1 62 BREATHING OF DEEP-WATER LIMNAEA chap. 



strongly of opinion ^ that there is no absolute necessity for Lim- 

 naea to obtain air by rising to the surface, and that, if pre- 

 vented from emerging, it can obtain air from the water. When 

 covered in by a roof of ice, Limnaea has not been observed to 

 suffer any inconvenience. Moquin-Tandon kept L. glabra and 

 Planorhis rotundatus in good health under 20 mm. of water 

 for eighteen and nineteen days, and relates a case in which 

 Physa was kept alive under water for 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 opinion that certain long and narrow lamellae, situated 

 within the pulmonary 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 transfor- 

 mation of organs, no appearance of a branchia, to meet the 

 changed circumstances 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 for months. The oxy- 

 gen 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 

 surface 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 breath- 

 1 Mollusques de France, i. p. 81. - N. Denk. Schio. Ges. xxix. (2) p. 196 f. 



