186 THE ANATOMY OF INVERTEBRATED ANIMALS. 



out, and the embryo has the form of a phial, of which the 

 evaginated caecum forms the neck. Round its apex are the 

 hooks, and below these the suckers, forming a complete ces- 

 toid head ; while the sac answers to the body of the phial. 

 The original hooks of the embryo are cast off in the course 

 of the process. 



If the eggs of the Tape-worm have passed into the aliment- 

 ary canal of an animal in which the worm is unable to attain 

 its sexual condition, the hooked embryo, as soon as it is 

 hatched, bores its way through the walls of the alimentary 

 canal, and eventually becomes lodged in the connective tissue 

 between the muscles, or in the liver, or in the brain or eye. 

 Here it goes through the changes which have been described, 

 and, generally, the sac undergoes very great dilatation. The 

 region of the' wall of the sac to which the cestoid head is at- 

 tached becomes invaginated, and thus is inclosed within a 

 chamber, the parietes of which are really constituted by the 

 outside of its own body. In this condition, the animal is 

 what is termed a Cystic worm, or bladder-worm ; and when 

 there is only one head it is a Cysticercus. In the genera 

 Coenurus and Eohinococcus the cystic worm has many heads; 

 and, in Echinococcus, the structure of the cystic worm is still 

 further complicated by its proliferation, the result of which is 

 the formation of many bladder-worms inclosed one within 

 the other, and contained in a strong laminated sac or cyst, ap- 

 parently of a chitinous nature, secreted by the parasite (Fig. 

 48). 



In the cystic condition, the Tape-worms never acquire 

 sexual organs ; but, if transported into the alimentary canal 

 of their appropriate hosts, the heads become detached from 

 the cysts, and, rapidly growing, give rise to segments, which 

 become sexual proglottides. The Tape-worms are rarely met 

 with in both the cystic and cestoid conditions in the same 

 animal ; but the cystic form is found in some creature which 

 serves as prey to the animal in which the cestoid form occurs. 

 Thus: 



Cystic Form. Cestoid Form. 



Cysticercus cellulosce. Tcenia solium. 



(Muscles of the Pig) (Man) 



Cysticercus 1 Tcenia mediocanellata. 



(Muscles of the Ox) (Man) 



Cysticercus pisiformis. Tcenia serrata. 



(Liver of the Rabbit) (Dog, Fox) 



