ANIMAL PARASITES. 135 



that they constitute a whole worm. When eggs con- 

 taining embryos are taken into the stomach of a suitable 

 host — usually in water or food — the embryos are liber- 

 ated, pass into the small intestines, the walls of which 

 they penetrate, and reach various tissues and organs, 

 the liver, muscle, brain, etc. Here they become encysted 

 and develop into cysticerci or '' bladder- worms," that 

 is to say, they are converted after a few months into a 

 cyst full of fluid. From a point on the inner wall of 

 each cyst a little bud projects, which in time is converted 

 into a tape- worm head or scolex, and a sac containing 

 it. A cyst containing a tape-worm head is known as a 

 ^^ measles ^^ or cysticercus celhdosce. Flesh of this kind 

 is said to be measled. As cysticerci the parasites live 

 indefinitely until the flesh containing them is eaten by 

 another host, in the intestinal tract of which they then 

 grow into mature tape-worms. The cycle of the cestodes 

 is therefore in two hosts, with possibly a short interval 

 between spent in water. Tape- worms parasitic in man 

 belong to two orders — the Tceniadce and the Bothrio- 

 cephalidcB. The first occurs in man either as ''measles " 

 or as tape- worms, the latter only as tape- worms. Ten 

 species of tape-worms have been described, of which 

 three, only, are known definitely to be connected with 

 food. These are Tcenia saginata (T. mediocanellata) 

 due to measly beef, Tcenia solium, to measly pork, and 

 Bothriocephalus latus, to infested fish, such as sturgeon, 

 pike, perch, and salmon. 



