FLAT-WORMS, ETC. 43 
Class I—FLat-Worms, etc. (Platyhelminthes). 
The Planarian worms are the 
common forms of brooks and 
streams, found clinging to the 
floating twigs and leaves, resem- 
bling slugs. Some are brown, 
with from two to thirty’ black 
eye-spots. The marine forms at- 
tain a large size. Many are cov- 
ered with delicate cilia and pro- 
tective bristles, either spiral or 
straight rods held in cells and 
shot out like so many arrows as 
a defense. They multiply in sev- 
eral ways, that of self-division be-  F!6._41.— Liver- fluke of 
ing the most curious: if divided 5 a oer sey 
: : size. @, mouth ; c, diges- 
into several pieces, each one be- tive tube; ¢, abdominal 
comes a perfect worm. The flat- sucker, 
worms are remarkable for their 
changes during growth, some passing through as many as 
seven. Many are parasitic, infesting other animals, as the 
liver-fluke of the sheep (Fig. 41). 
The metamorphosis of an allied form, Monostomum mu- 
tabile, is as follows: When the embryo escapes from the 
egg, it is a ciliated form (Fig. 42, A). It swims about, 
soon entering the body of some animal—in the case of 
Fig. 41, the snail (Zimax agrestis). It now produces a sac- 
like larva called the nurse; later, this is called the Reda, 
then having a tadpole-shape, C, in which are seen germs, 
a. The animal grows until it assumes the appearance of 
D, when the young burst out as tadpole-like creatures 
called Cercarie, E. They now leave their host, swim 
about, are swallowed by some animal, as the sheep, and 
make their way to the liver. Here the tail is lost, and 
they become encysted, in time appearing as a perfect 
