26 INTRODUCTION. 



perfect, but is either more or less altered, or else entirely 

 destroyed. 



Other encysted intestinal worms succeed in obtaining nourish- 

 ment through the walls of thek prison, and thus go on growing. 

 Those, however, amongst the encysted entozoa, which are in- 

 tended by nature to attain their sexual maturity only in the 

 digestive organs of certain animals, cannot arrive at this condition 

 in their cysts, and must, in spite of their further growth, fail in 

 the attainment of the power of sexual propagation, until the 

 animal they inhabit is devoured by the predacious creature, 

 whose intestine is alone fitted to allow of the passage of these 

 asexual intestinal worms into the last stage of their development. 

 I may cite here, as examples, various Nematoidea and Cestoidea. 

 In many marine fishes the liver is covered with capsules, which 

 often contain a well-grown nematoid worm more than an inch 

 long. Naturalists have arranged this parasite among the in- 

 testinal worms as Ascaris capsularis, Filaria piscium, Filaria 

 cystica. I have never met with one of these round worms con- 

 taining developed sexual organs. As in their further organization 

 no less than in their whole form, these ascarids most strikingly 

 resemble certain sexually-mature nematoid worms, namely, 

 Ascaris osculata, spiculigera, angulata, aucta, and others, which 

 infest the alimentary canal of seals, cormorants, divers, gulls, and 



predacious fishes, the idea presents 

 Fig. 17. itself that these encysted, not yet 



fully developed Nematoidea belong to 

 either one species or another of the 

 last-named Ascaridae. More par- 

 ticular inquiries into the subject will 

 instruct us what species of these 

 round worms, which are now con- 

 sidered to be distinct species, will 

 hereafter have to be united into a 



single group, as younger or older individuals of one and the 

 same species. The sexless Ascaris incisa, represented in fig. 17, 



Fig. 17. A convoluted piece of the intestine of the mole (nat. size), with many flat- 

 tened, pedunculated cysts, each enclosing a little thread- worm, attached to its peritoneal 

 investment. * * Such cysts viewed edgewise, b. A single capsule much magnified, 

 so as to render the enclosed thread-worm more clearly visible. This parasite belongs to 

 that group of the Ascaridce whose intestine is provided anteriorly with a ca3cum directed 

 upwards. 



