Sharks come from the family Elasmobranchii within the class Chondrichthyes. Members of the Elasmobrandchii family have skeletons made out of cartilage instead of bone. Aside from sharks, this family include rays, skates and chimaera.
Rays belong to the order Batoidei. They have "dorsoventral compression," which means they are squished or flattened in comparison with the standard fish body plan. They usually have a thin, whiplike tail. Most are bottom-dwellers, though some, like the manta ray, swim through open water. They vary in length from a few inches to more than 20 feet. Some have defenses, such as the spines and barbs of stingrays and thorny-back rays. Others, like the torpedo ray, can even generate a painful shock. Most species live in the ocean, but freshwater species do exist.
Skates and rays are closely related. Skates belong to the order Rajiformes, which some scientists consider to be a subclass of Batoidei, making them a specialized group of ray. In any case, they have the same flattened body shape rays have. In general, though, rays give live birth and skates lay eggs. Skates' eggs come in horned, hard cases, similar to the eggs of egg-laying sharks.
Guitarfish are an odd-looking group of fish in the shark/ray/skate family. Their bizarre appearance has given rise to numerous common names like fiddler ray, banjo shark and sharkray. They look like crosses between sharks and rays. The fronts of their bodies look compressed and flattened, like rays. However, the back halves of their bodies look more like traditional sharks' rear halves do. They live in warm, shallow tropical waters and grow to 6 feet in lenght.
Chimaeras, also called ghost sharks or ratfish, belong to the Elasmobranchii family but are more distantly related to the rest of the group. They possess a single gill-slit, unlike most other members of the family, which have five to six gills. Chimaeras live in rivers, estuaries and the deep sea. They eat fish and crustaceans. Some deep-sea Chimaeras posses bioluminescent organs, allowing them to glow in the dark.Their thin, ratlike tails give rise to their common name.
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