PARTS.] (fig. 4). In Myxine only the gill-opening is at a great distance from the head ; in that family (Cydostomi) it is either single or there are six or more on each side .{fig. 1), as in the lampreys. FIG. 4 Lower aspect of head of Raia lemprieri. The Trunk and Tail. In the trunk are distinguished the back, sides, and 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 may certain abdominal organs, such as the sexual, extend to between the muscles of the tail, but the intestinal tract itself may pass far backwards, or it may even 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 fins, leaving fin less only a small portion between .the dorsal, caudal, and anal fins ; this finless part is called the free portion or the peduncle of the tail. The. Fins. The fins are divided into vertical or unpaired, and horizontal or paired fins. Any of them may be pre sent 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 further forwards, ami it may reach even the head and the vent. In this embryonic condition the fin is generally supported by fine ays, vhich are the continuations of, or articulated to, other stronger rays supported by the processes or apophyses of the vertebral FIG. 5. i, simple ray , 2, column. This form of the vertical fin tS^hf^.^ is very common, for instance in the branched ray (soft), eels, and many Gadoid, Blennioid, and Ganoid fishes, in which the rays have ceased, besides, to be simple rods, show ing more or less numerous joints (simple articulated rays, 637 fig. 5). Branched rays are dichotomously 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 are then distin guished : 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. The caudal fin is rarely symmetrical so that the upper half is -equal to the lower; the greatest degree of asym metry obtains in fishes with a heterocercal termination of the vertebral column. In fishes in which it is nearly sym metrical it is frequently prolonged into an upper and lower FIG. 6. Heterocercal tail of Acipenfer. a, fulcra: b, osseous bucklers. lobe, its hind margin being concave or more or less deeply excised ; in others the hind margin is rounded, and, when the middle rays greatly exceed in length the outer ones, the fin assumes a pointed form. Many and systematically important differences are ob served in the dorsal fin, which is either spiny-rayed (spinous) (Acanthopterygian], or soft-rayed (Malacopterygian). In the former, a smaller or greater number of the rays are simple and without transverse joints ; they may be flexible, or so much osseous matter is deposited in them that they appear hard and truly spinous (fig. 7) ; these FIG 7. Labrax lupus (Bass), an Acanthopterygian with anterior spinous and posterior soft dorsal fin. spines form- always the anterior portion of the fin, which is either detached from or continuous with the remaining jointed rays. The spines can be erected or depressed at the will of the fish ; if in the depressed position the spines cover one another completely, their points lying in the same line, the fish is called homacanth ; but if the spines are asymmetrical, alternately broader on one side than on FIG. 8. Saiirus undosquamis, n Malacopterygian wrtn anterior soft dorsal and additional adipose fin. the other, the fish is called heteracanth. The spinous division, as well as the one consisting of jointed rays, may
again be subdivided. In the Malacopterygian type all the
