192 FACTS AND FANCIES. 



machinery connected 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 opera- 

 tion 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 former geological ages, more numerous, 

 as well as larger and more complex, forms 

 existed. Further, this contrivance is very old. 

 We find in the Orthoceratites and their allies of 

 the earliest Silurian formations these arrange- 

 ments 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 

 mollusks, but there is another group of some- 

 what lower grade, that of the lanthitUBt or vio- 

 let snails, in which flotation is provided for in 

 another way (see Fig. 16). 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 animal, and which are in- 

 creased in number to suit its increasing weight 

 as it grows in size. There are some reasons 



♦ As Pihcereu, iot example. 



