Pearly Nautilus.
The tetrabranchiate cephalopods, or Nautili, are very differently constructed from their dibranchiate relations. Here, instead of mighty muscular arms, furnished with suckers or raptorial claws, we find a number of small, sheathed, and retractile tentacles (f), surrounding the mouth in successive series, and amounting to little short of a hundred. The head is further provided with a large muscular disk (g), which, besides acting as a defence to the opening of the shell, serves also in all probability as an organ for creeping along the ground, like the foot in the Gasteropods. The mandibles are strengthened by a dense calcareous substance fit to break up the defensive armour of the crustacean or shell-fish on which the animal feeds. There is no ink-bag, no organ of hearing, and the eyes (h) are pedunculated, and of a more simple structure. The handsome pearl-mother and spirally wound shell is divided by transverse partitions (a), perforated in the centre, into numerous chambers (b). The animal takes up its abode in the foremost and largest (b′), but sends a communicating tube or siphon (c) through all the holes of the partitions to the very extremity of the spirally wound shell. Though the empty conch was frequently found swimming on the waters of the Indian Ocean, or cast ashore on the Moluccas or New Guinea, yet it was only in 1829 that the animal was known with any certainty, one having been caught alive by Mr. George Bennett, near the New Hebrides, which, preserved in spirits, is now in the museum of the College of Surgeons. Since then three different species have been found to abound in the waters of the above-named archipelago, of New Caledonia, and of the Feejee and Solomon Islands, where they principally sojourn among the coral reefs at depths of from three to six fathoms. They usually remain at the bottom of the water, where they creep along rather quickly, supporting themselves upon their 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, Ammonites) 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 under its inferior border on both sides of the body, or upon one side only, while in the Tectibranchiata they are placed, as in the Nudibranchiata, upon the dorsal aspect of the body, but are protected by a fold of the skin. In the Cyclobranchiata they form a fringe round the margin of the body, between the edge of the mantle and the foot, and in the Scutibranchiata and Pectinibranchiata they are pectinated, or shaped like the teeth of a comb, and placed in a large hollow chamber, which opens externally at the side of the body or above the head.
Tiara. | Glaucus. |
Scyllæa.