176 THE PEARLY NAUTILUS. 



The shell of the nautilus is camerated, as it is termed, 

 or chambered, instead of being simply hollow ; and if 

 a section be made, (see the sketch on p. 175,) it will be 

 found to be divided internally by numerous regular 

 partitions into distinct compartments, the last or largest 

 of which only encloses the body of the animal ; but 

 besides these, we find a tube, or siphuncle, partly com- 

 posed of shelly matter, partly membraneous, traversing 

 through these compartments, to the last. This tube is 

 continued membraneous into a cavity on the body of 

 the animal, termed by Cuvier the venous cavity, and by 

 Professor Owen the pericardium, which freely commu- 

 nicates with the branchial cavities, and which, receiving 

 water from those cavities, can, by its contraction, trans- 

 mit it through the tube into the chambers of the shell. 



These chambers, as Dr. Hooke suggests, naturally 

 contain air, generated by the nautilus, and being thus 

 filled with a fluid more buoyant than water, enable the 

 animal to float, notwithstanding the density of the shell 

 itself; but when the animal wishes to sink, it forces 

 water through the tube or syphon, thereby compressing 

 the air, and thus at once becomes heavier than the sur- 

 rounding medium. It would appear, that the retraction 

 of the head and tentacula into the shell, involve the 

 contraction of the pericardium, and consequently the 



