90 JEAN DAWSON 



upon its velocity. If it is moving slowly it stops before its 

 head touches the film; if in rapid motion, its head may come 

 against the film before it can stop itself to take air. The ten- 

 tacles were removed but the snail, upon reaching the surface 

 of the water, took air as before, the only apparent difference 

 being that the snail's head came in contact with the film before 

 the siphon began to protrude, or the shell to turn. 



The snail's body was tested for its sensitiveness to oxygen 

 in the following way: The animal was placed in a bacteria 

 dish filled with water and a glass cover placed over it so that 

 there were no visible air bubbles. When a snail was crawling 

 upon the glass cover, a bubble of air was allowed to enter 

 and was then moved so as to touch the different parts of the 

 animal. If the bubble came in contact with the head region, 

 the snail turned its shell at just the angle necessary for the 

 siphon to open into the bubble and took it into its lung. Very 



"small bubbles of air in the water do not produce this reaction; 

 but if the snail's lung is empty, bubbles about two mm. in 



diameter are sufficient to cause the siphon to open. If the air 

 bubble is brought in contact with the region of the lung, the 

 siphon immediately opens and takes in the air ; if it is touched 

 to other parts of surface of the animal there is no response. 



The fact that the siphon can sense oxygen independently 

 of the head is demonstrated by holding the snail with its head 

 under water and its lung region exposed to the air. The moist 

 siphon opens almost immediately when it is left exposed to 

 the air; if the snail is accustomed to being handled; but if not, 

 the process may be delayed. The siphon opens and closes 

 at more or less regular intervals much as does the lung 

 of the land snails Limax or Polygyra. The lung continues to 

 open and close until drying occurs; the snail then withdraws 

 into its shell and all visible respiration ceases. 



It is therefore seen that the snail can pick up air bubbles 

 which are under the water, if either the head or the siphon 

 chances to touch them. There is a biological advantage 

 in the fact that the head and siphon region can separately 

 sense air. The continuous exposure of the foot to the air while 

 the animal is on the film precludes its functioning to release 

 the pulmonary reflex. The head and siphon which are sub- 

 merged seem to release this reflex whenever they come in con- 



