BIOLOGY OF PHYSA 57 



the chances of the snail's foot touching the film depend largely 

 upon the distance the animal is from the film when it stops 

 to get air. Approximation to the film, in turn, depends upon 

 the length of the animal's tentacles and the quickness with 

 which it stops when the film is perceived. This latter has to 

 do largely with the rate of movement of the animal in its up- 

 ward ascent on the thread. The turning of a snail to descend 

 its thread is very characteristic and usually consists of a doubling 

 over of the foot which brings together the extremities of its 

 ventral surface. The snail descends its thread head first with 

 its tentacles straight out as in the ascent. The foot is wrapped 

 about the thread in the same relative position that it had when 

 the animal ascended. When the animal crawls down the thread, 

 it thus adds to it a coat of mucus that gives it almost double 

 its former diameter. One may always tell whether a thread 

 is attached at one or both ends by the position of the snail in 

 descending the thread. If the thread is attached to both film 

 and substratum, the long axis of the snail's body is nearly per- 

 pendicular to the two parallel surfaces to which the taut thread 

 is fastened, since the foot is wrapped about the thread. If 

 the thread is not fastened to the film the posterior part of the 

 foot is bent over into the form of a hook. This phenomenon 

 is brought about by the thread falling down as soon as the 

 snail crawls over it and thus bending the posterior part of the 

 foot. The snail upon nearing the bottom strikes its tentacles 

 on the substratum and lifts its head and crawls off, leaving 

 its thread where it has fallen on the substratum. 



It has been shown that it is possible for a snail to spin a thread 

 upward because the animal's specific gravity is less than water, 

 so that it is buoyed upward as soon as its foot is free from the 

 substratum. It is clearly impossible for a snail to spin a thread 

 down through the water when its lung is full of air. It will 

 be shown later that the snail, on the surface film, takes air fre- 

 quently, consequently, it nearly always has a specific gravity 

 less than the water. This means that the animal on the film 

 -with its lung full cannot descend beneath the water unless 

 upon some solid or upon a previously spun thread. Its only 

 other method of leaving the surface is to give up the air from 

 its lung and sink. 



That a downward thread may be spun by Physa is illustrated 



