GOD IN NATURE 185 



Its coiled shell is divided by partitions into air- 

 chambers so proportioned that the buoyancy of the 

 air is sufficient to counterpoise in sea-water the weight 

 of the animal. There are also contrivances by which 

 the density of the contained air and of the body of 

 the animal can be so modified as slightly to disturb 

 this equilibrium and to enable the creature to rise or 

 sink in the waters. It would be tedious to describe, , 

 without adequate illustrations, all the machinery con 

 nected with these adjustments. It is sufficient for 

 our purpose to know that they are provided in such 

 a manner that the animal is practically exempted 

 from the operation of the force of gravity. In the 

 modern seas these provisions are enjoyed by only a 

 few species of the genera Nautilus and Spirula, but 

 in past geological ages far more complex forms 

 existed. Further, this contrivance is very old. We 

 find in the Orthoceratites and their allies of the earliest 

 formations these arrangements in their full perfection, 

 and in some forms even more complex than in later 

 types. 



The peculiar contrivances observed in the nautilus 

 and its allies are possessed by no other molluscs ; but 

 there is another group of somewhat lower grade, that 

 of the Ianthincs y or violet snails, in which flotation 

 is provided for in a different way. In these animals 

 the shell is perfectly simple, though light, and the 

 floating apparatus consists in a series of horny air- 

 vesicles attached to what is termed the foot of the 



