AND THE LOWEE ANIMALS. 203 



generally provided with one or more suckers, by- 

 means of which they attach themselves after the 

 fashion of a leech. The liver-fliilve, so common among 

 sheep, may serve as an example of this group. The 

 second, of which the Tcenice (improperly called solitary 

 worms) may be taken as the type, occasionally attain 

 a length of several metres. In these worms the so- 

 called body is composed of flattened joints, very small 

 and slightly marked in front, but becoming gradually 

 wider and more distinct. A rounded button-like 

 expansion, sometimes provided with suckers, and at 

 others with hooklets, terminates the slender extremity 

 of this festooned ribbon. It is this expansion which 

 is termed the head. Finally, the Cystic worms are 

 like little vesicles, from some portions of whose 

 surface are borne the heads of teenige attached to a 

 short footstalk. The Cestoids inhabit the digestive 

 tube only ; the Trematodes are found in almost all the 

 viscera. The Cystic worms seem to prefer the tissues, 

 and we find them in the midst of muscles, in the 

 centre of the brain, &c. 



All these worms are nourished, and what is more, 

 respire, only through the intermediation of the animal 

 which encloses them. From this fact, we can even 

 now draw a very important conclusion, the application 

 of which we shall find further on. 



Every species of animal having its own proper form 

 of nutriment, its special temperature and its peculiar 

 liquids, it follows that each of them presents a series 

 of conditions different from the others, and conse- 

 quently constitutes a special world for the Helminthes. 

 Therefore, these parasites must distribute themselves 

 each according to its own requirements, and cannot 



