58 JEAN DAWSON 



in the following experiment: A very thin sheet of collodion 

 was placed on the water in a bacteria dish and a snail without 

 air in its lung was made to crawl on the underside of this arti- 

 ficial film. Every time the snail approached the edge of the 

 collodion and attempted to take air it was made to turn back 

 by stimulating it on the head with a glass rod. After this had 

 been repeated several times, the snail had increased its speed, 

 through being thus disturbed, so that when it came to the edge 

 of the collodion it was moving so rapidly that it crawled off 

 the collodion film and went to the bottom, and as it descended 

 it spun a thread the end of which was fastened to the edge of 

 the collodion film. Another instance of a snail's spinning a 

 thread downward, but not from the surface of the water, is 

 seen when the animal crawls upon a straw, or filament of alga 

 which extends but part way to the surface of the water. If the 

 animal is coming up at a quite rapid rate the fore part of its foot 

 may leave the end of the filament before the snail has slackened 

 its speed. In case the animal's specific gravity is less than 

 water, the snail continues upward on a thread, but if the snail's 

 specific gravity is greater than water it sinks to the bottom 

 suspended by a thread. Its descent differs from that observed 

 when the animal gives up its air and drops to the bottom in 

 that its foot is not contracted into its shell, and it goes down 

 with an easy gliding motion. Ordinarily Physa does not crawl 

 off the edge of a solid, as does the slug Arion ater; but it follows 

 the different surfaces of the solid, keeping its head in close 

 contact with them. It has been observed that threads are not 

 spun downward frequently. It happens sometimes that the 

 thread is not spun to the surface of the water, but that the 

 snail's upward progress is hindered for a brief time or that it 

 is obliged to turn around and go back to the sub-stratum, owing 

 to the failure of the thread to increase in length. It also may 

 happen that the thread breaks when the animal is part way up, 

 thus throwing it to the surface with some violence. In the first 

 case where the spinning ceases temporarily and the snail turns 

 and goes down the unfinished thread, its behavior may possibly 

 be made necessary by a failure of the mucus to flow. 



The thread that is attached to the surface film is left standing 

 when the animal descends and crawls away and may serve for 

 other snails to crawl to the surface upon or to descend upon. 



