52 THE LEECH. 



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

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

 blood-sinus, which runs 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. 



Clean the dorsal wall of the alimentary * canal along its 

 whole length, talcing 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 frequently 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 

 radially, one being dorsal and anterior, and the other 



Fig. 22. Hirudo medicinalis, seen from the ventral surface. (After 

 A. G. Bourne.) 



The numbers 1 to 23 indicate the somites ; the numbers I to V the five 

 annuli of the twelfth somite. 



A, anterior sucker, with the mouth. B, posterior sucker. G, male aperture. 

 H, female aperture. N, nephridial apertures. 



Fig. 23. 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-cesophageal ganglion. EP, left epididymis. G 1, the first of 

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

 L, lateral vessel. LA, latero-ventral branch of lateral vessel. |_D, latero- 

 dorsal branch. |_L, latero-lateral branch. N 1, the first of the seventeen 

 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. 



