i88 



PRACTICAL ZOOLOGY 



then encyst on a leaf or blade of grass. If the leaf or grass is 

 eaten by a sheep, the cercariae escape from their cyst wall and 

 make their way from the sheep's alimentary canal to the bile 

 ducts, where they develop into mature flukes in about six weeks. 



The great number of eggs produced 

 by a single fluke is necessary, because 

 the majority of the larvae do not find 

 the particular kind of snail, and the 

 cercariae to which the successful larvae 

 give rise have little chance of being 

 devoured by a sheep. The genera- 

 tions within the snail, of course, in- 

 crease the number of larvae which 

 may develop from a single egg. This 

 complicated life history should also 

 be looked upon as enabling the fluke 

 to gain access to new hosts. The liver 

 fluke is not so prevalent in the sheep 

 of this country as in those of Europe. 

 The Tapeworm. - 

 The tapeworm, Tcenia 

 solium, is a common 

 parasite which lives as 

 an adult in the alimen- 

 tary canal of man. A 

 nearly related species, 

 Tania saginata, is also 

 a parasite of man. 

 Tcsnia, as shown in 

 Figure 109, is a long 



flatworm consisting of a knoblike head, the scolex (Fig. 109, B), 

 and a great number of similar parts, the proglottides, arranged 

 in a linear series. The animal clings to the wall of the alimen- 

 tary canal by means of hooks (Fig. 109, B, 2) and suckers (j) on 

 the scolex. Behind the scolex is a short neck (4) followed by a 



FIG. 109. A, tapeworm. The lengths of parts 



omitted in the figure are indicated. 



B, head or scolex of tapeworm. (From Shipley 



and MacBride.) 



