372 THE CRAYFISH chap. 



PRACTICAL DIRECTIONS. 



CRAYFISH 



In a living specimen nole the cephalcthorax, abdomen, jointed 

 appendages, and exoskeleton, as well as the mode of walking and 

 swimming. Holding the animal out of water between the thumb and 

 finger, observe that bubbles are continually being formed on either side 

 of the lower part of the head (respiratory movements, see § B, l). Kill 

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

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



A. External Characters. 



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

 consists of seven movable segments ox metameres. Examine the ventral 

 side of the cephalothorax, and note that it also is composed of a number of 

 segments all fused together, 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 calcified. Distinguish between the dorsal convex 

 tergum, the ventral sternum, and \he pleuron projecting downwards on 

 either side from the tergum. - _ " ,'' 



3. Examine the appendages of the same segment : th'ey 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 endopodite, and an outer exopodite. 

 The cuticle covering the segments of the limb, or podomeres, 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 (see p. 350). 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. prostomitim and 

 12 metameres, which are completely fused together dorsally and laterally, 

 forming a large calcified shield — the carapace. Thus the entire number 

 of segments is 20 (prostomium -I- 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 



