Etymology
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mollusk (n.)

"soft-bodied invertebrate animal, usually with an external shell," 1783, mollusque (modern spelling from 1839), from French mollusque, from Modern Latin Mollusca (see Mollusca), the phylum name. Related: Molluscuous; molluscan.

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*mel- (1)
Proto-Indo-European root meaning "soft," with derivatives referring to soft or softened materials.

It forms all or part of: amblyopia; bland; blandish; blenny; emollient; enamel; malacia; malaxation; malt; melt; mild; Mildred; milt; moil; mollify; Mollusca; mollusk; mulch; mullein; mutton; schmaltz; smelt (v.); smelt (n.).

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit mrdh "to neglect," also "to be moist;" Greek malakos "soft," malthon "weakling;" Latin mollire "soften," mollis "soft;" Old Irish meldach "tender."
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winkle (n.)
edible mollusk, 1580s, shortening of periwinkle (n.2).
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squid (n.)
marine mollusk, cuttlefish, 1610s, of unknown origin; perhaps a sailors' variant of squirt, so called for the "ink" it squirts out.
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nudibranch (n.)

type of mollusk having naked gills and no shell, 1844, literally "having naked gills," from nudi- "naked" + Latin branchae, from Greek brankhia "gills," plural of brankhion "fin." 

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radula (n.)

1753, a type of surgical instrument, from Latin radula "scraper, scraping iron," from radere "to scrape" (see raze (v.).  As "tongue or lingual ribbon of a mollusk," by 1853. Related: Radular.

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bivalve (adj.)
1660s in reference to mollusks with hinged double shells; 1670s in reference to shutters or doors having two folding parts; from bi- + valve. The noun is 1680s in the mollusk sense.
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brachiopod (n.)
type of bivalve mollusk of the class Brachiopoda, 1836, Modern Latin, from Greek brakhion "an arm" (see brachio-) + pous "foot" (from PIE root *ped- "foot"). They develop long spiral "arms" from either side of their mouths.
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limpet (n.)
type of marine gastropod mollusk, early 14c., earlier lempet (early 14c.), alteration of Old English lempedu, which apparently originally meant "a lamprey" (both cling by sucking), from Medieval Latin lampreda "lamprey; limpet," from Late Latin lampetra "lamprey" (see lamprey). Limpin was a 16c. variant that survived in dialects.
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