v PHYLUM PLATYHELMINTHES 143 



the case of the ciliated embryo of the Trematodes, the egg 

 being received into the enteric canal of the second host with 

 the water or food. The digestive fluids of this second host 

 dissolve the egg-shell and set free the contained embryo, 

 which bores its way by means of its hooks to some part of 

 the body in which it is destined to pass through the next 

 phase of its life-history, and there becomes encysted (B). 

 The phase which follows presents two main varieties. In 

 cases in which the second host is an invertebrate animal, the 

 hooked embryo develops into a form to which the name of 

 Cysticercoid is given ; when, on the other hand, the inter- 

 mediate host is a vertebrate, the form assumed is nearly 

 always that termed Cysticercus or bladder-worm. In both 

 cases a tape- worm head is developed, with the rostellum, 

 hooks, and suckers of the adult. In the Cysticercus (C-H) 

 this is formed from the wall of a relatively large cyst or 

 bladder into which the hooked embryo develops. 



In a very small number both of Cysticercoids and of 

 Cysticerci more than one tape-worm head is formed. Thus 

 Tcznia ccenurus of the dog has a bladder-worm stage 

 occurring in the sheep and rabbit, which gives rise to several 

 tape-worm heads. But the most striking instance of mul- 

 tiple production of tape-worm heads in a bladder-worm is 

 Tcenia echinococcus, well known as the cause of the disease 

 termed hydatids, common in man and in various domestic 

 animals. In this case the hooked embryo develops into a 

 large mother-cyst, from the interior of which daughter-cysts 

 are budded off. Eventually from the walls of these daughter- 

 cysts (Fig. 78) are formed numerous tape-worm heads. 



The transference to the first or final host is effected by the 

 second or intermediate host, or the part of it containing the 

 Cyticercus or Cysticercoid, being taken into the enteric canal 

 of the final host. Sometimes, if the intermediate host is 



