624 TREMATODE ENTOZOA. TURBELLARIA. 



it be kept warm and moist, will be found after a few days to swarm 

 with these curious little creatures. 



486. Besides the foregoing Orders of Entozoa, the Trematode 

 group must be named ; of which the Distoma hepaticum, or 'Fluke,' 

 found in the Livers of Sheep affected with the 'Rot,' is a typical 

 example. Into the details of the structure of this animal, which 

 has the general form of a Sole, there is no occasion for us here to 

 enter : it is remarkable, however, for the branching form of its 

 digestive cavity, which extends throughout almost the entire body, 

 very much as in the Planarise (Fig. 326) ; and also for the curious 

 phenomena of its development, several distinct forms being passed 

 through between one Sexual Generation and another. These have 

 been especially studied in the Distoma which infests the Lymnceus ; 

 the Ova of which are not developed into the likeness of their 

 parents, but into minute Worm-like bodies, which seem to be little 

 else than masses of cells enclosed in a contractile integument, no 

 formed organs being found in them ; these cells, in their turn, are 

 developed into independent Zooids, which escape from their con- 

 taining cyst in the condition of free Ciliated Animalcules ; in this 

 condition they remain for some time, and then imbed themselves 

 in the mucus that covers the tail of the Mollusk, in which they 

 undergo a gradual development into true Distomata ; and having 

 thus acquired their perfect form, they penetrate the soft integu- 

 ment, and take-up their habitation in the interior of the body. 

 Thus a considerable number of Distomata may be produced from 

 a single Ovxim, by a process of Cell-multiplication in an early stage 

 of its development. In some instances the free ciliated Larva 

 possesses distinct Eyes ; although they are wanting in the fully- 

 developed Distoma, the peculiar ' habitat' of which would render 

 them useless. 



487. Turbellama. — This group of animals, which is distin- 

 guished by the presence of Cilia over the entire surface of the 

 body, seems intermediate in some respects between the Trematode 

 Entozoa and the Leech-tribe among Annelida. It deserves special 

 notice here, chiefly on account of the frequency with which the 

 Worms of the Planarian tribe present themselves among collec- 

 tions both of Marine and of Fresh-water animals (particular species 

 inhabiting either locality), and on account of the curious organi- 

 zation which many of these possess. Most of the members of 

 this tribe have elongated flattened bodies, and move by a sort of 

 gliding or crawling action over the surfaces of Aquatic Plants and 

 Animals. Some of the smaller kinds are sufficiently transparent 

 to allow of their internal structure being seen by transmitted 

 light, especially when they are slightly compressed; and the 

 accompanying figure (Fig. 326) displays the general conformation 

 of their principal organs, as thus shown. The body has the flat- 

 tened Sole-like shape of the Trematode Entozoa; its Mouth, which 

 is situated at a considerable distance from the anterior extremity 



