As the cells are mostly filled with various hard earthy substances, so as to form one solid mass, they used to be called snake-stones, because it was fancied they were petrified snakes. There are a great number of them on the coast of Yorkshire, and the story was once believed that the place had been infested with an immense quantity of snakes which were changed into stone at the prayer of St. Hilda. What I am going to tell you, relates both to the Ammonites and the Nautilus.
If you examine the first cut, you will see small openings connecting all the cells with each other, and the continuous passage thus formed through the whole of the shell, is called the siphuncle. The inhabitant of the shell is an animal of the Sepia kind, but without the webs which serve the Paper Nautilus for sails, and with its arms not so long. It always resides in the outermost chamber, and it is supposed that it forms a new partition every year, so that the age of the animal may be known by counting the number of chambers.
When the Pearly Nautilus wishes to rise to the surface, it pumps out the water from its shell through the siphuncle, and makes itself light. It floats often with its shell upwards, and at other times it moves along backwards for a considerable distance, by means of spouting out water over the front edge of the shell. It never floats with the same beautiful stateliness as the Argonaut.
There is also the pretty little shell of the Nautilus Spirula, not much larger than a shilling, [plate XVI], fig. 5, which does not contain a fish, but is merely annexed to the body of a Sepia, as is represented, fig. 6. Its use appears to be just the same as that of the shell of the Pearly Nautilus, to answer the purpose of a float.
These two kinds are very rarely seen floating on the surface, though there must be many of both of them in the ocean, from the number of their empty shells which are found. Perhaps this arises from their not being so buoyant as the Argonauts, and hence they more readily dip under the surface at the approach of a vessel.
It is now time for us to bid adieu to the sea and its wonderful inhabitants, since I have promised to tell you of some of the wonders of the sky, (though I remember a great many more things that I should like to tell you of).
