3294 THE MOLLUSKS OR SHELLFISH 



great distances by ocean currents, or the action of the tides and wind. On the con- 

 trary, land mollusks are creatures of slow progression, and are liable to have their 

 distribution hindered, either by rivers, mountains, or seas. Consequently we find 

 that island faunas, as regards the terrestrial species, are mostly peculiar. It should 

 be noticed that there are great differences in the molluskan 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 par- 

 ticular 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. 



The Molluska constitute one of the principal divisions (a subking- 

 dom) 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 con- 

 formation of the animals, but it is also worthy of notice that the shells of the differ- 

 ent classes differ widely in type. An important feature characteristic of the first 

 three 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 com- 

 minuting food. The armature of this radula, odontophore, or lingual ribbon, is 

 subject to great variation, and these differences have afforded characteristics for dis- 

 tinguishing various groups among the gastropods. There are a few genera of Gas- 

 tropoda which are peculiar on account of the want of this masticatory organ, and it 

 is also unknown among the headless bivalves. 



THE SQUIDS, CUTTLEFISHES, AND NAUTILI Class Cephalopoda 



The cephalopods are considered the most highly organized of all mollusks, 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 recognized 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 conse- 



