 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). 



c c 2 



