TREMATODA. 35 



Genus Distomum. 



In the worms composing this genns the body is 

 flattened or cylindrical, sometimes armed with hooks, 

 and furnished with two distinct isolated suckers ; of 

 these, one is situated anteriorly and contains the 

 mouth, whilst the posterior one is imperforate, and 

 placed upon the ventral surface, near the mid-line, 

 and at about one -sixth of the length of the worm 

 from the anterior extremity ; the intestine is divided 

 into two branches, which are simple in some species, 

 and ramified in others (as in the Distomum hepa- 

 ticum). 



This genus comprises a considerable number of 

 species, all of which Hve parasitically, either in cavi- 

 ties which have a more or less direct external 

 communication, or in closed cavities, or in cysts. 



In the larval condition they exist in crustaceous 

 and molluscous animals, or else in a state of freedom 

 in the water ; in the perfect condition, they are 

 found in the animals belonging to the four classes of 

 Yertebrata. 



Distomum Hepaticum. 



The body of this worm is of a whitish colour, 

 and is from one-half of a,n inch to upwards of an 

 inch in length, and from one-sixth to one-half of an 

 inch in breadth, when full-grown ; it is of an oval, 

 oblong, or lanceolate shape, and is large and rounded 

 in front, where it becomes suddenly contracted, so 

 as to form a kind of conical neck ; posteriorly it is 

 narrow, and presents a leaf-like appearance; the 

 integument is covered with flattened spines ; the 

 anterior sucker is terminal, and circular ; the posterior 



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