144 



LEECHES 



The existence of parasites with this type of Ufe history provides 

 evidence that water birds and fish are predators of leeches. Other 

 evidence is scanty since leeches seldom leave indigestible remains 

 so that they may be identified from stomach contents. From direct 

 observation of the feeding habits of various water birds and fish 

 and from occasional records from stomachs it is possible to say 

 that erpobdellids in particular are preyed upon by herons, swans, 





FiG.'^89. (a) diagram showing cysticercoid stages of the cestode 

 Hymenolepis parvula distributed in the tissues of a specimen 

 of the leech Erpobdella octoculata (drawn from a whole mount) ; 

 (b) a drawing of one cysticercoid, showing the ten characteristic 

 hooks; (c) details of one hook. Original. 



ducks, bitterns, trout, perch, tench, sticklebacks and eels. There 

 is no reason to suppose that they are rejected by aquatic amphibia 

 and mammals, and are no doubt taken by carnivorous inverte- 

 brates such as Hemiptera and Odonata. The mortality rates in 

 natural populations of Erpobdella octoculata^ Glossiphonia compla- 

 nata and Helohdella stagnalis have been roughly estimated for 

 certain habitats in Britain (Mann, 1953b, 1957a, b). In each case 

 there was a heavy mortality in the first three months of life, more 



