OKIGIN OF INTESTINAL WORMS. 29 



whilst other individuals of the same brood, which have found 

 their way into the intestine of the same animals, arrive at 

 maturity. Tricenophorus nodulosus, infesting fishes, offers an 

 example of this, developing into a long, sexually-mature tape- 

 worm, in the intestines of pikes and perch, whilst at the same 

 time these fishes often harbour other tape-worms, which are, 

 however, always sexless, in cysts in their liver. These last must 

 certainly be also regarded as strayed parasites. 



In these wanderings through the bodies of vertebrate animals, 

 the very small embryos of the intestinal worms, boring their way 

 through the walls of the blood-vessels, not unfrequently fall into 

 the current of the circulation, and so become distributed with the 

 blood. In fact, embryos of intestinal worms, to which the name 

 of H&matozoa has been given, have often been discovered in the 

 blood of birds, reptiles, and fishes. 1 These Hamatozoa neither be- 

 come further developed in the blood, nor increase in size ; but many 

 of them, whilst circulating in the vascular system, stick in the narrow 

 blood-vessels of certain organs which afford a more congenial soil 

 for their further growth ; such at least is the most natural way 

 of accounting for the appearance of intestinal worms in the 

 brain, in the spinal marrow, and in the eyeball of man and 

 animals. These organs are so completely enclosed, partly by 

 bones, and partly by dense fibrous membranes, that before the 

 existence of animals in the blood was known, it was supposed 

 quite impossible for parasites to penetrate into such well pro- 

 tected organs ; but that they must have originated then and 

 there through equivocal generation. The Cysticercus cellulosce, 

 the Ccenurus cerebralis, and the Echinococcus hominis and Veteri- 

 norum, have long been known as occasional denizens of the brain 

 and of the spinal marrow in men and animals, and have, up 

 to the very latest times, served as a stronghold for the sup- 

 porters of the doctrine of equivocal generation. Having sub- 

 jected these very cystic worms to particularly close inves- 

 tigation, in order to confute this fabulous hypothesis as to 



1 I have collected together the different observations on haematozoa in the article 

 " Parasiten" in Wagner's ' Hand-worterbuch' already referred to (p. 648) ; subsequently, 

 new facts of the same kind have been published by Ecker (Mailer's 'Archiv.,' 1845, 

 p. 501), Wedl (in his ' Beitrage zu Lehre von den Haraatozoen,' Wien, 1849), and 

 Leydig (in Mailer's ' Archiv./ 1851, p. 227). 



[See also the remarkable observations of Bilharz, ' Ueber das Distoma hoematobium,' 

 ' Zeitschrift fur Wiss. Zoloogie,' 1852. This dioecious haematode is found in the portal 

 Hood of man.] [ED.] 



