WORMS 209 



plumage of migrating birds and in this fashion must be carried far 

 afield. Leeches generally attack the mucous membrane of the head, 

 especially the nostrils, and domestic duck are not infrequently choked 

 by an accumulation in their trachea which thus blocks the air passages. 

 Leeches are also occasionally found in the digestive tract which they 

 gain via the vent. The odoriferous greasy secretion of the uropy- 

 gidial glands of water-birds is said to attract them strongly. 



The duck leech, which is extremely active and restless, is quite small, 

 only about 16 mm. long and barely 2 mm. in width. It is a beautiful 

 olive green and pale grey in colour, finely sprinkled with black star-like 

 pigment spots. There is a sucker at each end of the body, although in 

 the preserved specimen figured on p. 210 only the large posterior disc 

 shows up clearly. The mouth is provided with a protractile sucking tube 

 which inflicts a small circular wound. Some leeches which have jaws arm- 

 ed with pointed teeth leave a wound like a three-pointed star. The 

 digestive system is highly developed. The stomach has an acid re- 

 action and is provided with voluminous caeca in which blood can be 

 stored almost unchanged for many months. This enables the leech to 

 undergo long periods of fast. The intestine has an alkaline reaction and 

 in some species is also provided with lateral expansions. Glands which 

 secrete a powerful anti-coagulant are situated in the head. It is this 

 secretion which inhibits the clotting of blood in the wound made by 

 leeches and which can thus be the cause of fatal haemorrhages in the 

 host. 



The duck leech is hermaphrodite and after copulation and fertilisa- 

 tion both parties separate, lay eggs and rear young. They are admirable 

 parents, for not only do they brood their eggs but they also carry their 

 200-300 young about with them attached to their ventral surfaces. 

 The aquatic hirudinids as a group have many enemies and are preyed 

 upon by birds and mammals, frogs and newts and predatory insects 

 such as dragon-fly larvae and water beetles. On the other hand they 

 are carriers of various diseases of vertebrate animals ranging from a 

 fatal frog trypanosome to the virus of fowl-pox. They are also inter- 

 mediate hosts of several duck trematodes — a fact which demonstrates 

 how persistently they are eaten by these birds. 



The medicinal leech (Hirudo medicinalis) is a much larger species and, 

 apart from other differences, has bright red blood, whereas that of the 

 duck leech is colourless. Despite the observations of Pliny the presence 

 of leeches is often quite unsuspected by their victims. Their bite is 



