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- 

 ing the most curious : if divided 

 into several pieces, each one be- 

 comes a perfect worm. The flat- 

 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 (Ltmax agrestis). It now produces a sac- 

 like larva called the nurse ; later, this is called the Redia, 

 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 Cercaria, 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 



FIG. 41. Liver-fluke of 

 sheep twice the natural 

 size. <z, mouth ; <:, diges- 

 tive tube ; <f, abdominal 

 sucker. 



