Introduction to Marine Biology 4th Edition by George Karleskint,Richard Turner,James Small

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Skates and rays differ in the way they swim, the fleshiness of the tail, fin structure, and reproductive mode. Rays swim by moving their fins up and down, but skates swim by creating a wave that begins at the forward edge of the pectoral fins and sweeps down the edge to the back of the fins. The tails of skates are fleshier than those of rays and have small fins. The tails of rays are streamlined and contain serrated, hollow spines (barbs) connected to venom glands. Most skates release their eggs in a leathery case (mermaid's purse) but rays give birth to live young.