72 WORM PARASITES. 



attached. Nearly all Nematode worms lay eggs; a 

 few, however, such as the Trichina spiralis of pigs, pro- 

 duce living young. The free development is a meta- 

 morphosis which is often complicated by the necessity 

 of two hosts, or, anyhow, two different parts of the 

 same host. The larvas live generally in parenchy- 

 matous organs ; they may be free or in the form of 

 a cyst enveloped in a capsule. The adults nearly 

 always live in the alimentary canal. An intermediate 

 host is, however, not always essential ; for instance, 

 I have clearly shown in Gapes that the ova of the 

 Gape-worm given to Fowls produces Gapes at once. 

 As a rule, in these simplest cases, the embryos in the 

 egg-membrane are transported passively in the food 

 or water into the host. This is what I have tried 

 clearly to point out is the case in Syngamus trachealis 

 (fig. 22). Again, in small Nematodes (Spiropterw) , 

 some of which are found in Poultry, two quite distinct 

 hosts are necessary : for instance, the mouse Spiro- 

 ptera (S. obtusa) is found in the encysted stage in 

 the Meal-worm ; whilst as another type we may take 

 the Trichina sp>iralis of the pig and man, which is 

 sexually mature in the intestines, and passes its larval 

 state encysted in the muscle of the same individual. 

 Many remarkable cases of these complicated life- 

 histories could be given, but space forbids ; one other 

 instance must suffice, namely, the group of Eel- 

 worms, or Anguilhdidce, which are parasites not in 

 animals but plants. Thus it will be seen that the 

 life-histories of Nematodes are very varied. 



