OF INTESTINAL WOKMS. 25 



"V 



or else, having found a resting place in some organ, were tarrying 

 till the creature they infested should be swallowed by some other 

 animal, when the passive immigration for which they waited 

 would take place. 



Many wandering parasites are unresistingly suffered to bore 

 their way into and remain in, the organs of animals, whilst 

 on the other hand, certain kinds are arrested and finally stopped, 

 by becoming enclosed in a coagulable lymph thrown out by the 

 organs which they traverse. Hereafter we shall have to dis- 

 tinguish two kinds of encysted intestinal worms. In the one 

 kind the cyst is thrown out by the parasite itself, as I have 

 already explained in the case of the Cercaria ; in the other, the 

 organ in which the encysted parasite lies imbedded, furnishes the 

 walls of the cyst. These last ' ' extrinsic" cysts are easily recognised 

 in the passively encysted parasites of vertebrate animals, being 

 immediately and intimately connected with the neighbouring 

 tissues and traversed by blood-vessels. 



In such capsules or cysts are found the most diverse kinds 

 of intestinal worms, whose further course may be very 

 various. 



Many of the encysted young of the intestinal worms experience 

 no further change, but only remain for a longer or a shorter period 

 until such time as they may, together with their host, pass into the 

 intestine of some animal of prey suitable for their future develop- 

 ment. To this kind belong the Cercarice I have already mentioned 

 (page 20). There is also a small, imperfectly developed, round 

 worm, hitherto always erroneously described as a perfect intesti- 

 nal worm, under the name of Trichina spiralis, which remains a 

 long time in its cyst without either growing or developing sexual 

 organs. This minute Trichina spiralis is not only met with in 

 the substance of the muscles of man, but also in the pleura and 

 peritoneum of the most widely different kinds of vertebrate 

 animals, enclosed in oval capsules about a quarter of a line in 

 length. Most probably a certain time of imprisonment is allotted 

 to the little worm, and after this period has elapsed, should its 

 deliverance not be effected by passive emigration, it dies, and its 

 body, which has not in the least increased in size, is, without 

 changing its outward form, transformed into a brittle glassy mass 

 composed of carbonate of lime. This process of calcareous de- 

 generation also takes place in other encysted and dead intestinal 

 worms, in which, however, the form does not always remain 



