I 9 2 



ELEMENTARY ZOOLOGY. 



grow into a form known as the bladder-worm (Fig. 55, A) and encyst 

 in the tissues. They do not reproduce in this second host, however, 



the case of the trematode. 



They have no power of further 

 development so long as they are 

 in the flesh of the new host. This 

 flesh must be eaten by the ap- 

 propriate carnivorous animal 

 before the worm can come back 

 to its adult condition again. It 

 will be seen that at least one 

 of the hosts of the tape-worm 

 must be a flesh eater. In one 

 species of the tape-worm this 

 cycle is completed by man and 

 the hog as hosts; in another by 

 man and the ox; there are 

 others that use the dog and 

 various ruminants; the cat and 

 the mouse; the dog and the 

 rabbit; .the shark and other 

 fishes, etc. 



FIG. 55. Diagram showing stages in the life history of a tape-worm (Tania). 

 A and B, bladder- worm stages. C.'the adult worm consisting of a chain of " segments" 

 or individuals in various stages of maturity. D, a young embryo such as fill the uterus 

 of a mature case, b, bladder; ex, excretory canals; g, reproductive pore; h, head, provided 

 with suckers and hooks; u, uterus in a mature individual; z, zone of fission where new 

 individuals are formed. 



Questions on the Figures. What arguments can you find for 

 considering' the whole object an individual? for considering it a 



colony? Where does non-sexual reproduction 



Where 



sexual? If the numbers represent the approximate position of the 

 segments, what proportion of the worm is omitted in the drawing? 



PHYLUM V. Nemathelminthes (or Round Worms). 



208. Practical Library Exercise. Consult several more extended 

 texts as to the life history of Trichina. What are its hosts? What 

 part of its life does it spend with each? To what extent does it 

 reproduce in each? In which is the sexual phase? Is there an 

 alternation of generation as well as a metamorphosis? What is 

 the nature of the disease called trichinosis? How is it to be avoided 

 with certainty? 



Similarly, study the life history of Ascaris. 



What are the chief differences in form and structure between the 

 round worms and the flat worms? Are the round worms all parasitic ? 



