VI EXPERIMENTS ON BREATHING OF HELIX 1 63 



ing air. But exactly the same phenomenon is shown in the 

 case of Limnaea from great depths. Placed in an aquarium, 

 they immediately begin rising to the surface and inspiring air ; 

 in other words, they experience instantaneously a complete 

 transformation of their respiratory system. 



In Onchidium, a land pulmonate which has retrogressed to an 

 amphibious or quasi-marine mode of life, there is no organ which 

 represents the pulmonary or branchial cavity, the so-called lung 

 being only a cavity of the kidney. Respiration is, however, 

 conducted by the skin as well, and by the dorsal papillae. ^ 



Land Mollusca can sustain, for a considerable time, complete 

 deprivation of atmospheric air. Helices placed in an exhausted 

 receiver show no signs of being inconvenienced for about 20 

 hours, and are able to survive for about two or three days. If 

 detained under water, they are very active for about 6 hours, 

 then become motionless, the body swells, owing to the water 

 absorbed, and death ensues in about 36 hours. Immersion for 

 only 24 hours is generally followed by recovery. In the latter 

 case, the cause of death is not so much deprivation of air as 

 compulsory absorption of water by the skin. The amount of 

 water thus taken up is surprising. Spallanzani found that a 

 ITelix which weighed 18 grammes increased in weight by 13|^ 

 grammes after a prolonged immersion. Even slugs enclosed in 

 moist paper gained more than 2 grammes in the course of half 

 an hour. Experiment has shown that the amount of carbonic 

 acid gas produced by respiration stands in direct relation to the 

 amount of food consumed. Four pairs of snails were taken 

 which had recently awakened from their winter sleep and had 

 eaten heartily, and an equal number, under the same circum- 

 stances, which had been prevented from eating. It was found 

 that the first four pairs produced, in consuming a given amount 

 of oxygen, 11, 9, 10, and 13 parts respectively of carbonic acid, 

 while the second set produced, in consuming the same amount 

 of oxygen, only 4, 8, 7, and 9 parts of carbonic acid.^ Hiber- 

 nating Helices, if weighed in December and again in April, will 

 be found to have lost weight, due to the expiration of carbonic 

 acid. Owing to the difficulty of experiment, opinions vary as 

 to the absolute temperature of snails. It appears to be estab- 

 lished that several snails, if placed together in a tube, raise the 

 1 Bergh, Morph. Jahrb. x, p. 172. 2 p, Fischer, Journ. de Conch, ix. p. 101. 



