21 



its shell forward it manufactured a new partition wall behind, thus 

 making an additional air-cell. 



The most curious, as it has been the most puzzling part of the 

 whole structure is this narrow tube which passes through the 

 centre of each partition back to the very beginning. It is nearly 

 all decayed now, but it was a membranous tube known as the 

 Siphuncle ; it passes through a little collar formed in each wall, and 

 so was strengthened and supported. We find remains of the same 

 organ in fossil specimens as shown on the diagram. What is its 

 function ? And why were the air chambers constructed at all ? 

 At first sight they would seem an incumbrance rather than a con- 

 venience to the animal. I do not know that these questions have 

 even yet been settled to the satisfaction of Zoologists. We can 

 easily see that the air chambers would reduce the specific gravity 

 of the whole shell and make it lighter for the animal to move with 

 in the water, though they might be a hindrance to it in its descent 

 and in its movements at the bottom. Some have imagined them 

 filled and emptied with water as occasion required, others that 

 nothing but air, or some kind of gas secreted by the animal, was in 

 them. The clearest and most satisfactory account seems to me to 

 be that given by Dr. Buckland in his Bridgewater Treatise. The 

 compartments are true air chambers, completely isolated from each 

 other, from the body of the nautilus, and from the water, therefore 

 they cannot be alternately filled or emptied with anything. The 

 siphuncle is the organ by means of which the specific gravity of 

 the shell is altered. This tube is organically connected with the 

 animal, commencing in a cavity surrounding the heart called the 

 pericardial cavity. This contains a quantity of fluid which can 

 at will be forced into the siphuncle along its whole length, thus 

 leaving the pericardium empty or nearly so, and by so much, in- 

 creasing the weight of the chamber portion of the shell ; or it may 

 be again withdrawn from the tube so as to make it easier for the 

 animal to ascend to the surface, The divisions assist in strengthen- 

 ing the walls of the chambers against the pressure of the waters, 

 on the same principle that the walls of ships sailing on Arctic voyage^ 

 are strengthened by cross beams against ice pressure. 



Thus we see here, as everywhere else throughout nature, an 

 adaptation of the mechanism to the surrounding circumstances — 

 which loses none of its beauty, none of its wonder, whether we 

 look upon it as designed by a direct creation, or as the result of a 

 long evolution from a more primitive form. 



A fossil form of nautilus is represented on the diagram, from 

 which we gather that the ancient nautilus in all respects resembled 

 its modern descendant. The same air chambers, the same siphuncle 

 are preserved. The specimen figured illustrates in an admirable 



