SECTION VI 

 PHYLUM NEMATHELMINTHES 



THE members of the preceding phylum are characterised, as a 

 whole by a marked dorso-ventral flattening In the Worms in- 

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

 whence their general name of Round- or Thread- won us. 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. 



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 Chaetognatha 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 la.mbricoides). 



Ascaris lumbricoides is a common parasite in the human intes- 

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

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

 description will apply to any of these. The female worm is about 

 12-28 mm. (5-11 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 



T 2 



