CLASS I.—The CEPHALOPODA (Head-footed) contains those mollusca that, like the common Octopus, have a number of feet (or arms) set round the mouth, and is divided into those having two gills. (Order I. Dibranchiata); and those with four (Order II. Tetrabranchiata). Order I. is again divided into: (a.) Those with eight feet like the Argonaut (or Paper-nautilus, [Fig. 4]), which fable has so long endowed with the power of sailing on the surface of the ocean, (it is even represented in one book as propelling itself through the air!) and the common Octopus. (b.) Those with ten feet, such as the Loligo (or Squid, [Fig. 6]), whose delicate internal shell so much resembles a pen in shape; the Cuttle-fish (Sepia, Figs. [5] & [7]), whose so-called "bone" (once largely used as an ink eraser) is frequently found on our southern coasts; and the pretty little Spirula ([Fig. 8]).
The only representative of the four-gilled order now living is the well-known Pearly Nautilus; but in former times the Tetrabranchiata were extremely numerous, especially the Ammonites.
|
Fig. 6. Loligo vulgaris, and "Pen." |
Fig. 7. Sepia officinalis. |
Fig. 8. Spirula.
CLASS II.—GASTEROPODA (Belly-footed) comprises those mollusca which, like the common snail, creep on the under-surface of the body, and with one exception (Chiton, [Fig. 20]) their shells are univalve (i.e., composed of one piece). But before we go further, it may be well to point out the names given to different parts of a univalve shell. The aperture whence the animal issues is called the mouth, and its outer edge the lip; each turn of the shell is a whorl; the last and biggest, the body-whorl, the whorls, from the point at the top, or apex, down to the mouth form the spire; and the line where the whorls join each other is called the suture. The axis of the shell around which the whorls are coiled is sometimes open or hollow, and the shell is then said to be umbilicated (as in Fig. 3b); when closely coiled, a pillar of shell, or columella, is left (as in [Fig. 9]). Sometimes the corner of the mouth farthest from the spire and next the columella, is produced into a channel, the anterior canal (as in [Fig. 9]); whilst where the mouth meets the base of the spire there may be a kind of notch which is termed the posterior canal. Most Gasteropods are dextral, that is to say, the mouth is to the right of the axis as you look at it; a few, however, are sinistral, or wound to the left (like Physa); whilst reversed varieties of both kinds are met with.
Gasteropods of the first order have comb-like gills placed in advance of the heart, and are hence termed Prosobranchiata. They are divided into two groups: (a) Siphonostomata (Tube-mouthed), in which the animal has a long proboscis, and a tube, or siphon, from the breathing-chamber that passes along the anterior canal of the shell, which in this group is well developed. They have a horny operculum, or lid, with which to close the aperture. (b) Holostomata (or Whole-mouthed). In these the siphon is not so produced, and does not want to be protected; accordingly the mouth of the shell is entire, i.e. has no canal. The operculum is horny or shelly. The former (group a) includes several families:
1. Strombidæ, comprising shells, like the huge Strombus, or "Fountain-shell," which is so often used to adorn the mantelpiece or rockery, and from which cameos are cut.