254 



PLATYHELMINTHES. 



So long as the tape-worm head (scolex) remains attached to the 

 body of the bladder-worm and in the host of the latter, it never 

 develops into a sexually mature tape-worm ; although in many cases 

 it grows to a considerable length (Cysticercus fasciolaris of the 

 house-mouse). The bladder-worm must enter the alimentary canal 

 of another animal before the head (scolex) can, after separation from 

 the body of the bladder-worm, develop into the sexually mature 

 tape-worm. This transportation is effected passively, the new host 

 eating the flesh or organs of the Animal infected with Cysticerci. 

 The tape-worms, therefore, are principally found in the Carnivora, 



the Insectivora, and the Omnivora, which receive the 



bladder-worms in the flesh of the animals on which 



they feed. The vesicles are digested in the stomach, 



and the cestode head becomes free as a scolex. The 



latter is, perhaps, protected from the too intense 



action of the gastric juice by its 



calcareous concretions, and at 



once enters the small intestine, 



fastens itself to the intestinal 



wall, and grows by gradual seg- 



mentation into a tape -worm. 



From the scolex the chain of 



proglottides proceeds as the re- 



sult of a growth in length 



accompanied by segmentation, a 



Fig. 2Q7.-Taenia process which is to be looked FIG 208.-C^fcem>id of 



f aS6XUal 



Sr T Le7c- 



kart), magnified duction (budding in the direction 



12 to 15 times. Qf 



Taenia cucumerina, mag- 

 nified 60 times (after R. 

 Leuckart). 



ment of the scolex is then to be explained as a metamorphosis, 

 characterized by the individualization of certain stages of the 

 development. But the whole life-history is a case of metagenesis, 

 inasmuch as the sexual proglottides alternate with the asexual scolex. 

 The development of some tape-worms (Microtaeniinae) presents 

 considerable simplifications. In the cysticercus stage the vesicle is 

 represented by a small appendage (Fig. 209, b), in which the cavity 

 is much reduced or absent. Such cysticercus forms are called 

 Oysticercoids, in which an appendage bearing the embryonic hooks 

 is distinct from a larger part which represents the scolex (Fig. 210). 

 Cysticercoids are found principally in Invertebrates (Gammarids, 

 Cyclops, Insects, Slugs, Oligochaetes). 



