Fish scales are small, rigid plates that grow out of the skin of a fish and act as a shield against predators, disease, and habitat. Fish scales are part of the fishs integumentary system and are produced from the mesoderm layer of the dermis, which distinguishes them from reptile scales. The same genes involved in tooth and hair development in mammals are also involved in scale development. Fish scales vary significantly in size and form depending on the species, and not all fish have scales. The scales of bony fish are made of enamel and dentine, like teeth, and they can be detached individually from the skin. Most bony fishes are covered with the cycloid scales of salmon and carp, or the ctenoid scales of perch, or the ganoid scales of sturgeons and gars. Cartilaginous fishes (sharks and rays) are covered with placoid scales, which are small, spiny, irregular-shaped scales called dermal denticles or "skin teeth". Fish scales are composed of various useful materials, including organic components such as collagen, fat, lecithin, sclerotin, vitamins, and minerals.