ECOLOGY 139 



on the wave- washed shores of lakes. Hemiclepsis on the other hand 

 is found chiefly in standing water or in slowly running rivers. 



Some species of leech are better adapted to life in a strong 

 current of water than others. All are equipped with suckers and 

 so are hardly likely to be washed away in the normal course of 

 events, but during the breeding season some species are better 

 equipped to protect the eggs and young from being washed away 

 and lost. Erpohdella octoculata produces a low, smooth cocoon, 

 firmly fastened to a stone or plant and this species is always to be 

 found in running water. Glossiphonia complanata produces a soft, 

 gelatinous cocoon, but this is cemented down and the leech pro- 

 tects it with its body. This works well, and G. complanata is some- 

 times the most abundant species, especially in chalk streams. 

 G. heteroclita on the other hand does not cement its cocoon to the 

 substratum, but holds it loosely under its body. This may be the 

 reason why flourishing populations of this species are never found 

 in situations exposed to a current of any strength. 



2. Relations with the Animate Environment 



It seems that only in Europe have leeches been studied inten- 

 sively so that conclusions can be reached about their habitat 

 preferences. For the rest of the leech fauna of the world our. 

 information is mainly confined to brief notes about the food 

 organisms with which the leech is associated, with perhaps some 

 information about the habitats in which occasional specimens have 

 been found. A comprehensive list of the leeches of the world with 

 their hosts or food organisms is beyond the scope of this book, but 

 in Table 9 this information is given for a number of better known 

 genera. Here again the information from European sources is the 

 more reliable since many doubts have been resolved by the careful 

 serological, chemical and haemocytometric studies of the gut 

 contents of leeches by Jung (1955). 



Only a few leeches are restricted to one kind of host. Hemibdella 

 soleae is restricted to Solea spp. and Callohdella lophii feeds only 

 on the angler fish, Lophius piscatorius, but the majority of leeches, 

 while disposed to attack a particular kind of host, such as birds or 

 bony fish, will take a meal where they can find one. Hirudo, for 

 example, while predominantly an ectoparasite of mammals, is 



