172 PLATHELMINTHES 



the various organs. Several pairs of large longitudinal nerves pass to 

 the hinder part of the body. 



With a few exceptions all trematodes are hermaphroditic, the male 

 and female pores being either confluent, in which case a genital atrium 

 is often present, or side by side and near together in the ventral sur- 

 face. The arrangement of the genital organs is complex and varies 

 somewhat in the three orders of trematodes. The egg is composite in 

 structure, consisting of an ovum and several yolk cells. 



Habits and Distribution. The young trematode leads a free life for a 

 short time and then seeks its host. The most primitive trematodes are 

 external parasites and the entire life may be passed on a single host. 

 The higher forms, on the other hand, are internal parasites and live in 

 two or more hosts, the adult host being different from the larval host, 

 and the passage from one host to the other being accompanied by a 

 metamorphosis. The former are called monogenetic and the latter dige- 

 netic trematodes. Many of the latter are dangerous parasites to man 

 and his domestic animals. 



History. This class was established in 1808 by Rudolphi, who 

 included in it the genera, Monostoma Zeder, Amphistoma Rudolphi, 

 Distoma Retzius, and Polystoma Zeder. It was not until 1858 that the 

 distinction between the ectoparasitic and the entoparasitic forms found 

 expression in the classification, when P. J. van Beneden formed the groups 

 Monogenea to include the former 'and the Digenea for the latter. In 1892 

 Monticelli showed the need of subdividing the Digenea and established the 

 following suborders : the Heterocotylea, the Aspidocotylea, and the Malaco- 

 cotylea, the first of which coincides with the Monogenea. This subdivision 

 was generally adopted, but is now being abandoned in favor of the simpler 

 one of van Beneden. The explanation of the complex metamorphosis of 

 trematodes was first given by Steenstrup in 1842. Thomas and Leuckart 

 discovered almost simultaneously in 1881 the life history of the common 

 liver fluke (Fasciola hepatica), and the latter author and Looss have 

 played the principal part in the investigation of the entire group. 



About 2,500 species of trematodes are known, which may be 

 grouped in 3 orders. 



Key to the orders of Trematodes: 



Oj Ectoparasitic trematodes (except Polystoma) ; hooks usually present in 



the suckers or sucking discs 1. MONOGENEA 



o 2 Mostly entoparasitic trematodes; no hooks in the suckers or sucking 



discs. 

 61 Either a large ventral sucking disc or a midventral row of suckers ; no 



oral sucker 2. ASPIDOCOTYLEA 



5 a Usually either 1 or 2 median suckers; oral sucker present (except in 



Bucephalus) 3. DIGENEA 



