their Respiration, 111 



The purpose of the respiratory organs, and of the currents 

 just described, is, to expose the blood freely to the purificative 

 action of the atmospherical air, that it may be purged of 

 some noxious qualities which it has acquired during its 

 circulation through the venous system, and fitted again for 

 the continuance of the life of the individual. In the ver- 

 tebrate animals the blood is altered, even in its outward ap- 

 pearance, by this process ; from a dark it becomes a bright 

 red fluid: but no perceptible change is operated on the white 

 serous blood of the Mollusca, yet that it has experienced a 

 similar purification is not to be doubted ; for the air breathed 

 by these creatures is similarly deteriorated, as it would have 

 been had it been breathed by the quadruped or bird ; the 

 oxygen has disappeared, and its place become occupied by 

 an equal bulk of carbonic acid gas. This had been proved 

 by the well known experiments of Spallanzani and other 

 physiologists ; and though, in general, the proportion holds 

 good, yet it appears, from the recent experiments of Tre- 

 viranus, that the absorption of oxygen is not always pro- 

 portional to the excretion of carbonic acid, the proportion 

 of the one to the other depending on the strength of the 

 respiration, the time of its continuance while the respir- 

 ability of the air is diminishing, and the volume of the air 

 in which the respiration is performed. " The more car- 

 bonic acid," says Treviranus, " there is developed while 

 breathing in the open air, and the less the power of con- 

 tinuing in a medium deficient in oxygen, the less is the 

 proportion of the consumption of oxygen to the production 

 of carbonic acid gas, whence a small quantity of atmospheric 

 air is respired for a moderate period. But when the re- 

 spiration is continued for a longer period in the same air^ 

 and the strength of the individual begins to sink, the excre- 

 tion of the latter diminishes more rapidly than the absorption 

 of the former. We know that the higher classes of animals, 

 when enclosed in a certain quantity of air, die long before all 

 its oxygen has been exhausted. The case is very different 

 with many of the Mollusca under the same circumstances; 

 for they not only consume all the oxygen, but actually con- 

 tinue afterwards to exspire carbonic acid gas : consequently, 

 after the respiration has been continued for some time, there 

 has been more of the latter excreted than there has been con- 

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

 all the oxygen has been consumed." [Edin. New Phil. Jouni., 

 April, 1833, p. 383.*) These observations may serve to 



* The Rev. Mr. Guilding has conjectured that some Mollusca may even 

 purify water: — " AMeritinae are destroyed with great difficulty : some. 



