sessed of it to so?ne degree. It is more than a mere 

 automaton. Indeed, its predatory habits would suf- 

 fice to indicate that; for oftentimes it swims 

 around in search of food — a method of swimming, 

 by the way, which is unique in this worm, for un- 

 like the lateral movements of higher members of 

 the group which swim in a serpentine fashion, its 

 motions are up and down in the manner of the 

 gliding course of the ray. 



More often, however, it searches out its meals 

 by crawling after them. Its locomotion is effected 

 like that of a snail, that is, it is accomplished by 

 successively raising, putting forward and affixing 

 successively minute parts of its under surface; but 

 the operation is attended by a feature more remin- 

 iscent of a caterpillar tractor than of a mollusk. 

 The flatworm lays its own track. It exudes a slime 

 over which its body travels without friction — 

 moving like a flowing stream. Now, as to whether 

 our species of polyclad, S. littoralis, may be re- 

 garded as an ancestral form requires more knowl- 

 edge of its relationships than at present prevails, 

 but it is at any rate very obviously a primitive 

 type. And herein is found the first important struc- 

 tural feature in the evolutionary scale of animal 



[175] 



