534 RAYMOND PEARL. 



We have, then, here a case of what appears superficially to 

 be " social instinct." 



e. Movement on Surface Film. — As is well known, 

 flat-worms and a number of other animals frequently move 

 about on the under side of the surface film at the top of the 

 water. On account of the flexibility of the support, motion 

 under these conditions is very slow, and usually, after having 

 been on the surface film for a short time, the worm will 

 loosen its hold and pass down to the bottom in the way which 

 has been described above. The worms do not remain 

 customarily in the angles formed by the surface film with the 

 side of the dish, as does Stichostemma (Child, :01), but 

 instead pass out at once on to the free surface. Further, the 

 flat-worms never push through the fllm at the side of the 

 dish and pass up out of the water as the nemertean does. 

 The occurrence of planarians on the surface film is not the 

 result of any thigmotactic reaction (using thigmotactic in the 

 sense ordinarily understood), but is brought about by a 

 simple reflex act, and is tlie result of the configuration of the 

 surface of the water and the side of the dish. This will be 

 brought out in more detail later. It is probable that fresh- 

 water planarians, in their normal habitat, very rarely take up 

 a position on the surface fihu. Among other organisms 

 (Entomostraca, Hydra, etc.) this habit probably has a much 

 greater biological significance than in planarians (cf. 

 Scourfield, 'Of, : 00, :01). When on the surface film the 

 worm behaves in nearly every respect as it does when on the 

 bottom. The head is frequently raised (with reference to 

 the worm) and waved about in the water just as occurs in 

 the normal movement. That the situation is a more or less 

 abnormal one, however, is shown by the fact that, so far as I 

 have observed, the worni never comes to rest on the surface 

 film, but instead, always keeps in active movement till it 

 leaves it. 



The means by which the animals maintain their position on 

 the under side of the film is undoubtedly the mucous secre- 

 tion from the ventral surface. This is very sticky, and holds 



