CESTODES 191 



the body of the host. It imbeds itself in a muscle or other organs and 

 develops into a characteristic larva called a bladder worm. This larva 

 is in some cases exactly like the scolex of the adult worm and is then 

 called a plerocercus. In other cases it con- 

 tains besides the scolex some or all of the 

 strobila, sometimes with the genital organs, 

 and is called a plerocercoid. In still others 

 it is an ovoid vesicle filled with a fluid and 



containing one or more scolices and no ^ig. 311 ^A, Egg of Twnia 



proglottids and is called a cysticercus, or ffiked^'^emb^JoY^Bf same ?gg 

 a minute vesicle completely filled with an wUhout^the external membrane 



invaginated scolex and called a cysticer- 



coid. This larva remains quiescent in the intermediate host, but if 

 this animal be devoured by the final host the larva is transferred to the 

 intestine of the latter and, at once attaching itself to the intestinal wall, 

 begins to produce the strobila. In a few weeks or months the entire worm 

 is usually formed. 



Cestodes are among the most pronounced animal parasites and are 

 found in all comitries and in all of the larger animals. Man and his 

 domestic animals are especially liable to infection and may be the hosts 

 of some very dangerous tapeworms. 



History.— The common tapeworms of man, including both the adult 

 and the larval worm, have been known to science from the time of the 

 Greeks. The name Tcenia for a tapeworm occurs in Pliny, and the name 

 Tcenia solium has been employed since the Middle Ages, when it was given 

 to all the common tapeworms. The order Cestodes was established in 1808 

 by Rudolphi. It inckided however only the adult worms, the larval worms 

 being placed by Eudolphi in the separate order Cystici. The relation of 

 the adult to the lan^al worms was not then understood, notwithstanding 

 the fact that Goeze and Pallas in the previous centuiy had both clearly 

 indicated it, Rudolphi and all the important helmmthologists of his time 

 believing that tapeworms may develop by spontaneous generation in the 

 places where they are found. In fact it" was not until 1851 that it was 

 finally demonstrated by Klichenmeister that the bladder worm is the larval 

 stage of the adult worm. This investigator showed that if Cysticercus 

 pisiformis be fed to a dog Tcenia serrata will veiy shortly appear in the 

 dog's intestine. He and others afterwards repeated the experiment with 

 the tapeworms of the cat and of man and of other animals. In the case 

 of the human tapeworms the cystieerci were given to condemned criminals 

 and the adult worms were invariably found in their intestines after 

 death. Rudolf Leuckart has since been the most active in the study of the 

 entire group. 



