NATURE OF REPRODUCTION. 675 



and most mature articulations separate spontaneously from the rest of 

 the colony, and either find their way out by the anus singly, or are dis- 

 charged with the evacuations. They have, while still living, a con- 

 siderable degree of contractility and power of locomotion ; and thus 

 become accidentally transferred to the surface of neighboring vegetable 

 matters, and are devoured by the pig with his food. In the stomach 

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

 but the embryos, which are 33 mmm. in diameter, and armed with three 

 pairs of calcareous spines, make their way through the intestinal walls, 

 and thence are dispersed, either by a continuance of the same movement 

 or by the bloodvessels, throughout the connective tissue, where they 

 are afterward found. Here they become encysted, and go through with 

 a partial development, remaining in the condition of C} T sticerus in the 

 flesh of the pig until this flesh is used for food, when they finally be- 

 come converted into tsenia solium. Thus the entire round of generation 

 and development is completed, and the original form of the parasite 

 reproduced. A similar relation has been shown by Kiichenmeister 

 and Siebold 1 to exist between certain other species of taenia and cysti- 

 cercus. 



2. Trichina spiralis This is a sexless, encysted, worm-like para- 

 site, found in the muscular tissue of the pig, and sometimes in that of 

 the rat, the cat, and the human species. Each worm lies closely coiled, 

 in a spiral form, in the interior of its 



enveloping cyst. It is about 0.75 Fig. 216. 



millimetre in length, of a tapering 

 form, with a slender anterior and 

 rounded posterior extremity. It pre- 

 sents a nearly straight intestine ex- 

 tending through its whole length, 

 and rudimentary sexual organs which 

 are entirely inactive. The worm has TRICHINA SPIRALIS, encysted, 



from muscular tissue of a trichinous cat. 

 been known Since 1835, as Occasion- Magnified 76 diameters. 



ally found in the human muscular 



tissue in the encysted form; but it is only since 1860, principally from 

 the investigations of Leuckart, 2 that the different stages of its growth 

 and development have been made known. If muscular flesh containing 

 encysted trichinae be administered with the food to a rabbit, cat, rat, 

 mouse, or pig, the cysts become digested and the worms liberated in 

 the small intestine. Here they rapidly increase in size and develop- 

 ment, the females becoming impregnated and filled with living young, 

 and attaining, at the end of a fortnight, three or four times their pre- 

 vious size. The young emb^os are now discharged from the body of 

 the parent, make their way through the walls of the intestine, and are 



1 Yon Siebold, On Tape and Cystic Worms. Sydenham edition. London, 

 1857, p. 57. 



2 Untersuclmngen liber Trichina spiralis. Leipzig und Heidelberg, 1860. 



