ORDER I. ACANTHOPTERYGII.
(Spiny-finned Fishes.)
The skeleton in this large and very natural Order is composed of bone; the first rays (counting from the head backward), of the dorsal fin, of the pectorals, and of the anal, and, generally the first ray of the ventrals are unjointed, inflexible, and spinous. When there is more than one dorsal, the anterior is entirely filled with spinous rays. In some cases, as in the common Sticklebacks, the spinous rays are unconnected by a common membrane, and form free spines. The ventrals are, for the most part beneath the pectorals, or even in advance of them. The body is clothed with scales formed of successive laminæ or layers of horn-like, unenamelled bone, which have their free hinder margin cut into teeth. The swimming-bladder is not furnished with an air-duct leading into the gullet.
Nearly two-thirds of the species belonging to the whole Class of Fishes are found in this Order, which are scattered over all parts of the world, both in fresh and salt waters. Many of them are distinguished for elegance of form and beauty of colour; nearly all are fit for food, and some, as the Mackerel family, including the Tunny, support important fisheries.
The form of the dorsal fin is subject to much variation in this Order. Nearly half of the species have it divided into two, a spinous and a
