Fig. 28.—Roccus lineatus. Branchial arches. (After Starks.)

The Pharyngeals.—The hindmost gill-arch, as above stated, is modified to form a sort of jaw. The tooth-bearing bones above, 2 to 4 pairs, are known as upper pharyngeals (45), those below, single pair, as lower pharyngeals (46). Of these the lower pharyngeals are most highly specialized and the most useful in classification. These are usually formed much as in the striped bass. Occasionally they are much enlarged, with large teeth for grinding. In many families the lower pharyngeals are grown together in one large bone. In the suckers (Catostomidæ) the lower pharyngeal preserves its resemblance to a gill-arch. In the carp family (Cyprinidæ) retaining this resemblance, it possesses highly specialized teeth.

Vertebral Column.—The vertebral column is composed of a series of vertebræ, 24 in number in the striped bass and in many of the higher fishes, but varying in different groups from 16 to 18 to upwards of 400, the higher numbers being evidence of unspecialized or more usually degenerate structure.

Each vertebra consists of a double concave body or centrum (66). Above it are two small projections often turned backward, zygapophyses (71), and two larger ones, neurapophyses (67), which join above to form the neural spine (68) and thus form the neural canal, through which passes the spinal cord from end to end of the body.

Fig. 29.—Pharyngeal bone and teeth of European Chub, Leuciscus cephalus (Linnæus). (After Seelye.)

Fig. 30.—Upper pharyngeals of a Parrot-fish, Scarus strongylocephalus.