From ZooTerms (Dictionary of Invertebrate Zoology)
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beak (noun; Old French bec, beak): 1. Anything projecting and ending in a point.

2. (Arthropoda) a. In Chelicerata, the gnathosome of acarines. b. In Crustacea, the anteroventral projection of the free margin of the carapace; not equivalent to rostrum. c. In Insecta, the snout, proboscis, or rostrum; in Homoptera, usually 3-segmented, arising from the ventroposterior of the head and directed backward under the body.

3. (Brachiopoda) The pedicel valve.

4. (Bryozoa) The avicularia.

5. (Mollusca) a. An angular projection of the apex of an intermediate valve; a mucronate valve; a similar projection of the upper surface of the valve anteriorly and between the sutural laminae is termed a false beak. b. In Bivalvia, noselike angle, along or above the hinge margin, marking the area of shell growth. c. In Cephalopoda, paired horny mandibles.