PHYLUM NEMATHELMINTHES 173 



cells are not derived from the coelomic epithelium. The body- 

 cavity of Ascaris, therefore, differs structurally and functionally 

 from that of a true ccelom, but nevertheless is similar in many 

 respects. 



2. NEMATHELMINTHES IN GENERAL 

 v 



Definition. Phylum NEMATHELMINTHES. ROUNDWORMS. 

 Bilaterally symmetrical, triploblastic animals with an elon- 

 gated cylindrical body; airmen tary canal has a mouth opening 

 at the anterior end and an anal opening on the ventral surface 

 near the posterior end, and lies in a body-cavity, which is prob- 

 ably a ccelom; no cilia present in any part of the body; both 

 free-living and parasitic; sexes separate. 



It has been customary to place the NEMATOMORPHA (see p. 

 179) and ACANTHOCEPHALA (see p. 180) in the Phylum NEMAT- 

 HELMINTHES, but the relationships of these animals are so ob- 

 scure that it is considered best to treat them separately. The 

 phylum, therefore, contains only one class, the NEMATODA, 

 whose members have all of the characteristics cited above. 



Ascaris lumbricoides is but one of the interesting and important 

 nematodes. It belongs with a number of other similar forms to 

 the family ASCARID^E. 



The family STRONGYLID^E contains several dangerous para- 

 sites. Ancylostoma duodenalis, the European hookworm, is 

 frequently very injurious and sometimes fatal. Nematodes of 

 this species are taken into the alimentary canal with drinking 

 water, or enter the body through the skin, and thousands are 

 sometimes present. Anaemia is caused by their biting into the 

 intestinal wall and destroying the capillaries. Syngamus is the 

 parasite that causes the disease known as gapes in poultry 

 and game birds. The birds swallow the young syngamids, 

 which soon become mature in the trachea and bronchi. 



To the family TRICHINELLID^E belongs Trichinella spiralis 

 (Fig. 113) which causes the disease of human beings, pigs, and 

 rats called trichinosis. The parasites enter the human body 



