II. Selachia.
The Selachians include a great number of cartilaginous fishes, varying much in form, including the rays, dog-fish, skate, torpedo, hammer-fish, sharks, and saw-fish; they have pectoral and ventral fins. On each side of the neck, on the lower surface, are five gill-openings, in form of a slit to each gill. Many of the species have two blow-holes in the upper part of the head. The order is divided into Raiadæ and Squalidæ.
RAIADÆ.
Of the Raiadæ there are several genera, and many species. In Cephaloptera the head is truncated, with large, lateral eyes. In Myliobates it is projecting, the pectoral fins extending like wings. In Trygon it is enclosed by the pectorals. In the Skate (Raia) the body is rhomboidal, tail without spine, but two small dorsals near the top. In the Torpedo the body is nearly round, the tail short and fleshy, with two dorsals and a caudal fin.
The White Ray, Raia batis (Fig. 349), reminds us of the lozenge shape, the point of the muzzle forming the lower angle, the longest ray of each pectoral forming the lateral angles, while the summit of the tail forms the last angle; the whole surface seems flat, but a swelling is distinguishable towards the head, on the upper surface, which bears, as it were, the contour of the body, properly so called, namely, the three cavities of the head, the throat, and the belly, which occupy the centre of the lozenge, beyond which the pectoral fins extend. These fins, though covered with a thick skin, permit the cartilaginous rays, with their articulations, to be very distinctly seen.
Fig. 349. The White Ray (Raia batis).
The head of the white ray, which terminates in a muzzle slightly pointed, is attached behind to the cavity of the breast. The mouth, placed in the lower part of the head and far from the extremity of the muzzle, is elongated; its edges are cartilaginous, and furnished with many rows of hooked and pointed teeth; the nostrils are placed in front of the mouth. The eyes, which open in the upper part of the head, are half projecting, and protected in part by a continuation of the soft, elastic, and retractile skin which covers the head. Immediately behind the eyes are two blow-holes, which communicate with the interior of the mouth. The animal is able to open and close these holes at pleasure, by means of an extensible membrane, which acts as a sort of valve. Through these holes it ejects the superabundant water beyond what is necessary for respiration. In its general colour the animal is ashy grey on its upper surface; white, with rows of black spots, below.
Fig. 350. The Lump-fish (Raia clavata).