Etymology
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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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abalone (n.)
Origin and meaning of abalone
type of large mollusk found on the California coast, 1850, American English, from Spanish abulon, a loan-word from Rumsen (an extinct native language in the Costanoan family), aluan, said to mean "red abalone." Prized for its meat (once an important California export) and for the mother-of-pearl in its large shells (also called ear-shells).
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nacre (n.)

1590s, "type of shellfish that yields mother-of-pearl," from French nacre (Old French nacaire, 14c.), from Italian naccaro (now nacchera), possibly from Arabic naqur "hunting horn" (from nakara "to hollow out"), in reference to the shape of the mollusk shell. Meaning "mother-of-pearl" is from 1718. The French adjectival form nacré was applied in English to decorative objects iridescent like mother of pearl (1895).

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ammonite (n.)

"fossil cephalopod mollusk," 1758, from French (Breyn, 1732), "better established" [Century Dictionary] by French zoologist Jean Guillaume Bruguière in 1789, from Medieval Latin (cornu) Ammonis "horn of Ammon," the Egyptian god of life and reproduction (see Ammon), who was depicted with ram's horns, which the fossils resemble. The resemblance also was noted in ancient times. They also were thought to be petrified snakes, hence snakestone.

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cockle (n.1)

type of edible European mollusk, early 14c., from Old French coquille (13c.) "scallop, scallop shell; mother of pearl; a kind of hat," altered (by influence of coque "shell") from Vulgar Latin *conchilia, from Latin conchylium "mussel, shellfish," from Greek konkhylion "little shellfish," from konkhē "mussel, conch." Phrase cockles of the heart "inmost recesses of one's spirit" (1660s) is perhaps from similar shape, or from Latin corculum, diminutive of cor "heart." Cockle-shell attested from early 15c.

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pearl (n.)

"nacreous mass formed in the shell of a bivalve mollusk as a result of irritation caused by some foreign body," early 14c., perle (mid-13c. as a surname), from Old French perle (13c.) and directly from Medieval Latin perla (mid-13c.), which is of unknown origin. Perhaps from Vulgar Latin *pernula, diminutive of Latin perna, which in Sicily meant "pearl," earlier "sea-mussel," literally "ham, haunch, gammon," so called for the shape of the mollusk shells.

Other theories connect it with the root of pear, also somehow based on shape, or Latin pilula "globule," with dissimilation. The usual Latin word for "pearl" was margarita (see margarite).

Used from 14c. of anything valuable or of the finest kind; from mid-15c. of something small, round, and glistening white. For pearls before swine, see swine. Pearl Harbor translates Hawaiian Wai Momi, literally "pearl waters," so named for the pearl oysters found there; transferred sense of "effective sudden attack" is attested from 1942 (in reference to Dec. 7, 1941).

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clam (n.)

bivalve mollusk, c. 1500 (in clam-shell), originally Scottish, apparently a particular use of Middle English clam "pincers, vice, clamp" (late 14c.), from Old English clamm "bond, fetter, grip, grasp," from Proto-Germanic *klam- "to press or squeeze together" (source also of Old High German klamma "cramp, fetter, constriction," German Klamm "a constriction"), possibly from a PIE *glem- or *glom- "contain, embrace" (see glebe).

If this is right then the original reference is to the shell. Clam-chowder attested from 1822. To be happy as a clam is from 1833, but the earliest uses do not elaborate on the notion behind it, unless it be self-containment.

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mussel (n.)

"edible bivalve mollusk," Middle English muscle, from Old English muscle, musscel, from Late Latin muscula (source of Old French musle, Modern French moule, Middle Dutch mosscele, Dutch mossel, Old High German muscula, German Muschel), from Latin musculus "mussel," literally "little mouse," also "muscle;" like muscle, derived from mus "mouse" on the perceived similarity of size and shape (see mouse (n.)). The modern spelling, distinguishing the word from muscle, is recorded from c. 1600 but was not fully established until 1870s.

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scallop (n.)

type of edible bivalve mollusk, mid-14c., scalop, from Old French escalope "shell (of a nut), carpace," a variant of eschalope, which probably is from a Germanic source (compare Old Norse skalpr "sheath," Middle Dutch schelpe "shell"), from PIE root *skel- (1) "to cut."

Extended 17c. to objects shaped or ornaments cut like scallop shells, especially in design and dress. The shells of the larger species, often colorfully marked, have been used as domestic utensils. It also was a symbol of St. James the Great, and the shells were worn or carried as by pilgrims who had been to his shrine in Compostella.

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oyster (n.)

"edible bivalve mollusk of the family Ostreidæ," late 13c., oistre, from Old French oistre, uistre (Modern French huître) and directly from Latin ostrea, plural or fem. of ostreum "oyster," from Greek ostreon, from PIE root *ost- "bone." It is thus related to Greek ostrakon "a hard shell" and to osteon "a bone." The h- in the modern French word is a regular development; compare huile "oil" (Latin oleum), huit "eight" (Latin octo).

Why then the world's mine Oyster, which I, with sword will open. [Shakespeare, "The Merry Wives of Windsor," II.ii.2]

Oyster-bed "place where oysters breed or are bred" is from c. 1600; oyster-knife, used for opening oysters, is recorded from 1690s. Oysterman "man engaged in rearing, taking, or selling oysters" is attested from 1550s. The common statement that edible oysters are in season only in months that end in -r is recorded by 1765.

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