296 FLAT WORMS 



The largest human fluke, Faciolopsis buskl, is from 2 to 3 inches (50 to 

 75 mm.) in length, while the Heterophyes heterophyes is less than ^ 2 inch (2 

 mm.) in length. The most important fluke, the liver fluke, Clonorchis endemicus, 

 is flat and almost transparent, while the almost equally important lung fluke, the 

 Paragonimus westermanii, is oval, almost round and reddish brown in color. With 

 the exception of the Schistosomidae, all flukes are hermaphrodites, and, with the 

 exception of this family, all flukes have operculated eggs. The only other opercu- 

 lated (with a lid) eggs we meet with in man are those of the Dibothriocephalidse. 



The three important families of flukes parasitic for man are: 



1. Paramphistomidae flukes with two suckers situated at either 

 extremity. 



2. Fasciolidae flukes with two suckers, one terminal, the other ad- 

 jacent to it and situated ventrally. This family includes the im- 

 portant genera Fasciola, Opisthorchis, Dicroccelium, Fasciolopsis, and 

 Paragonimm. In Paragonimus and Heterophyes the genital pore is 

 posterior to the acetabulum, in the other genera it is anterior. Fas- 

 ciola has a dendritic intestinal canal which is not the case with Clon- 

 orchis, Fasciolopsis, Fascioletta, Opisthorchis and Dicroccelium. 

 In Dicrocxlium the testicles are anterior to the uterus, in Opisthorchis, 

 Clonorchis, Fasciolopsis and Fasciohtta they are posterior. Fascio- 

 lopsis and Clonorchis have branched testicles (the former a very large 

 ftuke-Clonorchis of medium size) while those of Opisthorchis are lobed. 



3. Schistosomidae: In this family we have a leaf -like male which by 

 a folding in of its sides makes a channel for the thread-like female. The 

 sexes are separate, not hermaphroditic as with the Fasciolidae and 

 Paramaphistomidae. 



Flukes have two suckers which, except in the Paramphistomidae, are quite near 

 each other one is termed the oral sucker and the other the ventral sucker or acet- 

 abulum. The intestinal tract consists of a pharynx, proceeding from the oral sucker 

 which bifurcates and terminates in blind intestinal caeca. 



At the posterior extremity is an excretory pore which is at the termination of 

 a duct which divides into ramifying branches. This is the water-vascular system. 

 The testes, of various shapes and relations to the uterus, are more or less centrally 

 situated and have vasa deferentia. In some flukes the receptaculum seminis is a 

 conspicuous organ. The vitellaria are bilateral branching glands which pour 

 nutrient material into the ootype. It is in the ootype that the eggs are formed, 

 and opening into it we have the adjacent ovary. The shell gland is near the ovary. 



A canal, known as Laurer's canal, leads from the ootype to the exterior, the 

 function of which is in question. It is probable that as trematodes have no sperma- 

 theca, the spermatozoa from other flukes enter by way of this canal. The life his- 

 tdry of the important human flukes is unknown. It is supposed that this, in a meas- 

 ure, may resemble that of the common liver fluke of sheep (sheep rot). In this the 

 eggs containing a ciliated embryo (miracidium) pass out in the faeces. This embryo 



