CESTODES : GENERATIVE ORGANS 203 



suitable animals to enable their further development ; in only 

 very rare cases an active invasion might 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 oncospheres 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 proglottides of certain cestodes or intro- 

 ducing 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, experience has taught us 

 that to succeed in cultivating the oncospheres certain species of 

 animals are necessary. Thus we are aware that the oncospheres of 

 Tcenia solium, 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 a few other mam- 

 mals. The oncospheres of Tcenia - saginata develop further only 

 in the ox, those of Tcenia marginata (of the dog) in the pig and 

 sheep ; those of Tcenia serrata (of the dog) in hares and rabbits ; 

 those of Dipylidium caninum (of the dog and cat) in parasitic 

 insects of the dog and cat, &c., &c. It is not unusual that young 

 animals only appear to be capable of infection, while older animals 

 of the same species are not so. 



Once introduced into a suitable animal, which is only excep- 

 tionally the same individual or belongs to the same species to 

 the one which harbours the adult tapeworm, the oncosphere passes 

 into the cysticercus stage common to all cestodes, but varying in 

 structure according to the species ; in the simplest case such a 

 cysticercus resembles the scolex of the corresponding tapeworm, as 

 in Dibothriocephalus, only that the head, provided with suckers, is 

 retracted within the forepart of the neck. The conditions appear 

 to be similar in Ligula, Schistocephalml Tricenophorus, but here the 

 cysticercus is very large, indeed, as large in the first-mentioned 

 genera as the tapeworm originating from it, and the sexual 

 organs are already outlined ; doubtless, however, this stage is 

 preceded by one that corresponds to the scolex of the genus in 

 question, and which represents the actual cysticercus stage. In 

 such cases the development of the body of the tapeworm from the 

 scolex already sets in within the preceding or intermediary host ; 

 in other cases, except in the single-jointed cestodes, this only 



