Although the cartilaginous skeleton is more flexible than bone, it must also be sturdy enough to support the size and weight of the animal. In sharks, the cartilaginous skeleton undergoes a process of calcification to harden the vertebrae and jaws. The animals absorb calcium salts that fortify both internal and external cartilage structures. Cartilaginous fish possess no ribs and are in capable of supporting their own weight out of the water. Skates and stingrays also possess calcified spines that form the tail and are often a part of the animal's defense mechanism.
Like the skeleton and jaws, the teeth and skin of cartilaginous fish also undergo calcification. Much like human teeth, the teeth of cartilaginous fish contain a soft pulp covered by a hard, calcified layer called dentin. The skin of some cartilaginous fish shares the same characteristics as the teeth. Small tooth-like projections called denticles form on placoid scales, also consisting of a center pulp and a layer of dentin, giving the skin the feel of sandpaper. These denticles deter the attachment of marine parasites like barnacles.
Cartilaginous fish have five to seven gills used for breathing. Sharks have gill slits on either side of the head, while skates, sawfish and rays have gills located on their underbellies. Some shark species must swim continuously in order to move oxygenated water over their gills, while bottom dwelling species possess spiracles, openings that pump water through the gills during periods of rest on the bottom. Cartilaginous fish also possess nostrils and finely tuned olfactory organs for smelling prey from miles away.
Whether shark, skate or ray, all cartilaginous fish possess fins in one form or another. Some species possess both vertical and lateral fins, while others have only lateral fins. Vertical fins consist of one or two triangular dorsal fins on the back, an anal fin and a caudal, or tail, fin. Lateral fins include a pair of pectoral fins toward the front of the body and a pair of pelvic fins near the tail. Vertical fins keep the animal upright, while lateral fins assist in lift like the wings of an airplane.
The internal organs of cartilaginous fish differ little from other marine animals, possessing a heart, liver, spleen, stomach and intestines. Due to the absence of a swim bladder, cartilaginous fish possess an oil-rich liver used for buoyancy. The reproductive organs include cloacae and claspers. Both males and female possess a cloaca, used for excretion in males and for both excretion and reproduction in females. The males have penis-like organs called claspers, used for latching onto the females and depositing sperm.