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MANUAL OF ZOOLOGY 



kind of animal — a second or intermediate host — in order 

 that the embryo may be enabled to enter the next phase of 

 its life-history. In the case of some tape-worms, this second 

 or intermediate host is, like the first or permanent host, a 

 vertebrate animal : in the case of others it is some inver- 



Fig. 77. — Development of Tape-worm. A, hexacanth embryo; B, Proscolex of 

 Tteiiia sagiiitita; C-E, stages in the formation of the scolex of the same; C, 

 thr invagination before the books and suckers have become developed; D, after 

 thk. appearance of the hooks and suckers; E. partlv evaginated; F, fully 

 evaluated scolex of T. solium with caudal vesicle; G, scolex of T. scrrata 

 with the remains of the vesicle; H, voung tape-worm of T. serrata. (After 

 Leuckart.) 



tebrate animal such as an earth-worm, a centipede, or an 

 insect. This transference of the hexacanth embryo to the 

 second host is a passive migration, not an active one, as in 



