CH. l] EARTHWORMS AND LEECHES 29 



few and weak denticules. It subsists on worms and other 

 soft-bodied aquatic animals. 



The blood plasma is red in Hirudo and Aulostoma and 

 Nephelis y colourless in Glepsine and its relatives. It contains 

 numerous colourless amoeboid corpuscles. There is a 

 pair of large lateral longitudinal blood vessels in the 

 former group, and in the latter a dorsal and a ventral 

 vessel in addition. These all have muscular walls and are 

 contractile. Besides these well-defined blood vessels there 

 are also numerous irregular plexuses, capillary sinuses, 

 traversing the tissues. 



Respiration is effected not by any specialised organs 

 but through the general surface of the skin, which is richly 

 provided with capillary plexuses, whose branches extend 

 between the inner ends of the cells of the epidermis. 

 The waving motion that leeches frequently keep up when 

 at rest is probably for the purpose of moving the sur- 

 rounding water and so promoting respiration. 



Nervous system and special senses. The nervous system 

 consists of a pair of supra-cesophageal ganglia united to a 

 paired ventral chain with ganglia in each segment. At 

 the anterior and posterior ends concentrations of ganglia 

 have occurred. From this central nervous system nerves 

 pass to all parts of the body. There are scattered over the 

 surface of the body, especially in its anterior region, small 

 cup -shaped bodies to which nerves are supplied and which 

 probably serve as organs of 'touch and perhaps also of smell 

 a sense which leeches undoubtedly possess. Round the 

 edge of the anterior sucker there are in the Medicinal 

 Leech ten minute black dots which are modifications of the 



