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 heteropocls, and a few other gastropods, pass their lives far out at 



sea upon the surface of the ocean, and are termed pelagic species. 



The Mollusca constitute one of the principal divisions (a sub- 

 Classification. . . . ..... 



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 



