V] TREMATODA 105 



The DIGENEA commence life as larvae which become parasitic in 

 some animal, where they give rise by gemmation to several gener- 

 ations of secondary larvae, which develop into adult forms only 

 when they are swallowed by a second animal. The life-history 

 therefore includes an alternation of generations and is only com- 

 pleted in two hosts. The Digenea further differ- from the Mono- 

 genea in having as main adhesive organ a sucker situated on the 

 anterior part of the ventral surface, in having only a single "canal 

 of Laurer" which opens on the dorsal surface, and finally in the 

 fact that the main trunks of the excretory system or in other 

 words the tubes of the two branched nephridia coalesce to form a 

 single trunk which opens to the exterior by a median posterior pore. 

 The Liver-fluke, Distomum hepaticum, is an example of the Digenea; 

 it is parasitic in the liver and bile-ducts of the sheep, causing a 

 wasting disease called sheep-rot. It gives rise to a larva consisting 

 of a solid mass of cells, the outermost layer of which is ciliated. 

 This larva cannot survive unless it reaches a pond-snail of the 

 species Limnaea truncatula. In the pulmonary chamber of this 

 animal it loses its cilia, enlarges and becomes hollow, forming a 

 structure called the sporocyst, which sometimes divides into two 

 or more. Germ, cells are budded off from the wall of the sporocyst 

 into its cavity. These by division form masses which develop into 

 secondary larvae called rediae, which are provided with a muscular 

 pharynx and a sac-like alimentary canal. These larvae have a pair 

 of blunt processes on the under side near the posterior end, by the 

 aid of which they move. They force their way out of the sporocyst 

 and enter the tissues of the snail, being found especially in the 

 liver. From the inner surface of their body-wall germ cells are 

 budded off which give rise to other rediae, which escape from the 

 parent by an opening near the anterior end. After a time the 

 rediae give rise to larvae of a third kind called cercariae. 

 These have suckers like the adult, and a forked alimentary canal 

 with a pharynx ; they are provided with a tail stiffened by a rod of 

 gelatinous tissue recalling the Vertebrate notochord. By the aid of 

 the tail they work their way out of the snail and attach themselves 

 to blades of grass. The tail then falls off and they enclose them- 

 selves in a cyst of mucus, and remain there till they are eaten by a 

 sheep, from whose intestine they pass into its liver, where they 

 develop genital organs and become mature. 



