DEVELOPMENT OF THE TAPEWORMS 



299 



swum about in water by its means for a week or so. Sooner or later, 

 however, all the oncospheres leave the host that harbours the parental 

 tapeworm and reach the open, either still enclosed in the uterus of 

 the evacuated proglottids, after the disintegration of which they 

 then become free, or after being deposited as eggs in the intestine 

 of the host; they then leave it with the faeces. In the former case 

 also, the slightest injury to the mature proglottids while still in the 

 intestine suffices to allow a part of the oncospheres in their embryo- 

 phores to be released and mingled with the fasces. Here they are the 

 generally, but falsely, so-called Taeniae " eggs." For, as stated above, 

 the "yolk" envelope and the true shell deposited in the ootype have 

 before this disintegrated. 



In other cases, e.g., Hymenolepis spp., the uterine (ootype) shell 

 persists in faeces (fig. 230). 



In any case the oncospheres 

 must be transmitted into suitable 

 animals to effect their further de- 

 velopment ; in only very rare cases 

 might an active invasion be possible, 

 as, for instance, takes place with the 

 miracidia of many Trematodes. The 

 entry into an animal is, as a rule, 

 entirely passive, that is to say, the 

 oncospheres are swallowed with the 

 food or water. Many animals are 

 coprophagous and ingest the onco- 

 spheres direct with the faeces ; others 

 swallow them with water, mud, or 



food contaminated by such faeces. Infection is easily produced 

 artificially by feeding suitable animals with mature proglottids of 

 certain Cestodes or introducing the oncospheres with the food. As 

 the mature tapeworm frequently finds the conditions suitable for its 

 development in only one species of host, or in species nearly related, 

 and perishes when artificially introduced into other hosts, experiment 

 has taught us that to succeed in cultivating the oncospheres certain 

 species of animals are necessary. Thus we are aware that the onco- 

 spheres of Tcenia solinm, which lives in the intestine of man, develop 

 only in the pig, and only quite exceptionally develop into the stage 

 characteristic of all Cestodes the cysticercus in the wide sense of the 

 word in a few other mammals. The oncospheres of T. saginaia 

 develop further only in the ox ; those of T. inarginata (of the dog) in 

 the pig, goat, and sheep; those of T. scrrata (of the dog) in hares and 

 rabbits ; those of Dipylidiiim caninum (of the dog and cat) in parasitic 

 insects of the dog and cat, etc. It is not unusual that young animals 



FIG. 197. a., oncosphere, in its 

 radially striated embryophore (errone- 

 ously termed egg-shell) of Tcenia 

 africana. Greatly magnified. (After 

 von Linstow.) ., freed oncosphere of 

 Dipylidium caninum. (After Grassi 

 and Rovelli.) Both oncospheres show 

 six spines. 



