Fig. 209.—White Perch (Morone Americana).
Is the covering of the body like the covering of any animal yet studied? The scales are attached in little pockets, or folds, in the skin. Observe the shape and size of scales on different parts of the body. What parts of the fish are without scales? Examine a single scale; what is its shape? Do you see concentric lines of growth on a scale? Sketch a few of the scales to show their arrangement. What is the use of scales? Why are no scales needed on the head? How much of each scale is hidden? Is there a film over the scale? Are the colours in the scale or on it?
The Fins.—Are the movements of the fish active or sluggish? Can it remain stationary without using its fins? Can it move backward? How are the fins set in motion? What is the colour of the flesh, or muscles, of a fish? Count the fins. How many are in pairs? (Fig. [209].) How many are vertical? How many are on the side? How many are on the middle line? Are the paired or the unpaired fins more effective in balancing the fish? In turning it from side to side? In raising and lowering the fish? In propelling it forward? How are some of the fins useful to the fish besides for balancing and swimming?
The hard spines supporting the fins are called the fin rays. The fin on the dorsal line of the fish is called the dorsal fin. Are its rays larger or smaller than the rays of the other fins? The perch is sometimes said to have two dorsal fins, since it is divided into two parts. The fin forming the tail is called the tail fin, or caudal fin. Are its upper and its lower corners alike in all fishes? (Fig. [228].) On the ventral side, just behind the vent, is the ventral fin, also called the anal fin. The three fins mentioned are unpaired fins. Of the four-paired fins, the pair higher on the sides (and usually nearer the front) are the pectoral fins. The pair nearer the ventral line are the pelvic fins. They are close together, and in many fish are joined across the ventral line. The ventral fins are compared to the legs, and the pectoral fins to the arms, of higher vertebrates. (Fig. [244].) Compare fins of fish, pages [123], [124].
Make a drawing of the fish seen from the side, omitting the scales unless your drawing is very large.
Are the eyes on the top or on the sides of the head, or on both? Can a fish shut its eyes? Why, or why not? Is the eyeball bare, or covered by a membrane? Is the covering of the eyeball continuous with the skin of the head? Is there a fold or wrinkle in this membrane or the surrounding skin? Has the eye a pupil? An iris? Is the eye of the fish immovable, slightly movable, or freely movable? Can it look with both eyes at the same object? Is the range of vision more upward or downward? To the front or the side? In what direction is vision impossible? Can a fish close its eyes in sleep? Does the eyeball appear spherical or flattened in front? The ball is really spherical, the lens is very convex, and fish are nearsighted. Far sight would be useless in a dense medium like water. In what direction from the eyes are the nostrils (Fig. [211].) There are two pair of nostrils, but there is only one pair of nasal cavities, with two nostrils opening into each. There are no nasal passages to the mouth, as the test with a probe shows that the cavities do not open into the mouth. What two functions has the nose in man? What function has it in the fish?
Fig. 210.—Blackboard Outline of Fish.
There are no external ears. The ear sacs are embedded in the bones of the skull. Is hearing acute or dull? When you are fishing, is it more necessary not to talk or to step lightly, so as not to jar the boat or bank?