CESTODES : BIOLOGY 



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cysticerci, as, for instance, the mother or daughter cyst, and a 

 piece, sometimes segmented, that is present between the latter and 

 the scolex (fig. 76), with the exception of the scolex or scolices, die 

 off, are digested, absorbed, or perhaps even evacuated. It is not 

 certain whether the cysticerci of the Bothriocephalina lose any part, 

 probably they do not. 



The time required by the scolex to complete the entire chain 

 of proglottides does not depend only on the number it has to 

 produce, for Tcenia echinococcus, which, as a rule, only possesses 

 three or four segments, takes quite as long a time for their 

 growth (eleven to twelve weeks) as Tcenia solium with its 

 numerous segments ; Tcenia ccenurus is fully developed in three 

 to four weeks, and the same holds good for Dibothriocephalus latus, 

 which possesses many more segments than the above-mentioned 

 taenia of the dog. In a number of species it has been possible 

 to follow the average daily growth and register it almost accurately ; 

 for instance, in Dibothriocephalus latus the daily growth is 8 cm., 

 in Tcenia saginata, 7 cm., &c. 



The history of the development of the cestodes demonstrates 

 that human beings and animals that harbour tapeworms in their 

 intestines acquire them by ingesting the respective cysticercus stages 

 living in other animals (mostly with food) ; it also shows that persons 

 and beasts harbouring these cysticerci may have become infected 

 by having swallowed the oncospheres of the species of tapeworm 

 to which they belong. In regard to Tcenia murina alone, it* is 

 known that the introduction of the oncospheres into those species 

 of animals which may harbour the adult tapeworm leads to the 

 formation of the latter after the development of a cysticercus stage 

 in the intestinal wall ; nevertheless only young animals (rats) are 

 capable of infection, for a previous infection, or the presence of 

 mature tapeworms in the intestine, appears to produce a kind of 

 immunity. 



BIOLOGY. 



In their adult stage, the tapeworms inhabit almost exclusively 

 the alimentary canal of vertebrate animals, the small intestine 

 being the chief abode with but few exceptions ; a few species 

 select definite parts of the intestine. A small number of Rhyn- 

 thobothriidse of marine fishes live apparently always in the 

 stomach, while in rays and sharks the spiral intestine is their 

 special seat. Bothriocephali generally attach themselves with 



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