276 PLATHELMINTIIES. 



metamorphosis. To illustrate this the history of Distomum 

 hepatwum of the sheep is chosen (fig. 236). 



The eggs leave the maternal uterus before embryonic develop- 

 ment is begun, pass down the bile ducts and thence by the intes- 

 tine to the exterior. They must come into water and remain here 

 awhile before the ciliated larva (< miracidium/ A) escapes by a 

 lifting of the lid of the shell. This larva bores its way into a 

 small snail (sp. of Limncea), where it grows into a ' sporocyst' (B). 

 The sporocyst, a muscular sac with protonephridia but lacking all 

 other organs, produces in its interior eggs which develop into a 

 second reproductive sac, the ' redia ' (D). These are distinguished 

 from the sporocysts by the possession of pharynx and a tubular 

 intestine as well as a birth-opening for the escape of the young pro- 

 duced inside. According to the season these young are either 

 'cercariae' (F) t or several generations of rediae may follow before 

 the cercariae appear. The cercarias are adapted for an aquatic life, 

 since each has, besides the characteristic organs of a Distomum 

 (genitalia excepted), a strongly vibratile tail. The cercariae escape 

 from the snail, swim about in the water until the tail drops off, 

 when . they encyst on water plants. When these encysted young 

 are eaten by sheep along with the vegetation, infection follows. 



In general it can only be said of the life history of other Trematoda 

 that the miracidia must penetrate a mollusc, and that the different species 

 have many modifications : (1) Ordinarily development begins in the ma- 

 ternal uterus. (2) Many miracidia are naked or only partly ciliated. (3) 

 In many species the miracidia only hatch when the egg is taken into the 

 stomach of a snail along with food. (4) Very frequently the cercaria 

 passes from the water into a new host (mollusc, arthropod, or vertebrate) 

 and becomes encysted here. In such cases there are three hosts in the 

 cycle. (5) On the other hand the history may be simplified, as when the 

 sporocyst in the snail produces directly * cercariae without tails ' (i.e., small 

 Distoma), which only need to be eaten by the definitive host to reach the 

 sexually mature condition. (6) It is doubtful if the sporocyst may be 

 omitted and the miracidia develop directly into redia. 



As the adjacent scheme shows, the typical development is distributed 

 among three hosts by the intercalation of a second aquatic interval. It 

 consists of two generations ; one extends from the fertilized egg to the 

 sporocyst, the second begins with the unfertilized egg of the latter and de- 

 velops, through the cercaria and the encysted Distomum, into the sexually 

 mature individual. There is no sexual reproduction by fission or budding, 

 rather an alternation of sexual and parthogenetic generations or heter- 

 ogony. Columns a and c show how the history maybe simplified and com- 

 plicated. 



Best known of the Distomeae are the following: Distomum (Fasciolaria) 

 hepaticum, the liver fluke (fig. 232), about the size and shape of a pump- 



