ENCYSTATION OF ENTOZOA. 



19 



regarded as really an integral part of their host ; they are quite com- 

 parable, in respect of the way in which they are nourished (and 

 breathe), with a cell, or an embryo. They manufacture their food in 

 a precisely similar manner out of the juices surrounding them, which, 

 by chemical change, minister to the conditions of their life and growth 

 and remove their waste products. 



The presence of a mouth and intestine is not, however, rendered 

 superfluous by the universality of this method of taking in food by 

 endosmosis ; they not only enable their possessor to feed upon other 

 semi-fluid or solid matters, 1 but also serve the purpose of increasing 

 the absorbent surface in cases where solid food-matter is not utilised. 



All that has been said hitherto refers to Entozoa that live in 

 absolute contact with the tissues of their host, which is generally, 

 but by no means always, the case. In the parenchymatous organs, a 

 membranous cyst usually surrounds and isolates the parasite, with 

 which it has no direct connection. It is a part of the infected organ, 

 a hypertrophy of the surrounding connective tissue, which gradually 

 encloses the parasite completely; a similar cyst is, indeed, formed 

 round other foreign bodies introduced into the organ, and becomes 

 very like a serous membrane, owing to the development on its free 

 surface of a more or less thick epithelial layer (endothelium). 

 (Figs. 12-14) 



FIG. 12. Cysticercus p isiformis 

 (young). 



FIG. 13. Eckinococcus. 



This capsule is regarded, and no doubt rightly, as an organ for the 

 protection of the infected part ; but, at the same time, it must not be 



1 How great an influence the quality and abundance of the food has upon the parasite 

 is strikingly shown in the case of Polystomum integerrimum. This Trematode usually 

 inhabits for a short time the gill cavity of tadpoles, and then wanders into the bladder, 

 where it becomes sexually mature in about four years ; if it remain longer in the branchial 

 cavity, it only takes twenty-seven days to reach sexual maturity . It is not merely the 



