THE COASTS OF SAINTONGE. 285 



Having once determined the part played by these 

 organs, I had next to inquire what liquid it was that 

 was exposed in them to the action of the air. This 

 question may at first sight seem a singular one. 

 Without any profound knowledge of natural history, 

 and even without any acquaintance with medical 

 science, most persons are aware that the blood is 

 subjected to various alterations by the action of re- 

 spiration, which is performed by lungs in Mammals, 

 Birds, and Reptiles, and by branchiae in the case of 

 Fishes. Is there then in certain of the Invertebrata 

 some other nutrient fluid besides the blood ? and does 

 this liquid require to be vivified by being brought 

 in contact with the air ? We reply in the affirmative 

 to both these questions, and at once proceed to enter 

 into a few details which may render this very im- 

 portant fact more easy of comprehension. 



In all animals without exception the nutrient 

 fluid, whatever may be its true nature *, becomes 

 constantly exhausted by its sojourn in the organs, 

 and repairs its losses by the materials furnished to it 



of colour for more than forty-eight hours. This difference of time, 

 together with the appearances presented by the lateral lamella;, 

 places the respiratory nature of the latter beyond all doubt. 



The method which I employed to determine the function of 

 these lamellae might be adopted in many analogous circumstances. 

 Considered from this point of view it would, I think, be found to be 

 highly serviceable ; for we are still ignorant of the exact point at 

 which the respiratory process is effected in the case of a great 

 number of animals. 



* I have elsewhere endeavoured to show how the circulatory 

 apparatus becomes successively completed, and how this progressive 

 complication corresponds with the equally progressive character of 

 the nutrient fluids. 



