_n_, end of nerve of smell; _au_, eye; _v_, _z_, _m_, fore, mid, and hind brain; _h_, spinal bulb; _r_, spinal cord.] Notice the general shape of the =brain= (Fig. 222). Are its subdivisions distinct or indistinct? Are the lobes in pairs? The middle portion of the brain is the widest, and consists of the two _optic lobes_. From these lobes the optic nerves pass beneath the brain to the eyes (_Sn_, Fig. 223). In front of the optic lobes lie the two cerebral lobes, or the _cerebrum_. The small _olfactory lobes_ are seen (Fig. 224) in front of the cerebrum. The olfactory nerves may be traced to the nostrils. Back of the optic lobes (mid brain) is the cerebellum (hind brain), and back of it is the _medulla oblongata_, or beginning of the spinal cord. [Illustration: FIG. 223.--BRAIN OF PERCH, side view.] [Illustration: FIG. 224.--BRAIN OF PERCH, from below.] Taking the eyeball for comparison, is the whole brain as large as one eyeball? (Fig. 222.) Judging from the size of the parts of the brain, which is more important with the fish, thinking or perception? Which is the most important sense? The scales along a certain line on each side of the fish, called the lateral line, are perforated over a series of lateral line sense organs, supposed to be the chief organs of _touch_ (see Fig. 209). [Illustration: FIG. 225.--THE STICKLEBACK. Instead of depositing the eggs on the bottom, it makes a nest of water plants--the only fish that does so--and bravely defends it.] =Questions.=--Which of the fins of the fish have a use which corresponds to the keel of a boat? The rudder? A paddle for sculling? An oar? State several reasons why the head of the fish must be very large, although the brain is very small. Does all the blood go to the gills just after leaving the heart? Make a list of the different species of fish found in the waters of your neighborhood; in the markets of your town. [Illustration: FIG. 226.--ARTIFICIAL FECUNDATION. The egg-cells and sperm-cells are pressed out into a pan of water.] [Illustration: FIG. 227.--NEWLY HATCHED TROUT, with yolk-sac adhering, eyes large, and fins mere folds of the skin. (Enlarged.)]