order of marine mammals containing whales, 1795, Modern Latin, from Latin cetus "any large sea creature" (whales, seals, dolphins), from Greek kētos "a whale, a sea monster," which is of unknown origin, + -acea.
"waxy, fatty stuff in the head of certain whales," late 15c., from Medieval Latin sperma ceti "sperm of a whale" (it has when fresh something of the appearance of sperm), from Latin sperma "seed, semen" (see sperm) + ceti, genitive of cetus "whale, large sea animal" (see Cetacea). The substance in olden times was credited with medicinal properties, as well as being used for candle oil.
Use ... Sperma Cete ana with redd Wyne when ye wax old. [Sir George Ripley, "The Compound of Alchemy," 1471]
Scientists still are not sure exactly what it does. Sperm whale, short for spermaceti whale, is from 1830.
The names are thus formally adjectives, Latin animalia "animals" (a neuter plural noun) being understood. Thus Crustacea "shellfish" are *crustacea animalia "crusty animals."
In botany, the suffix is -aceae, from the fem. plural of -aceus, forming orders or families of plants (Rosaceae, etc.) with a presumed plantae "plants," which is a fem. plural.
"animals of the mammalian order Cetacea," Old English hwæl "whale," also "walrus," from Proto-Germanic *hwalaz (source also of Old Saxon hwal, Old Norse hvalr, hvalfiskr, Swedish val, Middle Dutch wal, walvisc, Dutch walvis, Old High German wal, German Wal), from PIE *(s)kwal-o- (source also of Latin squalus "a kind of large sea fish"). In popular use it was applied to any large sea animal. Phrase whale of a "excellent or large example" is c. 1900, student slang. Whale-oil attested from mid-15c.