SECTION VI. — PHYLUM NEMATHEL- 



MINTHES 



The Nemathelminthes or round-worms are so named 

 because the body instead of being compressed from above 

 downwards, as in the flat-worms, is rounded, i.e., cylindrical. 

 The majority of the members of the phylum belong to the 

 class of the Nematoda or round-worms in a more restricted 

 sense. A good example of these is the common round- 

 worm of man (Ascaris lumbricoides), which is a common 

 parasite in the human intestine ; or the nearly allied Ascaris 

 suilla of the pig. When fresh the animal is of a light 

 yellowish-brown colour ; it is marked with four longitudinal 

 streaks, two of which, very narrow and pure white in the 

 living worm, are respectively dorsal and ventral in position, 

 and are called the dorsal (Fig. 81, d. I) and ventral (v. I) 

 lines : the other two are lateral in position, thicker than the 

 former and brown in colour, and are distinguished as the 

 lateral lines. The mouth is anterior and terminal in position, 

 and is bounded by three lobes, or lips, one median dorsal 

 (d. Ip), the other two ventro-lateral (v. If). A very minute 

 aperture on the ventral side, about two millimetres from 

 the anterior end, is the excretory pore (ex. p). At about the 

 same distance from the pointed and down-turned posterior 

 end is a transverse aperture with thickened lips, the anus 

 (an), which in the male serves also as a reproductive 

 aperture, and gives exit to a pair of needle-like chitinoid 



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