A fin is a unique anatomical feature consisting of bony spines or rays protruding from a fish's body. They are covered in skin, either webbed against each other like most bony fishes, or fin-like like sharks. Except for the caudal fin or caudal fin, the fins of fish are not directly connected to the spine and are supported only by muscles. Fins on different parts of the fish serve different purposes such as advancing, turning, sustaining and stopping. Most fish use their fins to swim, flying fish use their pectoral fins to glide, and frogfish use their pectoral fins to crawl. Fins can also be used for other purposes. Male sharks and filefish use modified fins to release sperm, thresher sharks use their caudal fins to stun prey, reef otters have venom-injecting spines in their dorsal fins, and angler fish uses the first spine of the dorsal fin like a fishing rod to attract flies. Triggerfish avoid predators by shoving into coral crevices and grabbing with the spikes of their fins. Fins can be paired or unpaired. The pectoral and pelvic fins are paired, and the dorsal, anal, and caudal fins are unpaired and located along the midline of the body. For each fin type, there are many fish species that have lost specific fins during evolution. Some groups acquired additional unpaired fins during evolution (additional dorsal fin, adipose fin, etc.). In some Acanthodii ("spine sharks"), one or more pairs of 'intermediate' or 'anterior pelvic' spines are present between the pectoral and pelvic fins, but these are not associated with the fins. A pair of pectoral fins is on each side, usually folded just behind the operculum, and is homologous to the front legs of tetrapods. A particular function of the pectoral fins, which are highly developed in some fish, is the generation of dynamic buoyancy, which helps some fish, such as sharks, maintain depth and provide 'flight' to flying fish. Many fish have pectoral fins that help them walk. In particular, the lobed fins of some anglerfish and mudskippers help them to walk (see also Walking Fish). Certain pectoral fin rays can be transformed into finger-like projections, such as in sea robins and flying gurnards. In skating and rays, the pectoral fins are used for propulsion (radiform propulsion). The "horns" of the manta and her relatives are called cephalic fins. This is actually a modification of the front part of the pectoral fin. The paired pelvic or pelvic fins are usually positioned ventrally below and behind the pectoral fins, but may be positioned in front of the pectoral fins in many fish families (e.g. cod). Pelvic fins help the fish move up and down through the water, turn sharply, and stop quickly. In gobies, the pelvic fins are often fused so that he is one sucker. This can be used to attach to objects. Pelvic fins can occupy many positions along the ventral surface of the fish. Ancestral prone position can be seen in (for example) minnows. The breast position of the sunfish. Position of the neck when the pelvis is in front of the pectoral fins, as seen in burbot.