OPINIONS OF HOOK AND PARKINSON. 251 



102,) admitting that these chambers were not accessible to 

 water, thinks that the act of rising or sinking depends on 

 the alternate introduction of air or water into the siphuncle; 

 but he is at a loss to find the source from which this air 

 could be obtained at the bottom of the sea, or to explain 

 " in what manner the animal effected those modifications 

 of the tube and its contained air, on which the variation of 

 its buoyancy depended."* The theory which supposes the 

 chambers of the shell to be permanently filled with air alone, 

 and the siphuncle to be the organ which regulates the rising 

 or sinking of the animal, by changing the place of the peri- 

 cardial fluid, seems adequate to satisfy every hydraulic 

 condition of a Problem that has hitherto received no satis- 

 factory solution. 



I have dwelt thus long upon this subject, on account of 

 its importance, in explaining the complex structure, and 

 hitherto imperfectly understood functions, of all the nume- 

 rous and widely disseminated families of fossil chambered 

 shells, that possessed siphunculi.f If, in all these families, it 

 can be shown that the same principles of mechanism, under 

 various modifications, have prevailed from the first com- 

 mencement of organic life unto the present hour, we can 

 hardly avoid the conclusion which would refer such unity 

 of organizations to the will and agency of one and the same 

 intelligent First Cause, and lead us to regard them all as 

 " emanations of that Infinite Wisdom, that appears in the 

 shape and structure of all other created beings."J 



tending to fall flat upon its side; thus exposing itself to injury by fric- 

 tion, and the animal to attacks from its enemies. Rumphius states, that 

 at the bottom, He creeps with his boat above him, and with his head and 

 barbs (tentacula) on the ground, making a tolerably quick progress. I 

 have observed that a similar vertical position is maintained by the shell of 

 the Planorbis corneus, whilst in the act of crawling at the bottom. 



* The recent observations of Mr. Owen show, that there is no gland con- 

 nected with the siphuncle, similar to that which is supposed to secrete air in 

 the air-bladder of fishes. 



+ See Sup. Note. 



t Dr. Hook's Experiments, p. 306. 



