Fertilization is internal in rays. Most bear live young, but the skates lay flattened, rectangular eggs, enclosed in leathery shells, with tendrils at the corners for anchorage. Empty egg cases of this type are found on beaches and are known as mermaids' purses. Most ray families have a more or less cosmopolitan distribution in tropical and subtropical marine waters; some include temperate or cold-water species. Some rays can live in brackish bays and estuaries, and the sawfish enters freshwater rivers and lakes.