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. [Sidenote: 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. [Illustration: Fig. 3. 1. Simple ray. 2. Spine. 3. Simple articulated ray (soft). 4. Branched ray (soft).]