THE MEDICINAL LEECH 21 



probably in function with the chloragogen cells of oligochaetes. 

 The vaso-fibrous tissue consists of strands running in the con- 

 nective tissue which contain deposits of brown pigment. They 

 have a small lumen which was shown by Lankester (1880) to be 

 continuous with that of the botryoidal tissue. It is thought that 

 the vaso-fibrous tissue accumulates excretory products and is in 

 some way complementary to the botryoidal tissue (Bradbury, 1959). 



The Gut 



The buccal cavity and pharynx of Hirudo are of ectodermal 

 origin and are lined by cuticle. The pharynx has in its walls three 

 muscular ridges which are enlarged anteriorly to form the jaws. 

 In section each jaw is seen to have a core of closely packed salivary 

 gland ducts, then a layer of muscle cells, and finally an epidermis 

 surmounted by cuticle. On the cutting edge of the jaws the cuticle 

 is thickened to form the teeth and the openings of the salivary 

 ducts are between the teeth (Fig. 11). 



The endodermal gut comprises the oesophagus, crop, intestine 

 and rectal bladder. The wall of the crop is composed of prismatic 

 cells having a striated border, and their cytoplasm contains 

 numerous fat droplets. There is a crop musculature composed of 

 interlacing muscle cells lying in connective tissue just outside the 

 epithelium. The wall of the intestine is also a single layer of 

 prismatic cells, but these are taller than those of the crop and lack 

 musculature. The epithelium of the rectal bladder is ciliated. 

 Only the tube from the rectal bladder to the anus is formed from 

 proctodaem and has a cuticular lining. 



Nervous system 



The fine structure of the nervous system is discussed in 

 Chapter 7. A transverse section of a leech usually passes through 

 the ventral nerve cord and shows two bundles of nerve fibres 

 together with a small median unpaired nerve known as Faivre's 

 nerve. 



