456 SUPPLEMENTARY NOTES. 



whilst in the water Balloon, the membrane only at the top of the 

 glass is flexible, and a small part only of the water in the glass 

 can be forced into the Balloon. 



The principle which causes a change in the specific gravity, 

 by varying the quantity of matter, within the shell and within 

 the Balloon, without varying their respective magnitudes, is the 

 same. 



P. 249.* The tentacula which when expanded around the 

 head, would impede any progressive motion of the animal, would 

 follow the retrograde body and shell, without causing any 

 material retardation. The part of the shell also which is foremost 

 in all the retrograde movements of the animal, in the act of ascend- 

 ing and descending, and also in swimming at the surface, is that 

 which receives the least resistance from the fluid through which 

 it moves, and at the same time presents the strongest part or back 

 of the shell to any body against Avhich it may strike, either when 

 floating on the surface, or on arriving at the bottom of the sea. 



P. 250. Mr. Owen observes, that the Hood, or flattened mus- 

 cular disk of the Nautilus Pompilius, seems calculated to act as 

 the chief locomotive organ in creeping at the bottom; and in the 

 supine position of the animal, bears considerable analogy to the 

 foot of a Gasteropod; in a state of rest and retraction it would 

 serve, like an operculum, as a rigid defence at the outlet of the 

 shell. {See Owen on the Pearly Nautilus, p. 12.) The animal 

 may also assist its movements along, and adhesion to the bottom, 

 by some of its numerous tentacula. 



P. 251.t In the case of animals possessing a siphuncle and 

 chambered shell, but having no means to fill the siphuncle with 

 pericardial fluid, the admission and abstraction of any other 

 secreted fluid, or of water, to and from the siphuncle, would have 

 a similar effect to that of the pericardial fluid of the Nautilus, in 

 varying the specific gravity. It may perhaps be shown hereafter, 

 that in some of these genera an organization exists fitted to fill and 

 empty the siphuncle by other agency than that of the Pericardium, 

 and possibly with water admitted from the branchial cavity; 



