DIGESTIVE SYSTEM 41 



a triradiate manner, one being dorsal and anterior, 

 and the other two ventro-lateral : each cushion is 

 covered by a thin chitinous cuticle, which is thick- 

 ened along the free edge of the jaw, and notched 

 into the sharp teeth by which the bite of the leech 

 is produced. The mouth leads, by a very small 

 aperture, into the pharynx. 



Snip aivay the margin of the anterior sucker so as to 

 expose the jaws. Examine them from the ventral surface 

 with a pocket lens : remove one of them and examine it with 

 a low power of the microscope. 



2. The pharynx is an oval sac with very muscular walls. 

 It is connected with the body-wall by strong radial 

 muscles, which give it a villous appearance, and by 

 their contraction dilate its cavity and effect a sucking 

 action. 



8. The salivary glands are very large granular pyriform 

 cells surrounding the pharynx. Each cell is a 

 gland in itself, and is produced into a long stalk 

 or ductule opening on the side of one of the jaws. 

 The secretion has the power of preventing coagula- 

 tion of the blood, and so facilitates very greatly the 

 act of suction. 



Remove part of the wall of the pharynx, and tease it on a 

 slide in salt solution. The large pyriform gland-cells, with 

 their long ductules, will be readily seen with a low power. 



4. The oesophagus is a short narrow tube leading from the 



pharynx to the crop. 



5. The crop is by far the largest part of the alimentary 



tract. It is a straight thin-walled tube lying in the 

 somites from the fourth to the fourteenth, and giving 

 off eleven pairs of lateral diverticula corresponding 

 to these somites. 



Of these diverticula the anterior two or three 

 pairs are small ; the remainder increase gradually in 

 size from before backwards ; and the hindmost pair 



