VARIOUS FORMS OF TAPEWORMS. 47 



The typical development is then a true metamorphosis : it is 

 only when many scolices are produced from the one emhryo 

 asexually that the development is a case of the remarkahle 

 "alternation of generations." The simplest development is 

 found in the genus Boihriocephalus, where it is direct, the 

 embryo becoming immediately a scolex. In some other forms 

 we find that in the Cysticercus stage the vesicle or bladder be- 

 comes smaU and is nearly lost ; no segments are formed, the 

 Cysticercus becomes converted into the Cystioercoid form, in 

 which that portion of the scolex bearing the hooks is separate 

 from that part with the suckers. 



Positions in which Tapeworms and their Cysts 

 and Ova are found. 



1. Cysts are found in the liver, lungs, muscles, brain, spinal 

 cord, kidneys, eyes, thoracic cavity, body cavity generally, and 

 in the hepatic ducts. 



2. Tapeworms are found only in the small and large intestines. 



3. The ova are met with on the ground, on grass, on dung- 

 hills, and in water. 



The cyst stage is, as a rule, found in herbivorous animals, but 

 also in man, fish, and in various species of insects. The dog, 

 for instance, has one of its tapeworms living in the cystic stage 

 in the louse (THchodectes). 



Various Forms of Tapeworms of Economic Importance. 



Coenurus cerebralis = Tcenia ccenurus 

 (which causes the Sturdy, Gid, or Staggers in sheep). 



In dogs, especially in sheep-dogs, we commonly find a tape- 

 worm known as Tcenia eaenurus (fig. 12, c). This worm, which 

 may also exist in other animals, such as the fox, lives hooked on 



