DEVELOPMENT OF CESTODES. 53 



segments, formed by an asexual budding in the longitudinal 

 axis, and resulting in the formation of a mature sexual cestode 

 or tapeworm, which may in some species reach as much as thirty 

 feet in length. At one time tapeworms were considered colonial 

 animals, each proglottis being a distinct individual ; but since 

 it is the soolex which grows, we must consider that the whole 

 is an individual, and that the individuality of the proglottis is 

 subordinate to it, — and, moreover, we find one excretory and 

 one nervous system common to the whole. 



The typical development is then a true metamorphosis : it is 

 only when many scolices are produced from the one embryo 

 asexually that the development is a case of the remarkable 

 " alternation of generations." The simplest development is 

 found in the genus Bothriocephalus, where it is more or less 

 direct, the embryo becoming immediately a scolex. In some 

 other forms we find that in the Cysticercus stage the vesicle or 

 bladder becomes small and is nearly lost ; no segments are 

 formed, the Cysticercus becomes converted into the Cysticercoid 

 form, in which that portion of the scolex bearing the hooks is 

 separate from that part with the suckers. 



Positions in nhicJi 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 

 and stomach. 



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 lonse (Trichodectes). 



