282 THE INHABITANTS OF THE SEA. 
tentacula, with their head downwards and the shell raised above. 
After stormy weather, as it becomes more calm, they may be 
seen in great numbers floating upon the surface of the sea with 
the head protruded, and the tentacula resting upon the water, 
the shell at the same time being undermost; they remain, 
however, but a short time sailing in this manner, as they can 
easily return to their situation at the bottom of the sea, by 
merely drawing in their tentacles and upsetting the shell. They 
are caught in baskets by the natives, who eat them roasted as a 
great delicacy. 
What renders these animals peculiarly interesting is the 
circumstance that they are the only living representatives of a 
class which once filled in countless numbers the bosom of the 
primeval ocean, and whose fossil remains (Orthoceratites, Am- 
monites) furnish the naturalist with a series of historical 
documents, attesting the unmeasured age of our planet. What 
are the ruins, thirty or forty centuries old, that speak of the 
vanished glories of extinguished empires to these wonderful 
medals of creation that lead our thoughts through the dim 
vista of unnumbered centuries to the fathomless abyss of the 
past. 
In point of development of organisation the Gasteropods or 
snails rank immediately after the Cephalopods. They also have 
a head plainly distinguishable from the rest of the body, and to 
which two brilliant black eyes give an animated expression. 
But their nervous system is far less developed, and while the 
lively cephalopod is able to swim about, and rapidly to seize a 
distant prey, almost all the gasteropods creep slowly along 
upon a flat disk or foot situated below the digestive organs, a 
formation to which they owe their name of gasteropods or 
stomach-footers. 
The marine snails are divided into several groups according 
to the different position and arrangement of their gills. In 
some species these organs form naked or free-swimming tufts 
on the back (Nudibranchiata) but generally they are variously 
disposed either in special cavities or under the folds of the 
mantle. Thus in the Inferobranchiata they are arranged 
