88 SEA FABLES EXPLAINED. 



of the addition of new shell in the form of a circular curve 

 or coil around the older portion, the whole rapidly increases 

 in size, both in diameter, and in the length of the chamber. 

 The Nautilus, requiring to keep the secreting portion of its 

 mantle applied to the lip of the shell, finds the chamber in 

 which it dwells gradually becoming inconveniently long for 

 it, and therefore builds up a wall behind itself, and continues 

 its work of enlarging its premises in front. Each of these 

 walls, concave in front, towards the mouth of the shell, and 



' FIG. 31. — SHELL OF THE PAPER NAUTILUS {ArgOtiaitla Orgo). 



concave behind, acts as a strong girder and support of the 

 arch of the shell against the inward pressure of deep water : 

 and it was formerly supposed that each successive chamber 

 so constructed and vacated remained filled with air, and 

 th7ts became an additional float by which the constantly 

 increasing weight of the growing shell was counter-balanced. 

 By this beautiful adjustment of augmented floating power to 

 increased weight, the buoyancy of the shell would be secured 

 and its specific gravity maintained as nearly as possible equal 

 to that of the surrounding water. This adjustment does 



