MEDICINAL LEECH. 217 



wards to a point on a level with the commencement of the rectum, and 

 nearly as far as the end of the body. The form of the caeca, it should be 

 noted, depends very much on their state of distension. This region of the 

 digestive tract serves, first, as a crop or reservoir for the blood which forms 

 the sole food of the animal, and, secondly, as a place where slow changes 

 go on in the various constituents of that fluid. The oxy-haemoglobin is 

 extracted by degrees from the corpuscles : it is reduced to haemoglobin 

 and crystallised, and changes gradually take place in the corpuscles. 



Between the two last caeca lies the stomach or * gastro-ileal' (Gratiolet) 

 section of the alimentary canal. A black bristle has been passed into it. 

 It communicates by a narrow aperture with the crop, and at its commence- 

 ment there is a pair of small caeca, one on the right, the other on the left, 

 directed forwards. These caeca and the stomach are much larger in the 

 Horse-leech (Aulostoma gulo). The stomach is very vascular, and it has 

 a villous interior with a spirally arranged valve. The contents of the crop 

 enter into it very slowly, and the red colour of the blood then changes from 

 dark-red to green. The posterior end, colon or intestine, of this section of 

 the alimentary canal is little vascular and contains no valve. It is followed 

 by a short rectum of small calibre which terminates in a dorsally-placed 

 anus, as in all Leeches except Acanthobdella where it opens in the centfe of 

 the posterior sucker. From a developmental point of view the pharynx 

 and rectum must be regarded as invaginations from the exterior, i. e. as 

 stomodaeum and proctodaeum, while the rest of the canal is archenteron, 

 i.e. lined by endoderm or hypoblast. 



The ventral nerve chain may be seen in part through the walls of the 

 stomach. The spot where the walls of the caeca meet centrally (i. e. in the 

 median tube) corresponds very nearly in most instances with a ganglion. 



On either side of the crop and adherent to the inner surface of the 

 reflected body-walls may be seen remains of the botryoidal tissue. It is 

 arranged chiefly in four bands, two dorsal, two ventral, close to the walls of 

 the digestive tract but separated from it by a layer of vaso-fibrous tissue 

 and capillaries. 



A section taken through the pharyngeal region shows, according to Gibbs 

 Bourne, the following structures passing from within outwards: (i) Pharyngeal 

 epithelium composed of minute cells; (2) three ridges, one dorsal, two lateral, 

 composed of salivary ducts and radiating muscles, the branched ends of which abut 

 upon the hypodermis ; (3) a circular layer of muscles very dense and compact ; (4) 

 the longitudinal muscle layer of the body wall, in which occur blood sinuses, vessels, 

 and salivary glands ; (5) the diagonal and circular muscle layers of the body ; and 

 (6) the hypodermis and cuticle. To the three pharyngeal ridges correspond the 

 three muscular jaws. They bear at their edges in the medicinal Leech about 80- 

 90 fine chitinoid teeth. These teeth contain lime carbonate both in Hirudo and 



