WANDERINGS WITHIN THE HOST. 67 



to follow a more complicated path. The young of Trichina, for 

 example, perforate the intestinal wall, and bore their way into 

 the surrounding organs or tissues. The same holds good for many 

 species of Tcenia, Echinorhynchus, and Pentastomum, whose develop- 

 ment I have traced, and numerous thread- worms Spiroptera murina, 

 Ascaris incisa, Sclerostomum equinum, &c. If we recall and com- 

 pare with these facts the additional fact that the larvae of Distomum, 

 Boihriocephalus, &c. bore their way from the exterior into the body 

 of their host, and make their way into certain definite localities, we 

 may state, in a general way, that the embryos of Entozoa which have 

 found their way into the body of some host do not at once become quiescent, 

 but continue their wanderings, and traverse in various directions the 

 tissues and organs of its body. 1 



These wanderings are facilitated by the minute size and often elon- 

 gated needle-shaped body of the parasite, or by the possession of a 

 boring apparatus. It is, in fact, no harder for a Nematode to make its 

 way through the tissues of an animal than for a bird to move through 

 a thick covert, or a dog through a cornfield, and they leave as little 

 trace of their progress, inasmuch as they rather push between than 

 actually tear their way through the tissues. 



The wanderings of parasites in the larger animals are also often 

 assisted by their getting into the blood-vessels, and so being carried 

 into the remotest parts of the body. Many of them even live for a 

 time as Hematozoa, e.g., the embryos of certain Filarice (p. 49). In a 

 few cases the presence of embryos of Tcenia in the blood has been 

 actually observed (Leuckart, Eaum) ; in other cases, it has been sus- 

 pected from the wide and uniform distribution of the parasites in the 

 body of the host. This conclusion is, however, quite a necessary one, 

 for my researches on Trichina have proved that the connective tissues 



1 If such a migration take place into a pregnant female, the young Entozoa may 

 reach the body of the embryos. Ley dig (Mutter's Archiv f. Anat. u. Physiol., p. 227, 1851) 

 observed in the blood of Mustdus Icevis and its foetus the same Filarice. However, this 

 does not seem to occur always, since in the Mammalia the transference of Nematode 

 Haematozoa to the foetus has not been demonstrated (Chaussat). The wandering embryos 

 of Trichina avoid the body of the foetus. On the other hand, I found in a pregnant 

 Lacerta agilis that nearly all the embryos nine out of twelve contained active sexless 

 Nematodes in the pericardial cavity, in the cavities of the brain and spinal cord, and in 

 the amniotic fluid. Most of the embryos harboured two or three parasites, or even four, 

 and in different parts, without showing the least traces of how the worms made their way 

 in. In the organs of the mother I could not find any of the parasites, nor even the 

 sexual worms which had produced them. Rathke, I find, anticipated me in this obser- 

 vation (Archiv f. Naturgesch., Jahrg. iii., Bd. i., p. 335, 1837). The presence of Entozoa 

 in embryos under such circumstances need excite no wonder ; but the older assertions, 

 according to which the embryos occasionally harboured sexually mature Helminths in the 

 intestine and liver, seem most suspicious (Davaine, loc. cit., p. 11). 



