ANIMAL AXD VEGETABLE PARASITES. 535 



mucous canals. Thus the Coenurus cerebralis is found imbedded 

 in the substance of the brain, the Trichina spiralis between the 

 fibres of the voluntary muscles, and the Cysticercus cellulosse in the 

 areolar tissue of various parts of the body. They are also distin- 

 guished from all other parasites by two peculiar characters. First, 

 they are inclosed in a distinct cyst, with which they have no organic 

 connection and from which they may be readily separated ; and se- 

 condly, they have no genera- 

 tive organs, nor is there any Fj g- no. 



apparent difference between 

 the sexes. The Trichina spi- 

 ralis, for example (Fig. 170), 

 is inclosed in an ovoid or 

 spindle-shaped cyst, swollen 

 in the middle and tapering at 

 each extremity, with a round- 



f* TKICHIX A SPIRAMS: from rictus femoris uius- 



ed Cavity in itS Central por- cle uf human subject. Maguified 57 diameters. 



tion, in which the worm lies 



coiled up in a spiral form. The worm itself has neither testicles 



nor ovaries, nor does it present any trace of a sexual organization. 



Now we have seen that it is easy to account for the conveyance 

 of these or any other parasites into the interior of vascular organs 

 and tissues; the eggs from which they are produced being trans- 

 ported by the bloodvessels to any part of the body, and there 

 retained by a local arrest of the capillary circulation. In the case 

 of the encysted entozoa, however, we have a much greater diffi- 

 culty ; since these parasites are entirely without sexual organs or 

 generative apparatus of any sort, nor have they ever been dis- 

 covered in the act of producing eggs, or of developing in any 

 manner a progeny similar to themselves. It appears, accordingly, 

 difficult to understand how animals, which are without a sexual 

 apparatus, should have been produced by sexual generation. As 

 it is certain that they can have no progeny, it would seem equally 

 evident that they must have been produced without a parentage. 



This difficulty, however, serious as it at first appears, is susceptible 

 of a very simple explanation. The case is in many respects analogous 

 to that of the maggots, hatched from the eggs of flies in putrefying 

 meat. These maggots are also without sexual organs ; for they 

 are still imperfectly developed, and in a kind of embryonic condi- 

 tion. It is only after their metamorphosis into perfect insects, that 

 generative organs are developed and a distinction between the 



