l62 liKKATIIlNG OF DEia'-WATEK LIMNAEA chap. 



life. Moquin-Tiuidoii, on the other hand, is strongly of opinion ^ 

 that there is no absolute necessity for Limnaea to obtain air by 

 rising to the surface, and that, if prevented from emerging, it can 

 obtain air from the water. When covered in by a roof of ice, 

 Limnaea luis not been observinl to suller any inconvenience. 

 Moquin-Tandon kept L. glahra and FhviwrJiis rotimdatus in good 

 health under 20 nun. of water for eighteen and nineteen days, 

 and relates a case in which Physa was kept alive under water 

 l\)r four days, and Planorhis for twelve. Young specimens, both 

 of Limnaea and Planorhis, do not rise to the surface for a supply 

 of air ; they are hatched with the pulmonary cavity full of water. 



It is probable, therefore, that Limnaeidae are capable, on 

 occasion, of respiration through the skin. Some authorities are 

 of ()]>ini(>n that certain long and narrow lamellae, situated within 

 tlic pidiiionary sac, are employed for the purpose of aqueous 

 respiration. Ancylus, which never makes periodic excursions to 

 the surface, perhaps respires by receiving into its pulmonary 

 chamber the minute quantities of oxygen given off by the 

 vegetation on which it feeds. 



Limnaeidae taken from a great depth of water, e.g. from 130 

 fathoms in the lake of Geneva, have been examined by Forel.^ 

 The pulmonary sac is full of water, but there is no transformation 

 of organs, no appearance of a branchia, to meet the changed cir- 

 cumstances of their environment. Doubtless a good deal of 

 respiration is done by the skin ; being soft and vascular, 

 it respires the air dissolved in the water. Forel cites cases 

 of Limnaea living at much shallower depths, which come to 

 the surface once, and then remain below i'or months. The 

 oxygen of this supply must soon have become exhausted, and the 

 animals, discontinuing for a time the use of the pulmonary 

 chamber, must have respired through the skin. Shallow-water 

 Limnaea, according to the same authority, remain beneath the sur- 

 face during cold weather ; when warm weather returns they rise 

 to the surface to take in a supply of air. Since the water at great 

 depths is always very cold, there is no need for the Limnaea living 

 there to rise to the surface at all. 



It is a curious fact that Limnaea, which have been respiring 

 by the skin for the whole winter, should suddenly, on the first 

 warm days of summer, take to rising to the surface and breathing 



* MoUusqucs dc France, i. p. 81. - K. Dcnk. Schu: Ges. .\.\i.\. (2) j). lOti C. 



