HIGH SCHOOL ZOOLOGY. 



225 



muscles ( Trichina), or the respiratory organs (Syngamus, Strongylus), 

 or blood-vascular system (SclerostomumJ, and cause many serious 

 diseases. One of the most dangerous of these to which man is liable 

 is Trichiniasis, caused by eating insufficiently-cooked pork, in the 

 flesh of which the minute encapsuled Trichinae are living (Fig. 151). 

 The Acanthocephali are so-named, from a proboscis covered with hooks, 

 by which they fix themselves in the intestines of their hosts — for the 

 most part, lower Vertebrates. 



11. The Plathelminthes also include some free forms, such as the marine 

 Nemertinl, imsegmented worms, which sometimes attain a length of thirty 

 or forty feet (Lineus), and also the Turbe]laria, for the most part very 

 small creatures living in water or damp places. In both, the skin is covered 

 with ciliated cells, which are absent in the other (parasitic) orders. 

 The intestine is tubidar and complete in the Nemerteans, but in the 

 Turbellarians it is sometimes much branched, and always opens to the 

 outside only by the mouth. The commonest forms are species of Plan- 

 aria (Fig. 152 b) little flat, leech-like forms, 

 which are to be found clinging to stones or 

 creeping about in fresh-water ponds or 

 streams. They are rarely longer than half 

 an inch, but some marine forms attain a 

 much larger size. On the other hand, there 

 are fresh-water Turbellaria with a simple 

 rod-like intestine (Fig. 152 a), which rarely 

 exceed a line in length, and others, with 

 no intestine at all, in which the food-parti- 

 cles are simply admitted by the mouth into 

 tlie interior of the body, which is composed 

 Fiff. 152.-Turbellaria. of soft ceUs. 



lik"e; f/vr«XwiSanch: 12- The remaining orders of the Flat-worms 

 ed intestine ; vt., the mouth ; n, are the Trematodes, and Cestodes. In the 

 nerve-ffan^lia with eye-spots. . xi-ii- • riiii 



former, the intestnie is a torked tube open- 

 ing only by the mouth ; in the latter, it is entirely absent, and 

 there is no cojlom in either. Organs of fixation in the form of 

 suckers or hooks are always present. The Trematodes live either 

 as ectoparasites on various animals, such as fishes and molluscs, — 

 in which case they are well provided with hooks and suckers, and 

 form only a few eggs, which they fix directly to their hosts, — 

 or as entoparasites within various Vertebrates, — in which case they 



