VI EXPERIMENTS ON BREATHING OF HELIX 163 



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 trans- 

 formation of their respiratory system. 



In Oncliidium, 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 absorp- 

 tion of water by the skin. The amount of water thus taken up 

 is surprising. Spallanzani found that a Helix 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 respira- 

 tion 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 circumstances, 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.^ Hibernating 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 dilliculty of experi- 

 ment, opinions vary as to the absolute temperature of snails. It 

 appears to be established that several snails, if placed together in 

 a tube, raise the temperature one or two degrees C, but as a rule, 



^ Bergh, Morph. Jahrb. x. p. 172. - P. Fischer, Juurn. dc Conch, ix. p. 101. 



