144 PICTURESQUE SKETCHES 



tiful, adorning it with a succession of most graceful 

 forms, resembling festoons of foliage and elegant em- 

 broidery ; so that when, as is often the case, these 

 thin septa are converted into iron pyrites, their 

 edges appear like golden filigrane-work, meandering 

 amid the pellucid spar that fills the chambers of the 

 shell.* 



The mechanical advantage of this contrivance is 

 considerable, and arises from there being many more 

 points supported in the case of a wall of this kind, 

 than by a plane surface, the cause of which will 

 readily be seen in the wood-cut annexed (fig. 55). 

 The main object of the empty chambers in all 

 animals that possess them, is to serve as floats buoy- 

 ing up the animal and its shell, and reducing the 

 specific gravity of the whole mass to that of water ; 

 and the vacant space in the shell, answering the 

 purpose of a float, must be divided into closed 

 compartments, because it is built at successive 

 times as the animal grows and requires additional 

 support. 



The position of the siphuncle as the tube running 

 through the whole series of chambers is called is 

 another and not unimportant peculiarity in the am- 

 monite. In the case of the nautilus this tube is of 

 tolerably large size often thicker than a quill and 

 it passes through the middle of the septa or walls into 

 the pericardium of the animal. In the ammonite (fig. 5 5, 

 s) it is often exceedingly small, and always situated 

 quite on the outside of the shell. Whatever may be 

 the use of the siphuncle in the nautilus, and that use 

 is by no means clearly determined, there is no proof 



* Buckland's Bridg. Tr., vol. i. p. 347. 



