Osteichthyes

From Free net encyclopedia

{{Taxobox | color = pink | name = Bony fishes | image = Herring2.jpg | image_caption = Atlantic Herring | regnum = Animalia | phylum = Chordata | subphylum = Vertebrata | infraphylum = Gnathostomata | classis = Osteichthyes | classis_authority = Huxley, 1880 | subdivision_ranks = Orders | subdivision = See under classes: Actinopterygii and Sarcopterygii }}

Class Osteichthyes are the bony fish, a group paraphyletic to the land vertebrates, which are sometimes included. Most belong to the Actinopterygii. The other seven living species are called lobe-finned fish, and include lungfish and coelacanths. Some of species of lobe-finned fish have jointed bones. They are traditionally treated as a class of vertebrates, with subclasses Actinopterygii and Sarcopterygii, but newer schemes may divide them into several separate classes. The vast majority of fish are bony fish, and therefore belong to class Osteichthyes. Based on the number of species, Osteichthyes are the most successful group of vertebrates. They are the largest class of vertebrates with over 29,000 species.

Osteichthians are characterized by a relatively stable pattern of cranial bones, rooted teeth, medial insertion of mandibular muscle in lower jaw. The head and pectoral girdles are covered with large dermal bones. The eyeball is supported by a sclerotic ring of four small bones, although this characteristic has been lost or modified in many modern species. The labyrinth in the inner ear contains large otoliths. The braincase, or neurocranium, is frequently divided into anterior and posterior sections divided by fissure. Osteichthyans possess a lung or swim bladder. They do not have fin spines, but instead support the fin with lepidotrichia (bone fin rays). Bony fish also have operculum, which helps them breathe without having to swim.

One of the best-known innovations of the osteichthians is endochondral or "replacement" bone, i.e. bone ossified internally, by replacement of cartilage, as well as perichondrally, as "spongy bone." In the more general vertebrates there are various types of calcified tissues: dentine, enamel (or "enameloids") and bone, plus variants, characterized by their ontogeny, chemistry, form and location. However, endochondral bone is unique because it begins life as cartilage. In more basal vertebrates, cartilaginous structures can become superficially calcified. However, in osteichthians, the circulatory system actually invades the cartilaginous matrix. This permits the local osteoblasts (bone-forming cells) to continue bone formation within the cartilage and also recruits additional, circulating osteoblasts. Other cells gradually eat away at the surrounding cartilage. The net result is that the cartilage is replaced from within by a somewhat irregular vascularized network of bone. Structurally, the effect is to create a relatively lightweight, flexible, "spongy" bone interior, surrounded by an outline of dense, lamellar periostial bone (since this bone now surrounds other bone, rather than cartilage, it is referred to as periostial rather than perichondral). This is the unique endochondral bone from which the osteichthians derived their name, as well as countless structural advantages. However useful endochondral bone may be, it is also much heavier and less flexible than cartilage. Thus, many modern osteichian groups, including the extremely successful teleosts, have evolved away from extensive use of endochondral bone.

The dissection of a bony, or any other fish can prove quite useful to study internal organs.

The ocean sunfish is the most massive bony fish in the world (but not the longest one; that honor goes to the oarfish). Specimens of ocean sunfish have been observed up to 3.33 m (11 ft) in length and weighing up to 2,300 kg (5070 lbs). Other very large bony fish include the blue marlin, some specimens of which have been recorded as in excess of 820 kilograms, the black marlin, and some sturgeon species.

[edit]

Osteichthyes

From Free net encyclopedia

See also

Views

Personal tools

Search

Partner sites