Chap. XVII.] THE LIVER-FLUKE AND TAPEWORM. 331 



to the liver, and there gradually assume the form of an adult 

 liver-fluke. 



The fluke may be developed in other forms than the sheep, 

 such as the goat, and occasionally in the larger cattle, but since 

 they are less close feeders they less frequently take in the en- 

 cysted cercaria. 1 



By way of appendix to the foregoing life-history of the 

 liver-fluke, the life-history of another parasite, the tapeworm, 

 may here be given. 



One form of tapeworm (Tcenia solium) infests man as its host. 

 The adult worm may be many yards in length. Anteriorly there 

 is a minute scolex or head (resembling the upper part of Fig. 

 98, C.). It .has a double circlet of spines set close together, and 

 behind this four suckers. Then follow a great number of joints 

 or metameres. These are constantly forming behind the scolex, 

 and as new ones are budded off those previously formed are 

 pushed further away from the head. Thus the metameres are pro- 

 gressively older as we proceed from the anterior to the posterior 

 end. The youngest are scarcely differentiated, and form the 

 so-called neck. Behind this they have the form shown in Fig. 

 98, D. ; further down they are as shown in E. ; near the posterior 

 end they are as represented at F. The scolex and metameres 

 is called a strobila. From time to time some of the posterior 

 metameres are detached as proglottides, and pass out with the 

 excreta. 



Running down the whole strobila are canals of the excretory 

 or water-vascular system. In the scolex there is a circular 

 vessel. Four longitudinal trunks (Fig. 98, G., I. v.) run down, 

 two on each side, the whole series of metameres. At the hinder 

 end of each metamere there is a transverse vessel (tr. v.). At 

 the extremity of the strobila is a porus excretorius. From the 

 trunks fine branched vessels, with ciliated junctions, open out. 



1 Mr. Bolton, of Birmingham, has supplied me with liver-flukes and 

 with cercarise of allied forms. Prepared and mounted flukes may be 

 obtained of most dealers in microscopic objects. Messrs. Watson, of High 

 Holborn, supplied me with a very well-mounted specimen. 



