156 AGRICULTURAL EXPERIMENT. 



uniform conditions for two or three weeks prior to the commence- 

 ment of the experiment. 



5. In field trials no appreciable increase in reliability is gained 

 by enlarging the plots beyond l-40th of an acre. 



6. In interpreting the results of agricultural experiment the 

 "Probable Error" figure is useful, since it is a guide to the relia- 

 bility and variability of the results under consideration. 



THE LIFE-HISTORIES OF SOME TREMATODES 

 OCCURRING IN SOUTH AFRICA. 



By Annie Porter, D.Sc.Lond., F.L.S., F.R.S. (S.A.). 



Parasitologist, South African Institute for Medical Research, 



Johannesburg. 



Read July 11, 1921. 



Abstract. 



It is well known that the life-history of a parasitic Trematode 

 may usually be divided into two main phases, one passed in an 

 invertebrate host and the other in a vertebrate. A few Trematoda 

 are known in which two successive invertebrate hosts occur and 

 only one vertebrate host, but such have not been notified in the 

 Union of South Africa. Here the Trematoda whose life-histories 

 are known are digenetic. 



In South Africa, as in other parts of the world, the inverte- 

 brate hosts of digenetic Trematodes are usually Mollusca. When 

 South African Mollusca are examined for larval Trematoda it is 

 found that} a large number of different kinds of the larval forms 

 are present, and in only a very few cases have the adults correspond- 

 ing to the larval forms been worked out. Conversely, a number 

 of adult Trematoda from various vertebrate hosts have been noti- 

 fied, of whose early developmental stages and invertebrate hosts 

 nothing is known. 



It has been my good fortune to have been able to determine 

 the life-histories of several South African flukes during the last 

 three years*, some of which are now summarised, full details being 

 reserved for future publication. 



Schistosomes. 



(1) Schistosoma haematobium. 



Schistosoma haematobium is the fluke responsible for human 



urinary bilharziasis. The larval stages are passed usually in South 



Africa in the freshwater snail, Physopsis africana, and occasion- 



* See Med. Journ. S. Africa, 1920, vol. XV, pp. 128-133; Ibid., vol. XVI, pp. 75, 76; 

 S.A. Journ. of Science, 1920, vol. XVII, pp. 120-130. 



