224 



HIGH SCHOOL ZOOLOGY. 



also to be recognised. The development is often complicated 

 by a metamoi'phosis, and the creatures often pass through dif- 

 ferent stages of their parasitic life in diiferent hosts, a phenom- 

 enon known as hetercecism. 



Fig. 151.— Trichina spiralis. 



A, female; B, male; C, junction of oesophagus and intestine; D, encysted 

 Trichina-larva between the fibres of muscular tissue. 



10. Two orders of Nematelminthes are recognised, the Nematodes, 

 which have generally a complete intestine, and the Acanthocephali, which 

 have none. To the former order there belong some free microscopic forms, 

 which live in decaying matter in water, also some vegetable para- 

 sites, like the wheat- worm f' T^^feHc/ms^ and the beet-worm ^i/e^ercxZera^, 

 but the bulk of the order are parasites, either during a part of their 

 life (like the insect parasites Gordius and Mermls), or during the whole of 

 it. All groups of Vertebrates have special Nematode parasites, which live 

 iu the skin, or the eyes (Filaria), or iu the intestine (AscarisJ, or the 



