40 ELIZABETH LOCKWOOD THOMPSON 



in nearly all groups of vertebrates, less extensively among 

 invertebrates. Notable among the work on the latter group is 

 that of Yerkes (1902) on the crayfish Carcinus granulatus, 

 Yerkes and Huggins (1903) on habit formation in the crayfish 

 Camharus af finis, Fielde (1901) and Shepard (1916) on ants, 

 and Yerkes (1912) on earthworms. 



Just as the natural habits of rats in running about in narrow 

 horizontal passages suggested to Small (1899) a series of such 

 passages for experimental work, so the structure and habits of 

 Physa suggested an upright or vertical path. Physa breathes 

 air, but remains under water indefinitely when the so-called 

 lung is full of air. When a new sup|:»ly of oxygen is required 

 it commonly seeks the surface of the water, although bubbles 

 below the surface are sometimes " tapped " for air by the 

 snail (Dawson, 1911). In order to gain the surface it ordin- 

 arily crawls up the stems of water plants growing in its natural 

 habitat. It seemed then as though the tests might be placed on 

 a purely natural basis by mechanically depriving the snail of 

 air and placing it at the base of an upright path leading to the 

 surface of the water. The animal's need for air would furnish 

 the motive necessary to action. This need also suggested a 

 natural punishment and reward for a correct or incorrect 

 choice of paths, that is, the attainment of air at the end of the 

 path or failure to get it. 



2. Apparatus and Material* 



The labyrinth used had the form of a rectangular U on an 

 upright stem (fig. 14, Plate IV). One arm of the U reached the 

 surface of the water while the other was so much shorter that 

 the snail could not reach the surface from its tip. Various 

 materials were tried out in the construction of the apparatus, 

 such as cardboard coated with paraffin to protect it from the 

 water, metal, mica and celluloid. The latter was finally selected 

 because the surface of this material is like that of glass upon 

 which snails crawl readily and also because it could be easily 

 cut and manipulated should it be decided to alter the form 

 of the path. Transparent celluloid 2.5 mm. in thickness was 

 used in constructing the path (fig. 14, Plate IV). The upright 



* The author is indebted to Professor George R. La Rue for advice in the 

 mechanical construction of the labyrinth. 



