FL UKE- WORMS. 689 



The Distomeae, as their name, meaning two-mouthed, implies, are pro- 

 vided with only two suckers, and have no clinging hooks ; for, being internal 

 parasites, they have no such special need of instruments for maintaining a 

 secure hold as the more highly organised species of the previous group. 

 Their development, too, is often complicated by the most extraordinary 

 metamorphoses, and is accompanied by migrations from one host to another. 



A well-known example of this section is the Liver fluke (Distoma hepa- 

 ticum), which lives parasitically in the liver of herbivorous mammalia such 

 as sheep. The worm is less than an inch long, and is broad and flat, wider 

 in front than behind, and bearing on its front end a conical projection sup- 

 porting the mouth, which is lodged in the centre of a sucker. The second 

 sucker is situated on the ventral surface a short distance behind the first. 

 The eggs of the Fluke enter the intestines of the sheep through the bile 

 ducts, and are thence discharged with the droppings. Thereupon they hatch; 

 but the embryo perishes, unless by chance it is carried or washed into some 

 fresh-water pond or stream. If favoured by fortune in this particular, it 

 swims actively about by means of its long cilia in search of the particular 

 host in which it is forced for awhile to sojourn. This host is the water-snail 

 known as Limnceus truncatulus. Into the soft tissues of this mollusc the young 

 Fluke bores its way, and after losing its cilia becomes converted into an oval 

 sac known as the Fporocyst. Within this fresh individuals, known as Redice, 

 are developed, the Redice, being short cylindrical little worms, furnished, like 

 the parent form, with mouth, gullet, and stomach. After making their way 

 to the snails' liver, the Redice in turn take up the process of development, 

 and give rise to individuals as different from themselves as they are from the 

 ciliated embryo. These new forms, known as Cercarice, somewhat resemble 

 a tadpole in shape, consisting of a flattish, heart-shaped body, furnished with 

 two suckers, a mouth, a forked alimentary canal, and produced behind 

 into a long vibratile tail. These make their escape from the body of the 

 snail, and, after swimming about for a time, settle upon some plant, and 

 envelop themselves in a coating or cyst. In times of flood this takes place 

 upon the grass blades of some overflowed meadow, and in this case there is 

 a chance that the organism will be devoured by a grazing sheep, and will 

 become lodged in the bile ducts of its new host, where the development into 

 the adult Fluke will be completed (see Fig. 13, a-e). 



To be regarded in all probability as degenerate Flat-worms are the 

 minute organisms belonging to the Dicyemidce and Orihonedidce, which live 

 as parasites, the former in the branchial veins of cuttle-fish and the latter 

 upon Planarian and Nemertine worms and on sand-stars. They are ciliated 

 vermiform creatures, without trace of body cavity or alimentary canal, the 

 chief peculiarity of their organisation consisting in the fact that the solid 

 body consists of a central mass, composed in the Dicyemidse of one large 

 nucleated cell, and in the Orthonectidse of a cluster of cells, surrounded 

 externally by a single layer of ectoderm cells. 



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