Osteichthyes are the bony fish, a group paraphyletic to the land vertebrates, which are sometimes included. Most belong to the subclass Actinopterygii. The others are called lobe-finned fish, and include lungfish and coelacanths.
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).
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.