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Placoid scales cannot grow in size, but rather more scales are added as the fish increases in size. Similar scales can also be found under the head of the denticle herring . The amount of scale coverage is much less in rays.
These scales, also called denticles, are similar in structure to teeth, and have one median spine and two lateral spines. The modern jawed fish ancestors, the jawless ostracoderms and later jawed placoderms, may have had scales with the properties of both placoid and ganoid scales.
Placoid Scale. Super Smooth scales (dermal denticles) coat the skin of sharks, rays, and cartilaginous fishes due to the absence of dermal bone. These scales are present in the dermis, which has fibrous connective tissue components, and project through the epidermis, which contains secretory cells and stratified epidermal cells, to the surface.
Ganoid scales are flat, basal-looking scales. Derived from placoid scales, they have a thick coat of enamel, but without the underlying layer of dentin. These scales cover the fish's body with little overlapping. They are typical of gar and bichirs. Cycloid scales are small, oval-shaped scales with growth rings like the rings of a tree. They ...
Chondrichthyes are aquatic vertebrates with paired fins, paired nares, placoid scales, conus arteriosus in the heart, and a lack of opercula and swim bladders. Within the infraphylum Gnathostomata, cartilaginous fishes are distinct from all other jawed vertebrates.
A large portion of the skate's dorsal body is covered by rough skin made of placoid scales. Placoid scales have a pointed tip that is oriented caudally and are homologous to teeth. Their mouths are located on the underside of the body, with a jaw suspension common to Batoids known as euhyostyly. [11]
Bony fish do not have placoid scales like cartilaginous fish, instead they consist of three types of scales that do not penetrate the epidermis in the process. The three categories of scales for Osteichthyes which are cosmoid scales, ganoid scales, teleost scales.
placoid scales Teeth-like scales found in sharks and rays. posterior Towards the hind end of the fish. postorbital Behind the eye. precaudal Anterior to the tail portion. premaxillaries Two bones forming the front portion of the upper jaw. preocular spine A spine positioned above and in front of the eye. preopercle, preoperculum