306 THE ANIMAL PARASITES OP^ MAN 



The time required by the scolex to complete the entire chain 

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

 produce, for Tania echinococais, which, as a rule, only possesses 

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

 (eleven to twelve weeks) as T. solium with its numerous seg- 

 ments ; T. 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 determine fairly 

 accurately the average daily growth ; for instance, in Dibothriocephalus 

 latus the daily growth is 8 cm., in Tccnia saginata 7 cm., etc. 



The history of the development of the Cestodes demonstrates that 

 persons and beasts harbouring larval tapeworms have become 

 infected by having swallowed the oncospheres of the species of 

 tapeworm to which they belong. In regard to Hymenolepis murina 

 alone, it is known that the introduction of the oncospheres into those 

 species of animals which harbour the adult tapeworm leads to the 

 formation of the latter after the development of a larval stage in the 

 intestinal wall ; nevertheless, only young animals (rats) are capable of 

 infection, for a previous infection, or the presence of mature tape- 

 worms 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, with but few exceptions 

 the small intestine, and a few species select definite parts of it. A 

 small number of Rhynchobothriida of marine fishes live apparently 

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

 their exclusive site. Bothriocephali generally attach themselves with 

 their head on to the appendices of the pylorus of fishes ; other 

 species (Hymenolepis diminuta) occasionally fix their head in the 

 ductus choledochus, and this is more frequent still in the tapeworms 

 of the rock badger (Hyrax), which occasionally penetrate entirely 

 into the biliary ducts. Stilesia hepatica, Wolffh., has so far only been 

 found in the bile-ducts of its host (sheep and goat, East Africa). 



In the disease of sheep induced by Cestodes, the worms have 

 been observed also in the pancreas. Specimens found in the large 

 intestines were probably being evacuated. 



The Cestodes are looked upon as fairly inert creatures, this 

 opinion having been formed by observing their condition in the cold 

 cadavers of warm-blooded animals. Actually, however, they are 

 exceedingly active, and accomplish local movements within the 

 intestine, for they have been found in the ducts communicating with 



