SECTION VI. 



PHYLUM NEMATHELMINTHES. 



THE members of the preceding phylum are characterised, as a 

 whole, by a marked dorso-ventral flattening. In the Worms 

 included in the present group the body is elongated and cylindrical, 

 whence their general name of Round- or Thread-worms. The 

 phylum includes the following classes : 



Class 1. NEMATODA. The Round- worms in the strict sense of 

 the term. The best known forms are internal parasites, but many 

 genera and species are extremely abundant in fresh- and salt-water 

 as well as in the soil. 



Class 2. ACANTHOCEPHALA. The " Hook-headed Worms," a 

 group of formidable internal parasites. 



Class 3. CH^ETOGNATHA. The " Arrow-worms," a small group of 

 pelagic organisms. 



The affinities of the Acanthocephala and Chsetognatha with the 

 Nematoda are somewhat doubtful, and the association of the three 

 classes is largely a matter of convenience. 



CLASS I. NEMATODA. 



1. EXAMPLE OF THE CLASS THE COMMON ROUND-WORM OF 

 MAN. (Ascaris lumbricoides.) 



Ascaris lumbricoides is a common parasite in the human intes- 

 tine : a closely allied if not identical form (A. suillce) occurs in the 

 Pig, and another (A. megalocephakt) in the Horse. The following 

 description will apply to any of these. The female A. lumbricoides 

 is about 20-40 cm. (8-16 inches) long, and about 6-8 mm. (] inch) 

 in diameter ; the male is considerably smaller. 



External Characters. 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 



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