in animals, hard thorny formations, usually projecting above the body surface, and having a chiefly protective function.

In sponges the spines, or spicules, are uni-, tri-, tetra-, or multi-radiate formations of lime carbonate or silica that form the internal skeleton. In most echinoderms, the spines are external lime formations that are particularly diverse in sea urchins. In bony fish, the spines, or spikes, are hard, unsegmented, bony rays located mainly along the front edge of the fins; they are especially numerous on the dorsal fin of acanthopterygian fish (perch, ruff). In some mammals the horny spines are thick, modified hairs that come to a point. Examples among the monotremes are the echnidas— Tachyglossus and Zaglossus; among the insectivores, the tenrecs and hedgehogs; among the rodents, the porcupine.