450 NOTES. 



the oxygen contained in that air if it is kept under water, and instead 

 of it carbonic acid will be deposited in the lungs. This gas, being 

 positively injurious to the creature, must presently be expelled, and 

 consequently the lungs must soon become empty and so collapse, or 

 they must be replenished with air or water. In Pauly's experiments 

 the former was the case ; he expressly says the lung-cavity was empty, 

 as could be seen by its shrunken aspect from outside.' But he con- 

 ducted the experiment in a somewhat energetic fashion. He forced 

 the animal to expel all the air from its lung-cavity under water. Now, 

 he himself says that the expulsion of the air in consequence of irrita- 

 tion may sometimes even occasion the death of the animal ; hence the 

 question is allowable whether the persistent closing of the lung may 

 not have been a diseased result of the irritation in itself so unendurable. 

 The experiment must be repeated in some way differing from Pauly's 

 before it can be regarded as perfectly conclusive evidence of the infer- 

 ences given above in the text. 



CHAPTER VII. 



203. Some mollusca, as Patella and Navicella, are im- 

 moveably attached to the rock for the whole period of their existence ; 

 they never quit one spot, and not unfrequently make a more or less con- 

 spicuous impression in the stone. How ? This is not known. 



When they are not disturbed, they usually lift the fore-part of the 

 shell just so much as is requisite to admit a fresh supply of water to the 

 branchial cavity, and of food to the mouth. If they are touched they 

 shrink back, and the shell adheres so closely to the stone that it is im- 

 possible to loosen it from its hold without injuring it. I have often 

 endeavoured to loosen a Navicella hardly an inch in length from its 

 situation on a stone in a swift mountain torrent, by lateral pressure on 

 the shell, not by insinuating a knife under it ; but I most rarely suc- 

 ceeded never, indeed, unless I took the creature by surprise ; if it were 

 in any way on the alert, I could not do it but by application of the 

 knife, and a consequent injury to the shell. The case is the same 

 with Patella (limpet), and many other mollusca; even the creeping 

 kinds, as Chiton, can adhere uncommonly tightly by suction, and in 

 every case the foot is the organ employed. 



Nate 96, page 205. A few deep-water Siphonophora have lately been 

 described by Studei. Two species were dredged up from a depth of 

 from 800 to 1,000 fathoms, belonging to the genus Rhizophyga, which, 



