ECOLOGY 143 



such as Entamoeba ranarum occur in a variety of aquatic animals. 

 Possibly of greater interest are the parasites for which leeches are 

 the intermediate host by reason of their blood-sucking habits. 

 Many species of Trypanosoma from the blood of freshwater fishes 

 are transmitted by Piscicola and Hemiclepsis, and a similar role is 

 played by Pontohdella and Trachelohdella in respect of marine fish. 

 In these cases reinfection of the fish occurs during blood sucking, 

 the infective stages migrating to the proboscis. Barrow (1953, 

 1958) in an intensive study of the biology of Trypanosoma diemyctyli, 

 which has the newt Triturus as its primary host and the leech 

 Batracobdella picta as its secondary host, showed that the passage 

 of the infection from mother to offspring in the leech takes place 

 via the vertebrate host. When the young leave the mother to take 

 their first meal of vertebrate blood they may stay on the newt for 

 7-14 days. During this time the newt becomes infected by the 

 mother and the trypanosomes are picked up by the offspring 

 during feeding. Haemogregarines may also be carried by leeches, 

 Haemogregarina stepanowi of the pond tortoise Emys orbicularis 

 being transmitted by Placobdella costata. 



When handling Erpobdella octoculata it is often possible to see 

 numerous clear cysts in the mesenchyme or body wall. These 

 were called tetracotyles, one kind being given the name Tetracotyle 

 typica. Subsequently it was shown that they were metacercariae 

 of strigeid trematodes, and Szidat (1930) showed that they develop, 

 when the leeches are eaten by birds, into Cotylurus cornutus and 

 Apatemon gracilis. Other cysts which occur in Erpobdella clearly 

 show a ring of hooks characteristic of cestodes. One such cyst 

 has been shown to be the cysticercoid of Hymenolepis parvula, a 

 small cestode found in the intestine of ducks. 



Fig. 88. The life cycle of Haemogregarina stepanowi. a-h, 

 from the turtle Emys orbicularis; i-n, from the leech Placobdella 

 costata; a, schizont with fully formed merozoites, from bone 

 marrow; b, entry of merozoite into blood corpuscle; c, d, e, 

 further merozoite formation; /, formation of gamonts; ^i, 

 macrogametocyte ; hz, microgametocyte ; i, maturing of macro- 

 gametes ; k, fusion of gametes ; /, oocyst ; m, sporozoite forma- 

 tion; n, free sporozoite in proboscis blood vessel of leech. 

 From Herter, 1937, after Reichenow. 



