8 



LEECHES 



of the body. Every annulus has receptor organs which in a con- 

 tracted specimen may be seen to be raised on papillae. These are 

 the annular receptors which are thought to be tactile organs. On 

 every fifth annulus, the middle annulus of its segment, there are 

 also white circular areas which in a preserved specimen are not 

 raised on papillae. These contain cells which are thought to be 

 light-sensitive. They are a convenient outward indication of 

 internal segmentation, and are shown diagrammatically in Fig. 1. 

 They are known as the segmental receptors, or sensillae. Finally, 

 on segments I-V, there are five pairs of eyes which correspond in 

 position with segmental receptors. They have larger light- 

 sensitive cells, and are backed by a pigmented cup. They there- 

 fore appear as black dots on the head, but it may be necessary to 

 bleach the head in 5% caustic potash before they become visible. 



3. Alimentary Canal 



The cavity containing the jaws, the buccal cavity, is separated 

 from the cavity of the sucker by a low fold, the velum. When 



Sub-oesophageal 

 ganglion 



Salivary glands 



Velum 



Radial muscles 



Jaw 



Cut circum- 



')S,^f^ oesophageal 



connective 



Longitudinal 

 muscles 



Lining of pharynx 



5 mm 



Fig. 2. Ventral dissection of the head of Hirudo medicinalis 

 showing jaws protruded in biting position. 



