300 THE ANIMAL PARASITES OF MAN 



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 as the 

 one which harbours the adult tapeworm, the oncosphere passes 

 into the larval stage common to all Cestodes, but varying in structure 

 according to the species. In the simplest case as, e.g., in Dibothrio- 

 cepliahis such a larva resembles the scolex of the corresponding 

 tapeworm, only that the head, provided with suckers, is retracted 

 within the fore-part of the neck. Such a larval form is known as a 

 plerocercoid (7^77/9779, full ; /eepicos, tail). They differ from the cysticer- 

 coids in being solid larval forms, elongated, tape-like or oval, with the 

 head invaginated. The conditions appear to be similar in Li^nla, 

 Scliistocephaliis, Tricenoplionis, but here the larvae are very large, indeed 

 as large iii the first-mentioned genera as the tapeworms originating 

 from them, 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 larval stage. 

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

 scolex has already begun within the first or intermediate host ; in 

 other cases, except in the single-jointed (monozootic) Cestodes, this 

 only takes place in the definitive host. The direct metamorphosis 

 of the oncosphere into the larval forms termed PLEROCERCOID has 

 hitherto not been investigated, although Ligula, Schist ocephalus and 

 Botlirioceplialus are very common parasites, but many circumstances 

 point to the conclusions arrived at by us and by other observers. In 

 the larval stages of other tapeworms we can always distinguish the 

 scolex and a caudal- like appendage, vesicular in the cysticerci (fig. 200), 

 compact in the cysticercoids (fig. 231). The scolex alone forms the 

 future tapeworm, the variously formed appendage perishing. 



It has now been proved that the appendage, the caudal vesicle, 

 originates direct from the body of the oncosphere, and therefore is 

 primary, and that the scolex only subsequently forms through pro- 

 liferation on the surface of this appendage. On account of this 

 origin the scolex is generally regarded as the daughter, and the part 

 usually designated as the appendage as the mother, originating from 

 the oncosphere. 



Accordingly, two modes of development of the larval stage may 

 be distinguished ; in the one case, plerocerci and plerocercoids, the 

 oncosphere changes directly into the scolex, thus forming the body 

 of the tapeworm within the primary host ; in the other case, cysticerci 

 and cysticercoids, the scolex only forms secondarily in the trans- 

 formed body of the oncosphere, which later on perishes, the scolex 

 alone remaining as the originator of the tapeworm colony. 



