in order to more effectually close the gill-opening; and this fringe is supported by one or several or many bony rays, the branchiostegals. The space on the chest between the two rami of the lower jaw and between the gill-openings is called the isthmus.
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).
Fig. 2.—Head of Mordacia mordax, showing the
single nostril, and seven branchial openings.
In the Trunk are distinguished the back, the sides, and the abdomen. It gradually passes in all fishes into the Tail; 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.
