DIGESTIVE SYSTEM 41 



alimentary canal, which Ues close beneath the skin. Carefully 

 dissect off the integument from the alimentary canal along the 

 whole length of the amimal, and pin out the flaps right and 

 left. Wash thoroughly under the tap. Note at once the 

 dorsal blood-sinus, which rims along the dorsal wall of the 

 alimentary canal. 



A. The Digestive System. 



The alimentary canal of the leech runs straight from 

 mouth to anus. Along the greater part of its length it is a 

 "wide tube, whose capacity is much increased by paired lateral 

 diverticula, and is further capable of great distension. 



A leech can draw as much as three times its own weight 

 of blood, a great part of which is often spontaneously dis- 

 charged. The digestion of a full meal may take as long as 

 nine months in an adult animal. 



Dissect clean the dorsal wall of the alimentary canal 

 ■along its whole length, taking care not to damage the supra- 

 oesophageal ganglia. If the canal is empty its walls are white 

 and readily distinguished ; if full, it appears red from the 

 contained blood, and is less easy to dissect. Wash freguentVy 

 "under the tap during the dissection. 



1. The mouth is a conical depression in the anterior sucker, 

 at the bottom of which are the jaws. These are three 

 laterally compressed muscular cushions arranged in 



Fig. 20. — Hirudo medicinalis, seen from the ventral surface. (After A. 

 G. Bourne.) 



The numbers 1 to 23 indicate tlie somites ; the numbers I. to V. the fire 

 annul! of the twelfth somite. 



A, anterior sucker, with the mouth. B, posterior sucker. G? male aper- 

 ture. H, female aperture. K", laephridlal apertures. 



I'm. 21. — Hirudo medicinalis. A diagrammatic figure of the renal, ner- 

 vous, and reproductive systems. The animal has been opened along 

 the mid-dorsal line, pinned out, and the alimentary canal removed. 

 (After A. G. Bourne.) 



The somites are numbered 1 to 23, and their boundaries are indicated by 

 the dotted lines. 



CG, supra-oesophageal ganglion. EP, left epididymis. Or 1, the first 

 of the twenty-three post-oral ganglia. GL, glandular enlargement of the 

 oviducts. Ij, lateral vessel. LA, latero-ventral branch of lateral vessel. LD, 

 latero-dorsal branch. LL, latero-lateral branch. U" 3, the first of the seven- 

 teen nephridia of the left side. O, nerve-collar. OV, ovisac containing 

 the left ovary. PE, penis. T 4, the third testis of the left side. VD, vas 

 deferens. 



