• WordNet 3.6
    • n Mollusca gastropods; bivalves; cephalopods; chitons
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Webster's Revised Unabridged Dictionary
    • prop. n. pl Mollusca (Zoöl) One of the grand divisions of the animal kingdom, a phylum including the classes Cephalopoda Gastropoda Pteropoda Scaphopoda, and Pelecyopodasyn. Bivalvia, formerly called Lamellibranchiata, or Conchifera). These animals have an unsegmented bilateral body, with most of the organs and parts paired, but not repeated longitudinally. Most of them develop a mantle, which incloses either a branchial or a pulmonary cavity. They are generally more or less covered and protected by a calcareous shell, which may be univalve, bivalve, or multivalve.☞ Formerly the Brachiopoda, Bryzoa, and Tunicata were united with the Lamellibranchiata in an artificial group called Acephala, which was also included under Mollusca. See Molluscoidea.
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Century Dictionary and Cyclopedia
    • n mollusca One of the leading divisions of invertebrated animals; an extensive series of invertebrates whose bodies are soft, without any jointed legs, and commonly covered with a hard shell in one, two, or more pieces, and whose principal parts are neither segmented into a series of longitudinal rings, as in insects, crustaceans, and worms, nor radiately arranged, as in echinoderms; the mollusks, as the univalve or bivalve shell-fish of ordinary language. Mollusks have no trace of a notochord or urochord, which distinguishes them from certain organisms, as ascidians, formerly classed with them. They are primitively bilaterally symmetrical, or have a right and left “side” along a main axis; this form is best expressed in the chitons, and is evident in bivalves, slugs, etc., but its expression is often obscured by a twisting to which the body is subjected in various univalves, as those whose shells are spiral. (See Isopleura, Anisopleura.) There is always a well-defined alimentary canal, with definite walls. A nervous system is well developed as a set of ganglia with connecting commissures, one characteristic feature of which is the formation of a nervous ring or collar around the gullet, and another is the torsion of the visceral commissures in those forms whose bodies are twisted as above said. (See Euthyneura, Streptoneura.). Most mollusks have a distinct head, which, however, is not apparent in bivalves, leading to a division of headless mollusks (Acephala or Lipocephala). A characteristic organ of Glossophora or mollusks with heads is the odontophore, buccal mass, or lingual ribbon, whose radula serves as a rasping-organ in a mouth otherwise soft and toothless. Various modifications of the radular teeth give rise to several descriptive terms. (See ptenoglossate, rachiglossate, rhipidoglossate, tænioglossate.) There is always a heart, with a ventricle and at least one auricle, and dorsal in position. Its relative situation with respect to the gills differs in certain groups of mollusks. (See opisthobranchiate, prosobranchiate.) The circulation is double. The respiratory system is branchial, and in some cases, as of snails and slugs, modified for breathing air into a kind of lung. (See Pulmonata, Gasteropoda.) The primitive typical gills are paired organs called ctenidia; but these undergo many modifications, and their function of respiration may be assumed vicariously by other parts of the body not homologous with them. These modifications give rise to the names of many subordinate groups of molusks, especially of gastropods, besides that of the great series Lamellibranchiata. The renal organs of mollusks are technically called nephridia, or organs of Bojanus. (See cut under Lamellibranchiata.) The sexual organs are developed, either in the same individuals, or in different individuals of opposite sexes. The characteristic organ of locomotion is the foot or podium, a development of the under surface of the body, which may be a broad fiat sole (see cut under Gasteropoda), upon which the mollusk creeps, or otherwise shaped. It is often wanting, as in the oyster, or may give rise to a thready byssus by which the animal is rooted, as in the mussel. Forms of the podium give names to most of the leading groups of mollusks, as cephalopods, pteropods, scaphopods, heteropods, gastropods, and pelecypods. A large part of the soft integument of mollusks forms what is called the mantle or pallium, from which the shell, when present, is developed (see integropalliate, sinupalliate), and the impression of the edge of the mantle on the inside of the shell is the pallial line. Some mollusks are entirely naked, or have only a rudimentary and concealed shell, as land-slugs and sea-slugs, and also most of the living cephalopods. The body of cephalopods is strengthened by an internal skeleton, the calamary or cuttlebone, though no mollusk has an articulated internal skeleton. But the great majority of mollusks have a hard shell (whence the old names Testacea, Ostracodermata), of a horny or chitinous or more decidedly calcareous substance. Those whose shell is single are called univalves; those in which it forms a hinged pair of shells are bivalves; but the former may have an additional shelly piece, closing the aperture, the operculum; and the two main valves of the latter may be supplemented by accessory valves (see cut under accessory). Bivalves are the natural group of headless or lamellibranch mollusks; but univalves include several orders, though the word is chiefly used of the numerous and conspicuous gastropods. A few mollusks are technically multivalve; such are the chitons, hence called Polyplacophora, having several segments of the shell in lengthwise series. (See cut under chiton.) Cirripeds used to be considered multivalve mollusks. The shell is usually covered outside with a rough skin or epidermis; inside it may be beautifully lustrous, as with mother-of-pearl. Most mollusks live either in salt, brackish, or fresh water; land-mollusks are mostly found in damp places. Most are locomotory, either by creeping or by swimming; some swim by flapping their shells, others by moving various appendages; many adhere to or even burrow deeply in rocks; a few are parasitic. Some are carnivorous, others herbivorous; most are oviparous, a few ovoviviparous. Many are important as food, and the shells of many are put to useful or ornamental purposes. Certain bivalves furnish pearls. The Mollusca have been variously rated, limited, and classified; at one time the bodies of the animals were differently named from their shells.
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Webster's Revised Unabridged Dictionary
NL. See Mollusk


In literature:

They existed, few or many, as early as any other of the mollusca.
"The Christian Foundation, Or, Scientific and Religious Journal, Volume I, No. 7, July, 1880" by Various
LIME, effect of, upon shells of the mollusca, ii.
"The Variation of Animals and Plants Under Domestication, Volume II (of 2)" by Charles Darwin
In variety, in this connexion, the Mollusca must perhaps be given the first place.
"Encyclopaedia Britannica, 11th Edition, Volume 9, Slice 1" by Various
Such beds exist; there is, however, not a trace in any of them of any American mollusca.
"The History of the European Fauna" by R. F. Scharff
No structure comparable to this siphuncular pedicle is known in any other Mollusca.
"Encyclopaedia Britannica, 11th Edition, Volume 5, Slice 6" by Various
His attention was given not only to geology but to zoology, and especially to the land-mollusca and to the vertebrates.
"Encyclopaedia Britannica, 11th Edition, Volume 4, Slice 1" by Various
These animals have not an internal skeleton, like the vertebrata; nor are they wholly destitute of a skeleton, as are the mollusca.
"Illustrative Anecdotes of the Animal Kingdom" by Samuel Griswold Goodrich
Some of them have feet, as the mollusca.
"Lives of Eminent Zoologists, from Aristotle to Linnæus" by William MacGillivray
What various methods of locomotion are found among mollusca?
"A Guide for the Study of Animals" by Worrallo Whitney
This animal is a genus of the mollusca tribe, which glitters in the night on the crest of every bursting wave.
"The South-West" by Joseph Holt Ingraham