NEGATIVE PHOTOTAXIS ETC. OF Littorina. 



17 



Note. This property enables the mollusc to creep up from 

 the sea towards the higher level which corresponds in 

 most cases with land, but in cases of detached rocks may 

 be away from the general mass of land. 



2. Littorina exigua shows a disinclination to be submerged. This is 



proved by the fact that a rise of water drives the mollusc up- 

 ward, as well as by the fact that if the mollusc touches the 

 water's edge in coming down, it hesitates to plunge in, and in 

 many cases travels horizontally along the edge as if in search 

 for a better or more comfortable route. In Experiment 11, 

 two actually turned back and went upwards again. 



Note. When negative phototaxis and negative hydrotaxis 

 act together, the mollusc is driven upward without hesita- 

 tion. When these two properties are acting against each 

 other as in the case of Experiments 3 and 7, or at D, Fig. 

 6, negative phototaxis seems to be stronger and to over- 

 come negative hydrotaxis, at least in small depths. In 

 greater depths, the result is not entirely clear, as at B. 

 Fig. 6, where the mollusc did not stir. 



3. Littorina exigua are in nature scattered over rocks, because there 



are unevennesses in rocks which provide them with holes and 

 crevices to settle down in. On smooth glass-plates, they move 

 on until they are stopped by some obstacle which they can not 

 overcome. 



4. When Littorina exigua is splashed on by water, for some length of 



time, as by waves in rising tide or artificially by a jet of water, 

 and then is left quiet, it may become positively phototaxic and 

 then it goes down from land so ward the sea. That this is phor 

 totaxis and not hydrotaxis, is shown by the hesitating manner 

 which these exhibit on touching water's edge, in one case at 

 least, such dislike seeming to send the animal upward again (Exp. 

 11). This property is not as unfailing and strong as negative 



