The Sharks and Rays differ from the Teleosteous and Ganoid fishes in having five branchial slits (six or seven in Hexanchus and Heptanchus), which are lateral in the Sharks, and at the lower surface of the head in the Rays (Fig. [1], p. 34). In Myxine only the gill-opening is at a great distance from the head; it is either single in this family (Cyclostomi), or there are six and more on each side (Fig. [2]).
Tail.
In the Trunk are distinguished the back, the sides, and the abdomen. It gradually passes in all fishes into the Tail; the termination of the abdominal cavity and the commencement of the tail being generally indicated by the position of the vent. The exceptions are numerous: not only certain abdominal organs, like the sexual, may extend to between the muscles of the tail, but the intestinal tract itself may pass far backwards, or, singularly, it may be reflected forwards, so that the position of the vent may be either close to the extremity of the tail or to the foremost part of the trunk.
In many fishes the greater part of the tail is surrounded by the fins, leaving only a small portion (between dorsal, caudal, and anal fins) finless; this part is called the free portion or the peduncle of the tail.
Fins.
The Fins are divided into vertical or unpaired, and into horizontal or paired fins. Any of them may be present or absent; and their position, number, and form are most important guides in determining the affinities of fishes.
The vertical fins are situated in the median dorsal line, from the head to the extremity of the tail, and in the ventral line of the tail. In fishes in which they are least developed or most embryonic, the vertical fin appears as a simple fold of the skin surrounding the extremity of the tail In its further progress of development in the series of fishes, it gradually extends more forwards, and may reach even the head and vent. Even in this embryonic condition the fin is generally supported by fine rays, which are the continuations of, or articulated to, other stronger rays supported by the processes or apophyses of the vertebral column. This form of the vertical fin is very common, for instance in the Eels, many Gadoid, Blennioid and Ganoid fishes in which, besides, the rays have ceased to be simple rods, showing more or less numerous joints (simple articulated rays; Fig. [3]). Branched rays are dichotomically split, the joints increasing in number towards the extremity.
The continuity of the vertical fin, however, is interrupted in the majority of fishes; and three fins then are distinguished: one in the dorsal line—the dorsal fin; one in the ventral line behind the anus—the anal fin; and one confined to the extremity of the tail—the caudal fin.
Fig. 3.