II. The Encephalous Mollusca.—These possess a head, with feelers or soft tentacula, eyes, and a mouth with jaws; they are arranged in classes, according to the modification of their locomotive organs; for, with but few exceptions, they are free animals, and can crawl, climb, or swim. Their shells are, for the most part, composed of one piece, or valve, hence they are termed Univalves. In some genera the shell is a simple cavity, spirally disposed, as in the Snail; in others, it is conical, consisting of one or many pieces, as in the Limpet and Chiton. In the Cephalopoda it is internally divided into cells, or chambers, as, for example, in the Nautilus.
The Encephalous Mollusca are subdivided into the following classes; viz.—
a. Pteropoda (wing-feet).—In these the organs of progression are two wing-like muscular expansions, proceeding from the sides of the neck, by which they can swim and float in the open sea: all the species are of small size.
b. Gasteropoda (feet under the body).—These crawl by means of a muscular disk, or foot, which is attached to the under-part of the body; most of the species are marine, but some are terrestrial, and others inhabit fresh-water. They are very widely distributed; the garden snail is a familiar instance of a terrestrial Gasteropod.
c. Cephalopoda (feet around the head).—The mollusca of this order have powerful muscular arms, or tentacula, which surround the head, or upper part of the body; some genera have no shell, but possess an internal skeleton, as the recent Sepiadæ and the fossil Belemnitidæ. Most of the testaceous Cephalopoda have a discoidal, univalve shell, which is divided internally by septa or partitions; as the Nautilus.
FOSSIL MOLLUSCA.
In many univalves the aperture or opening is entire, that is, without any notch or groove; in others it is notched or extended into a canal, or siphon, and this character has relation to the respiratory organs: thus the Gasteropods, in which the water is conducted to the interior by a muscular tube, or siphon, have the margin of the aperture of the shell channelled; as in the Whelk, or Buccinum. Many of the land and fresh-water species have entire openings, and are, for the most part, herbivorous; while the greater number of the marine univalves have the aperture indented or notched, and are carnivorous.[349] Some of these mollusca, too, have a retractile proboscis, armed with minute teeth, by which they can rasp or bore into the shells of the species on which they prey. There are some exceptions to the above rules, but the prevalence of the characters specified afford pretty certain indications of the fluviatile or marine nature of the originals. The application of these data to geological investigations will be considered hereafter.
[349] The form of the aperture does not necessarily indicate fresh-water genera. Melanopsis, Pirena, and most of the Melaniæ have a channelled or notched aperture. Fresh-water univalves frequently have the spire corroded; in a fossil state they can only be determined [to be fresh-water species] by their analogy to recent genera and sub-genera.—Note by Mr. Woodward.
In the generic distinctions of the simple univalves, the form of the mouth is an important character; while in the bivalves, the configuration of the hinge affords an equally convenient aid for their classification.