Chap. III.] 



ROUND- WORMS. 



with a group, the great majority of the members of 

 which are, in their sexual state at least, endo-parasitic. 

 These are the round-worms or thread-worms (Nemato- 

 lielmi lit lies). They are remarkable, as compared 

 with the soft-bodied Turbellarians, for the great 

 development of that horny material which is, as chi- 

 tin, so richly present in the integuments of many 

 Metazoa. The intestine forms a straight tube, and 

 is surrounded by a comparatively spacious body cavity. 

 The whole body is, as 

 their popular name implies, 

 greatly elongated. Examples 

 of this group are Gordius, 

 Ascaris, Filaria, and Tri- 

 china. 



More closely allied to the 

 round- worms than to any 

 other worms are the Acan- 

 thocephali, of which Echi- 

 norhynchus (Fig. 18) is an 

 example. They are internal 

 parasites, which, like most 

 tape-worms and flukes, live, 

 at different stages of their 

 life-history, in different hosts. They are provided with 

 a protrusible proboscis, which is armed with recurved 

 hooks of considerable strength. 



The Rotatoria or Wheel- Animalcules exhibit 

 certain characters which we shall again meet with in the 

 larval stages of some of the higher forms. The anterior 

 end carries a disc, the edge of which is ciliated (this is 

 the so-called "wheel-organ"), and in the centre of 

 which the mouth is placed (Fig. 19). A special 

 apparatus for comminuting the food is found in the 

 stomach. The " water- vessels," or organs by means 

 of which, in all probability, waste nitrogenous matters 

 are excreted, are very distinct, and are provided with 



Fig. 18. Echinorliynclius no- 

 dulatus (nat. size and en- 

 larged). (After Busk.) 



