54 JEAN DAWSON 



Any mechanical stimulus on the ventral side of the head and 

 foot causes, at the point of contact, an indentation of the sur- 

 face. If the animal is still, the muscles of the foot contract 

 so as to form a cup-like depression about an object, but if the 

 animal is moving, the depression becomes trough-like as the 

 foot moves along under or over the object. The depth of the 

 depression corresponds to the strength of the stimulus. If a 

 pencil point is run along the foot of the snail with some firm- 

 ness, a deep furrow is produced, but if the foot is more lightly 

 touched, a corresponding shallow furrow is formed. This also 

 is true of any object laid upon the ventral surface of the foot 

 when the animal is crawling upon the film. If it is light and 

 floats on the film, no perceptible indentation follows, as the 

 foot moves along it; but if the object is heavier and depresses 

 the surface film, a deeper indentation occurs. This is also 

 true for the edges of the foot, and it would seem that the muscles 

 in the snail's foot sense weight much as the human hand rec- 

 ognizes it. 



The thread is invisible when freshly spun and can be seen 

 only after it has remained in the water some time. Long 

 before it becomes visible, however, the snail has left the thread 

 and all opportunity for studying its behavior on it is lost. 

 Warington (1853) gives the method of passing a rod under the 

 creature. This shows the existence of the thread by swaying 

 the animal to and fro. In the present study the following 

 method was emplo3^ed to study spinning: When the snail 

 began to spin, carmine mixed in water was injected from a 

 pipette into the region of the snail. Great care was taken not 

 to disturb the snail when the carmine left the pipette. The 

 particles of carmine settled down upon the freshly spun thread 

 and enabled one to watch the process. If snails are used that 

 are not accustomed to being handled, they react when spinning, 

 much as they do upon the substratum. The slightest disturbance 

 causes them to either stop or retract somewhat into the shell, 

 and if there is no further disturbance they may then continue 

 the spinning. The chances are that they will either turn about 

 and descend the thread if thus brought to a standstill or expel 

 the air from the lung and sink to the bottom. A slight raising 

 of the anterior part of the snail's foot from the substratum 

 is the first indication that the snail is about to spin a thread. 



