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 away 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 examijie 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. 



3. 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 he readily seen with a low power. 



i. 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 



