Teeth.
The teeth of fish[40] are subject to a very large amount of variation, perhaps to more variation than are those of any other class of animals. Sometimes, as in adult Sturgeons, they are entirely absent, sometimes they are found on all the bones of the mouth, and also on the hyoid and branchial arches. The teeth are all originally developed in the mucous membrane of the mouth, but they afterwards generally become attached to firmer structures, especially to the jaws. In Elasmobranchs, however, they are generally simply imbedded in the tough fibrous integument of the mouth. Their attachment to the jaws may take place in three different ways.
Fig. 14. Diagram of a section through the jaw of a Shark (Odontaspis
americanus) showing the succession of teeth (Brit. Mus. from
specimen and diagram).
| 1. teeth in use. | cartilage. |
| 2. teeth in reserve. | 6. connective tissue. |
| 3. skin. | 7. mucous membrane of the |
| 4. cartilage of the jaw. | mouth. |
| 5. encrusting calcification of |
(1) By an elastic hinge-joint, as in the Angler (Lophius), and the Pike (Esox lucius). In the Angler the tooth is held by a fibrous band attaching its posterior end to the subjacent bone, in the Pike by uncalcified elastic rods in the pulp cavity.
(2) By ankylosis, i.e. by the complete union of the calcified tooth substance with the subjacent bone. This is the commonest method among fish.
(3) By implantation in sockets. This method is not very common among fish. The teeth are sometimes, as in Lepidosteus, ankylosed to the base of the socket. In this genus there is along each ramus of the mandible a median row of large teeth placed in perfect sockets, and two irregular lateral rows of small teeth ankylosed to the jaw.
Dentine, enamel and cement are all represented in the teeth of fishes, but the enamel is generally very thin, and cement is but rarely developed. Dentine forms the main bulk of the teeth; it is sometimes of the normal type, but generally differs from that in higher vertebrates in being vascular, and is known as vasodentine. A third type occurs, known as osteodentine; it is traversed by canals occupied by marrow, and is closely allied to bone.
