272 



THE ANIMAL PARASITES OF MAN 



species. In many the conveyance into the definitive host is effected 

 direct after the larvae have developed within the eggs ; thus, for 

 instance, the feeding of suitable animals with the embryo-containing 

 eggs of species of Trichocephalus and Ascaris, leads to the settle- 

 ment of the worms in the intestine ; for the young Trichocephali 

 or Ascarides only leave the egg shell when they have attained the 

 intestine of the final host, in which they become adult. 



In other cases (Dochmius, Sclerostomum, Strongylus, &c.), the 

 larvae hatch in the open, and live for a time free in another shape ; 

 they grow, cast their skin, and finally gain the intestine of the 



host by means of water or 

 some other vehicle, when they 

 lose their larval characters and 

 assume the structure of the 

 parents. 



Frequently, however, the 

 larvae of Nematodes make 

 use of one or even two inter- 

 mediary hosts ; their condition 

 then resembles that of Ces- 

 todes or Trematodes, except- 

 ing that there is never a 

 multiplication within the in- 

 termediary hosts. The larvae 

 become encapsulated amongst 

 the tissues of the intermediary 

 host, and wait till they are 

 introduced with the latter into 



FIG. 190. A piece of the trunk mus- the final hosti For ms tance, 

 cular system of t the pig with capsules 

 containing trichinae (magnified). OllulcinilS tricUSplS, the adult 



form of which is found in 



cats, lives previously encysted in the muscular system of mice. 

 Cucullanus elegans, which attains the adult stage in fishes (Perch, 

 &c.), is found encysted in species of Cyclops. 



!J Peculiar conditions prevail in Trichinella spiralis. This species, 

 which in its adult state lives in the intestine of man and of 

 various mammals, is viviparous ; the young Trichinae, however, 

 do not leave the intestine, but attain the intestinal wall (Cerfon- 

 taine, Askanazy). The female intestinal Trichinae bore themselves 

 into the intestinal wall, where they are found within the sub- 

 mucosa, or in the lumen of the dilated lacteal vessels. Here the 

 young are born, within the intestinal wall, and leave this posi- 



