The general shape, and the suitability of the shape for swimming.
The surface of the body and the protection it affords. Note the scales and the slime, the latter a protection against the growth of fungi, etc.
The gills—two openings behind the flaps at the rear of the head. The colours, and their value in concealing the fish. The dark upper surface makes it inconspicuous from above; the light under surface blends with the shadow and dims it.
The divisions of the body—head, trunk, and tail.
Movements of the fish and the part that the various fins play in these movements.
Note that the broad tail fin is the most useful fin for locomotion, the others act as balancers or as brakes, or for causing currents of water near the gills. Observe the movements of the pair of fins nearest the gills, the movements of the mouth, and the currents of water entering the mouth and passing through the gill slits. When a fish is kept in a very small quantity of water, observe the effect produced on the movements of the mouth and gill flaps. What are the uses of these movements? The pupils will thus discover the nature of the respiration of the fish. Why do fish die if many are kept in a jar of water?
II
By supplying various foods learn what kinds are preferred. Find in the actions or habits of the living fish evidences of a sense of smell, of sight, of hearing, and of taste.
Nearly all the following points of detailed study can be observed from the living fish: shape; size; tongue; teeth; gill slits leading from the mouth to the gills; nostrils, number and position; eyes, absence of eyelids; fins, size, build; the arrangement of the scales.