576 REPRODUCTION. 



find their way out singly by the anus, or are discharged with the 

 evacuations. They have, while living, considerable power of con- 

 tractility and locomotion ; and thus become transferred to neighboring- 

 vegetable substances, which are devoured by the pig. In the pig's 

 stomach and intestine, the substance of the articulation is digested ; 

 but the embryos, which are but little over 30 mmm. in diameter, and 

 armed with movable calcareous spines, make their way through the 

 intestinal walls, and are thence dispersed, either by active locomotion 

 or by the circulating blood, throughout the connective tissue. Here 

 they become encysted, and go through with a partial development, until 

 they acquire the form of cysticercus. In this condition they remain 

 until the flesh of the pig is used for food, when they are transformed, 

 as above described, into taenia solium, thus reproducing the original 

 form of the parasite. A similar relation has been found by Kiichen- 

 meister and Siebold between certain other species of taenia and cysti- 

 cercus. 



2. Trichina spiralis. This is an encysted, worm-like parasite, found 

 in the muscular tissue of the pig, and sometimes in that of the rat, the 



cat, and the human species. Each 

 FIG. 154. worm lies spirally coiled within its 



enveloping cyst. It is about 0.15 

 millimetre in length, with a tapering 

 anterior and a rounded posterior ex- 

 tremity. It has a nearly straight 

 intestine and rudimentary sexual 

 TRICHINA SPIRALIS, encysted, from muscu- organs. It has long been recognized 



lar tissue of a trichinous cat. Magnified ' ., . ,, c 



76 diameters. as an occasional parasite in the mus- 



cular tissue of man ; but it is only 



since the investigations of Leuckart* and Pagenstecherf that the his- 

 tory of its growth and development has been made known. If muscu- 

 lar flesh containing encysted trichinae be administered to a rabbit, cat, 

 rat, mouse, or pig, the cysts are digested and the worms liberated in 

 the intestine. Here they rapidly increase in development ; the females 

 becoming impregnated and filled with living young, and attaining, at 

 the end of a fortnight, three or four times their previous size. The 

 embryos are now discharged into the cavity of the intestine ; after 

 which they penetrate the intestinal walls, and thence disperse through- 

 out the body. On reaching the muscular tissue, they become encysted, 

 and thus remain quiescent until introduced into the intestine of another 

 animal or of man. The existence of such sexless and encysted parasites 

 is therefore analogous to that of the caterpillar or the maggot. They 

 are sexless, because they are still in the embryonic condition. But 

 they have been produced by generation from parents ; and they will, 

 at a subsequent period, themselves produce young by the same process. 



* Untersuchungen iiber Trichina spiralis. Leipzig und Heidelberg, 1860. 

 f Die Trichinen. Leipzig, 1866. 



