30 EARTHWORMS AND LEECHES [CH. I 



above-mentioned tactile bodies and are sensitive to light. 

 There is no evidence to show that there is any sense of 

 hearing. 



Excretion. Excretion is performed by means of ne- 

 phridial tubes of which there are, in the Medicinal Leech, 

 seventeen pairs, lying in the second to the eighteenth seg- 

 ments inclusive. The inner end of the tube is a spongy 

 mass of ciliated cells lying in a space which probably 

 represents a remnant of the reduced body-cavity : the 

 external opening is on the ventral side of the body. Urea 

 and uric acid have been found in the nephridia. It is 

 probable that the granular pigmented cells of the "bo- 

 tryoidal " tissue, which occurs in abundance immediately 

 within the muscular walls of the body, is also excretory in 

 function and comparable to the chloragogen cells of the 

 earthworm. 



Reproduction. Leeches are hermaphrodite and many 

 details of the processes of reproduction are yet to be dis- 

 covered. In some species the eggs are deposited in 

 cocoons formed by clitellar glands situated in the skin in 

 the region of the genital apertures : in others the young 

 are cared for by the parent and carried about attached to 

 its body for a time. Observations 1 on Hirudo geometra 

 (Linn.), a species parasitic on gudgeon, roach, and other 

 fish, kept in aquaria showed that mating frequently 

 followed the addition of fresh water to the vessels. The 

 eggs were deposited on the glass sides of the aquarium 

 about 24 hours after sexual union ; they are oval in shape 

 and reddish-brown in colour and covered by a white web- 

 1 Brightwell, Ann. Mag. Nat Hist. 1842. 



