PLATYELMIA 5TURBELLARIA : TREMATODA. 313 



neath the skin is a singular areolar structure, which is probably 

 connected with the absorption of fluid through the integument, 

 this being apparently the only way in which these parasites can 

 obtain their nourishment. Some species of the group arc para- 

 sitic on Mammalia, and one of these found in the Pig attains a 

 length of no less than eighteen inches. The so-called Eels of 

 vinegar and sour paste are referable to this group, in regard to 

 the simplicity of their structure and their worm-like form ; al- 

 though their habitation is so different. There is also a little 

 worm-like animal closely allied to these, which is found in dis- 

 eased ears of wheat, and which possesses remarkable tenacity of 

 life, being revived by moisture after having been dried for almost 

 any length of time. 



924. Section II. PLATYELMIA. Of the Flat-worms, we may 

 likewise distinguish three Orders. Of these the first, that of the 

 TURBELLARIA, includes the non-parasitic species, which are dis- 

 tinguished from their parasitic relatives by the possession of cilia 

 on the surface of the body. To this Order belong the Planaria 

 already referred to ( 921), together with a number of nearly 

 allied species, the majority of which are inhabitants of fresh 

 water, whilst a few are found in the sea, and several are ter- 

 restrial in their habits. The Nemertida or Ribbon-worms, which 

 are also placed with the Turbellaria, differ from the Planariae in 

 the elongated ribbon-like form of their bodies, and in the posses- 

 sion of an anal opening. They are predaceous in their habits, and 

 some of them attain a considerable length. The TREMATODA, 

 forming the first Order of the parasitic Flat-worms, have a 

 rounded, elliptical or oval body, covered with a soft skin, which 

 usually contains calcareous corpuscles, and furnished with one 

 or more sucking discs, which are often supported by a horny 

 framework. The intestine is generally forked at a little distance 

 behind the mouth, and the two branches sometimes end in blind 

 extremities, and sometimes meet at the posterior part of the body, 

 so as to form a complete circle ; in some cases the intestine is 

 minutely ramified. The Distoma hepaticum or Fluke, which 

 commonly infects the livers of sheep, and is also found in man, is 

 a well-known example of this group, most of the worms belong- 



