BIOLOGY OF PHYSA 91 



tact with the film. The snail thus receives a greater number 

 of stimuli to take air while it is under water or while crawling, 

 shell downward, on the surface film. In the latter position, 

 the head and siphon region are both beneath the film; these 

 regions are, however, quite frequently exposed to the film when 

 the animal reaches up for food which has passed back upon its 

 foot ; and also if it twists its shell from side to side. This twist- 

 ing or rocking of the shell is occasioned by a hindrance to the 

 snail's progress. In ordinary locomotion, the shell drags and 

 the snail brings it forward with a jerking motion. This occurs 

 whenever the snail moves, whether upon film or solid, but it 

 is accentuated and modified by anything which still further 

 impedes its movements. The ribbon of slime often becomes 

 thick and filled with foreign debris, and the extra weight causes 

 the snail occasionally to rock its shell from side to side. Thus 

 the siphon is exposed to the film. This behavior may be brought 

 about whenever the slime is sufficiently thick, by putting small 

 shavings of parafin in the mucous ribbon or by attaching a piece 

 of the parafin to the shell with collodion. 



The condition of the habitat and the general activity of the 

 snail greatly affect the number of times it is stimulated by 

 oxygen and, consequently, the amount of gaseous exchange 

 taking place in its lung. The solids upon which the snail may 

 crawl to the film, the mucous threads spun, and the amount 

 of food upon the film, are some of the factors which tend to keep 

 the small snail near the surface of the water so that it will more 

 frequently be stimulated by oxygen and consequently will 

 receive more air into its lung. 



' 4. Physiological state of the snail affects respiration. The 

 siphon acts reflexly to the stimulus from the surface film but 

 the head and tentacles do not respond as unerringly. There 

 are times when both head and tentacles touch the film, and the 

 snail responds to it without respiration following as though it 

 were a solid. This occasional indifference to the stimulus from 

 the film on the head and tentacles is due to the presence of air 

 in the lung. A snail that has for some time been crawling 

 on or about the film and has taken the air into its lung several 

 times at short intervals, is for a time indifferent to it, much 

 as it shows indifference to food after it has eaten until it is satis- 

 fied. In the majority of cases, however, when the head region 



