Etymology
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echidna (n.)

Australian egg-laying hedgehog-like mammal, 1810, said to have been named by Cuvier, usually explained as from Greek ekhidna "snake, viper" (also used metaphorically of a treacherous wife or friend), from ekhis "snake," from PIE *angwhi- "snake, eel" (source also of Norwegian igle, Old High German egala, German Egel "leech," Latin anguis "serpent, snake").

But this sense is difficult to reconcile with this animal (unless it is a reference to the ant-eating tongue). The name perhaps belongs to Latin echinus, Greek ekhinos "sea-urchin," originally "hedgehog" (in Greek also "sharp points"), which Watkins explains as "snake-eater," from ekhis "snake." The 1810 Encyclopaedia Britannica gives as the animal's alternative name "porcupine ant-eater." Or, more likely, the name refers to Echidna as the name of a serpent-nymph in Greek mythology, "a beautiful woman in the upper part of her body; but instead of legs and feet, she had from the waist downward, the form of a serpent," in which case the animal was so named for its mixed characteristics (early naturalists doubted whether it was mammal or amphibian).

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echinoderm (n.)
1834, from Modern Latin Echinodermata, name of the phylum that includes starfish and sea urchins, from Latinized form of Greek ekhinos "sea urchin," originally "porcupine, hedgehog" (see echidna) + derma (genitive dermatos) "skin," from PIE root *der- "to split, flay, peel," with derivatives referring to skin and leather. So called from its spiky shell. Related: Echinodermal.
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ant-eater (n.)
also anteater, 1764, in reference to the South American species; 1868 of the Australian echidna; from ant + agent noun from eat (v.).
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