326 
MOLLUSCS. 
are mostly peculiar. It should be noticed that there are great differences in the 
molluscan land-fauna of different areas; that of North America being, for instance 
quite distinct from that of Central and South America. 
It is a well-known fact that certain marine gastropods and bivalves inhabit 
particular parts of the sea-bottom. Some groups which occur between tide-marks, 
such as periwinkles and limpets, are termed littoral forms; others occurring below 
low-water mark, to about ten or twelve fathoms, are said to inhabit the laminarian 
zone, or the region where seaweed abounds. Below this, to about fifty fathoms, 
extends the coralline zone, so called from the abundance of corallines at this depth, 
which also furnishes a lurking-place for certain special forms. Beyond this is the 
deep-sea or abyssal region, of which certain species and genera are more or less 
characteristic. 
Other races, such as the squids among the cephalopods, the various forms of 
pteropods and heteropods, and a few other gastropods, pass their lives far out at 
sea upon the surface of the ocean, and are termed pelagic species. 
ci The Mollusca constitute one of the principal divisions (a sub¬ 
kingdom) of the animal kingdom, and it is subdivided into five 
principal sections or classes, namely, Cephalopoda, Gastropoda, Amphineura, 
Scaphopoda, and Pelecypoda. These divisions are founded on peculiarities in the 
general conformation of the animals, but it is also worthy of notice that the shells 
of the different classes differ widely in type. An important feature characteristic 
of the three first of these classes is a structure termed the radula. It is situated 
within the mouth, and is a kind of muscular tongue armed with teeth, and used in 
obtaining or comminuting food. The armature of this radula, odontophore, or 
lingual ribbon, is subject to great variation, and these differences have afforded 
characters for distinguishing various groups among the gastropods. There are a 
few genera of Gastropoda which are peculiar on account of the want of this 
masticatory organ, and it is also unknown among the headless bivalves. 
The Squids, Cuttle-Fishes, and Nautili, —Class Cephalopoda. 
The cephalopods are considered the most highly organised of all molluscs, and 
some of the species are remarkable for the enormous size they sometimes attain. 
They are exclusively marine animals, leading a predatory life out on the high 
seas, or among rocks in shallow water, or about low-water mark. The sexes are 
distinct. They may be recognised by the symmetry of their general conformation, 
the fleshy arms or tentacles situated around their mouths and in front of the 
head, and by their retrograde mode of progression, which is effected by the 
expulsion of water from a particular organ, termed the siphuncle or funnel. With 
one exception—the nautilus—none of the living cephalopods possess an external 
shell, and they are consequently termed naked molluscs. Nearly all decapods, 
that is, those species which are provided with ten so-called arms—have a straight 
calcareous or horny internal shell, which is a strengthening support to the back. 
Spirula, however, although a decapod, is an exception, and possesses a segmented 
shell, coiled up like a ram’s horn, and concealed within the hinder part of the 
animal’s body. The external shell wherein the female argonaut dwells is not the 
