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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seashell (n.)

also sea-shell, "shell of any salt-water mollusk; shell found on the seashore," Old English sæscel; see sea and shell (n.).

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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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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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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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