20 OCCURRENCE OF PARASITES. 



forgotten that it is of no less importance for the nourishment of the 

 contained parasite. The blood-vessels which traverse the capsule, 

 and occasionally form a definite system with afferent and efferent 

 vessels, supply fluid nutriment, which is absorbed by the parasite 

 through its mouth or skin, and which varies in quality according to 

 the structure of the capsule. On the whole, it appears that worms 

 encysted in parenchymatous organs do not receive a great deal of 

 nutriment, since they often remain unchanged for years, and even 

 longer periods, while the same worms, under other circumstances 

 by migration to the intestine, for instance rapidly grow and undergo 

 further development This capsule is most conspicuous in the so- 

 called bladder-worms, especially in those which grow to a large size, 

 and inhabit organs rich in connective tissue. In these cases (Echino- 

 coccv.s) the cyst becomes occasionally several millimetres in thickness, 

 and so firm that it can be easily removed without injury from the 



surrounding parenchyma. (Fig. 13.) 

 Traces of a cyst are, however, found 

 in all worms which remain for any 

 length of time in parenchymatous 

 organs, even when they only attain 

 to a small size. In these cases, how- 

 ever, the hypertrophy of the con- 

 nective tissue, caused apparently by 

 the irritation set up through the 

 presence of the parasite, can hardly 

 Fio. 14. Sclerostomum tetracanthum, be recognised as a continuous (inde- 



encysted. pendent) cyst. 



Many worms which inhabit parenchymatous organs secrete on the 

 inner surface of their connective tissue capsule a cuticular cyst, which 

 is, of course, sharply marked off from the former by its histological 

 structure. 



It appears in the form of a homogeneous membrane, consisting of 

 concentric layers ; it resists the action of alkalies, and belongs, ap- 

 parently, to the chitinous formation so generally met with in the 

 lower animals. 1 This chitinous cyst is most usually found in the 

 Trematodes, but is not wholly absent in the other groups, being 

 found, for example, in Tetrarhynclius> which lives in fishes, and even 

 in the muscle- Trichina (Fig. 15), the cysts of which are nothing 



rapidity of development which characterises these individuals ; they differ also from the 

 common form in a number of anatomical peculiarities, notably by the absence of copulatory 

 organs. See the interesting observations of Zeller, Zetischr. f. wiss. Zod., Bd. xxvii., p. 

 238 et seq., 1876. 



i Waldenburg is of opinion that this chitinous capsule is in some cases formed by 

 the host. See Archivf. pathd. Anat. u. Physiol., Bd. xxiv., p. 157, 1862. 



