While observing a living fish and admiring its beauty, it will probably occur to some of us that a fish consists only of a head and tail. Yet this is not all. Between the head and tail is a part that we may call the trunk. It contains the digestive and other organs. There is no indication of a neck in a fish. Any such constriction would destroy the regular outline of the animal's body and thus retard the speed with which it moves through the water. But head, trunk and tail are not all. There are attached to the outer side of the fish's body certain appendages that are called fins. Before discussing some of the different kinds of fishes and their habits, it will be necessary to learn something about fins, for the fins of all fishes are not alike. When a fish moves through the water, it bends its tail first to one side and then to the other. This undulatory movement, as it is called, pushes the fish's body ahead. One can observe the movements easily upon a specimen kept alive in an aquarium jar. At the extreme end of the tail there is a broad, notched fin which aids the tail in propelling and steering the body. We will call this the _tail_ or _caudal_ fin (Fig. 79 B). In most of our common fishes there are seven fins--six without the caudal. The first of these six is a large fin situated near the middle of the back. This is the _back_ or _dorsal_ fin (Fig. 79 A). Sometimes we may find a fish that has two dorsal fins. In this case the one nearest the head is called first dorsal and the next one behind it the second dorsal. Near the head, in a position corresponding to our arms, is a pair of fins which are called the _arm_ or _pectoral_ fins (Fig. 79 E). Farther back towards the tail, on the under side of the fish, is another pair, corresponding in position to the hind legs of a quadruped. This pair is called the _leg_ or _pelvic_ fins (Fig. 79 D). Just behind the pelvic fins is a single fin, situated on the middle line of the body. This is the _anal_ fin (Fig. 79 C). The pectoral and pelvic fins are called paired fins because they are in pairs. The others which are not in pairs are called median fins, because they are situated on the middle line of the body. The paired fins serve as delicate balancers to keep the body right side up and to regulate speed. They are also used to propel the body backwards. After naming the different fins of the fish in the schoolroom aquarium, it will be interesting to observe the uses of each. [Illustration: _Fig. 79. Diagram of a fish to show: A, dorsal fin; B, caudal fin; C, anal fin; D, pelvic fins; E, pectoral fins; L, lateral line._]