256 RESPIRATION. 



ration has been continvied for some time, there has been 

 more of the latter excreted than there has been consumed 

 of the former ; nay, sometimes this occurs even before all 

 the oxygen has been consumed."* These observations may 

 serve to explain, in some degree, the apparent apathy of 

 the mollusca generally to a temporary deprivation of their 

 respiratory media, for snails may be immersed in water for 

 many hours without injury;-]- and the purely aquatic spe- 

 cies will survive as long a time exposed to the atmosphere. 

 " The species of Voluta and Buccinum generally (particu- 

 larly B. oliva and B. harpa,") says Mr. C. Collier, " die 

 in a few hours ; those of Strombus and Murex survive 

 thirty-six, forty-eight, and even sixty hours ; Trochus nilo- 

 ticus and turritus live yet longer ; and Strombus palustris 

 will live several days." | Oysters and mussels, as every 

 one knows, and probably all the Conchifera, will live for 

 three or four days without any more water to breathe in 

 than what may lie in the concavity of their shells ; and 

 Mr. Boyle has some experiments which illustrate, in a re- 

 markable manner, their tenacity of life even in vacuum. 

 He found that two oysters put "into a very small receiver," 

 exhausted of air, were alive at the end of twenty-four 

 hours ; " but how long afterwards they continued so I did 

 not observe." § Another oyster was put into a vial full 

 of water before being enclosed in the receiver, " that, 

 through the liquor the motion of the (air) bubbles, ex- 

 pected^ from the fish, might be more pleasantly seen and 

 considered. This oyster proved so strong as to keej) it- 

 self close shut, and repressed the eruption of the bvibbles, 

 that in the other did force open the shells from time to 

 time ; and kept in its own air as long as we had occasion 

 to continue the trials." || Shelled snails (Helices) appeared 

 to be not more disordered in vacuity ; and even the slugs 

 (Limax) endured the privation for many hours. The same 

 illustrious philosopher included two of the latter " in a 



* Edinb. New Phil. Journ. April, 1833, p. 383. The Rev. Mr. Guilding 

 has conjectured that some mollusca may even purify water : — " Neritinaa 

 are destroyed with great difficulty : some, which were even kept close in 

 salt water, seemed to have the power of purifying it, and rendering it fit 

 for respiration ; while many large air-hubhles were generated in the glass. 

 Some power of this kind would be very valuable to those species which 

 inhabit maritime ponds, the waters of which, nearly dried up at certain sea- 

 sons, must be stagnant and unwholesome." — Zooloi^kal Journal, v. 33. 



f Miiller mentions that a variety of Helix nemoralis lived a whole sum- 

 mer at the bottom of a rivulet. — VeTin. Fliw. et Ter. Hist. ii. pre/', xi. 



J Edinb. New Phil. Journ. vii. 230. 



§ Phil. Trans. 1670, p. 2023. || Ibid. p. 2024. 



