Skates are relatives of the sharks, much flattened from back to belly, with a roughly triangularly shaped body, and a long, thin tail. The large triangular pectoral fins, extending out on either side of the body, are the principal means of locomotion. These fins are undulated from front to rear, not flapped like wings as are those of some rays. The more or less rigid tail, with two small fins attached at its end, acts as a steering device to a certain extent.
The eyes are located quite close together on top of the head, while the mouth is underneath. The gill slits are also on the underside. Instead of taking water in through the mouth and passing it out through the gill slits, as do the sharks during breathing, skates use their spiracles, which are two valved openings just behind the eyes. Water enters through the spiracles and passes out through the gill slits. Thus the problem of remaining on the bottom and inhaling without taking in debris – as would happen if the mouth were used – is solved.
The little skate is usually not more than twenty inches long, but may attain a length of two feet. Females lay eggs inclosed in a rectangular, blackish, leathery case, measuring about one and three-quarters by one and three-eighths inches, at each corner of which a thin, hook-shaped prong is attached. Similar egg cases are laid by other skates. They are sometimes found washed up on the beach and called “mermaids’ purses.” The eggs of the little skate are laid during the late spring and the summer and probably take several months to hatch.
There are seventy-five or more skates belonging to the genus Raja, and they inhabit most of the world’s cool seas. Some of them occur in quite deep water. The largest species are about eight feet long.