22 



C. Dissection. 



The leech having heen killed with chloroform, should be fixed under water, and 

 an incision carried along the dorsal surface from the third to the second last ring. 

 The skin should then he carefully reflected and fixed with pins at each side of the 

 middle line, in order to expose the alimentary canal and its appendages. In doing 

 this, note the dorsal and the lateral blood vessels and the absence of a distinct body 

 cavity. 



1. Examine and draw the digestive tract, indicating — 



(1) The thick-waUed tubular pharynx connected by muscular fibres to 



the body wall. 



(2) The thin-waUed stemach with 1 1 pairs of saccular diverticula (which 



correspond to the ganglia of the nerve-cord from the 4th to the 

 14th inclusive), and that the 11th pair of diverticula send long 

 projections backwards towards the end of the body. 



(3) The small tubular intestine running along between the projection of 



the 11th pair of diverticula to dilate slightly before opening to the 

 exterior between the last ring and the upper margin of the posterior 

 sucker. 



2. Indicate in the same sketch, the position of the dorsal and lateral blood 



vessels — the remains of the reduced body cavity. 



3. Examine (high power) the blood (which is coloured with haemoglobin) for the 



colourless corpuscles. 



4. Make an incision along the pharynx, stomach, and intestine, and note — 



(1) The position of the circular muscles which on contraction close both 



ends of the fusiform cavity, and the muscles which radiate to the 

 skin, and serve for producing the sucking action, and also the longi- 

 tudinal folds containing muscular fibres which serve for driving the 

 blood backwards into the gastric diverticula. 



(2) That the stomach is constricted by incomplete septa, which^ extend 



inwards between the diverticula, and that communicating by a small 

 aperture with the pharynx in front, it projects slightly into the in- 

 testine behind. 



(3) That the gastric diverticula consist of a small anterior and of a larger 



posterior chamber, the posterior chamber of the 11th extending, as 

 already seen, nearly to the end of the body. 



