106 JEAN DAWSON 



statements of v. Siebold in observations made upon L. auric- 

 ularia. The pulmonate snails taken from abyssal depths by 

 V. Siebold (1858), Forel (1869 and 1874), Pauly (1877), Forbes 

 (1888), Walker (1900), and Andre (1901), undoubtedly ab- 

 stracted their oxygen supply from the water through the walls 

 of their lungs, which were filled with water, and through their in tegu- 

 ment. Tryon (1882) observes, "The air-breathers that inhabit water 

 are obliged to visit the surface frequently; and stale water 

 is so inimical to the water-breathers that they soon attempt 

 to escape from ,the confinement of a glass or basin unless the 

 water is frequently renewed." Willem (1896) shows that, if 

 water' is aerated, Lymnaea and Planorhis can live without 

 atmospheric air. He regards the cutaneous respiration as m.uch 

 more essential than pulmonary. Walter (1906) observed that 

 Limnaea elodes came to the surface to take air twenty times 

 in an hour if kept in spring water and forty-four times in boiled 

 water; while he admits that pulmonate snails absorb a certain 

 amount of air from the water, yet he thinks they cannot take 

 the required amount in this way. He speaks of breathing 

 without coming to the surface as abnormal and says "in certain 

 rather rare instances, snails may be made to survive for certain 

 periods when forcibly kept under water. Death ensues pre- 

 maturely in such cases and the inability of the snail to adapt 

 itself suddenly to extracting its air supply from water alone 

 is clearly demonstrated." He evidently regards the absorption 

 of oxygen through the snail's integument as a new adaptation 

 and not as a process that is taking place continually, whether 

 the animal is using atmospheric air or not. He regards the 

 pulmonates that are living in deep water as those, "modified 

 to such a degree that they extract all their necessary oxygen from 

 air contained in water." 



1 2 . The reaction of Physa away from the deep water and toward 

 the shallow^ is probably due to the greater amount of oxygen 

 in the shallow- water. Tryon (1882) has made the following 

 observations: "The depth at which shell fish live is prob- 

 ably influenced by the quantity of oxygen which they require; 

 the most active and energetic races live only in shallow vvater, 

 or near the surface; those found in very deep water are the 

 lowest in their instincts and are especially organized for the 

 situation." 



