246 FLAT WORMS 



not have cilia on ectoderm. Very characteristic of them is the posses- 

 sion of suckers by which they hold on to the skin or alimentary system of 

 their host. 



They are divided into two orders: i. the Monogenea in which the egg gives rise 

 to a larva which later becomes the adult and 2. the Digenea. It is to this latter that 

 the flukes parasitic in man belong. This order is characterized by the fact that the 

 larva becomes parasitic in some second animal and then gives rise to a second gen- 

 eration of larvae which latter develop into adults. 



The largest human fluke, Fasciolopsis buski, is from two to three inches (50 to 

 75 mm.) in length, while the Heterophyes heterophyes is less than 1/12 of an 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 Schistosomidse, 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 Dibothriocephalidae. 



The three important families of flukes parasitic for man are: i. 

 Paramphistomidas flukes with two suckers situated at either extremity. 

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

 to it and situated ventrally. This family includes the important geneia 

 Fasciola, Opisthorchis, Dicroccelium, Fasciolopsis, and Paragonimus. 

 In Paragonimus and Heterophyes the genital pore is posterior to the 

 acetabulum, in the other genera it is anterior. Fasciola has a den- 

 dritic intestinal canal which is not the case with Clonorchis, Fascio- 

 lopsis, Fascioletta, Opisthorchis and Dicroccelium. In Dicroccelium 

 the testicles are anterior to the uterus, in Opisthorchis, Clonorchis, 

 Fasciolopsis and Fascioletta they are posterior. Fasciolopsis and 

 Clonorchis have branched testicles (the former a very large fluke- 

 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 acetabu- 

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

 which bifurcates and terminates in blind intestinal casca. 



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 



