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more easily if some powdered carmine is stirred into the water.)

What will the incoming current of water bring to the gill filaments?

9. Watch the fish as it closes its mouth.

a. Is the size of the mouth cavity now greater or less than it was before?

b. What forces open the gill covers?

c. What will the current of water carry away from the gill filaments?

101. Respiration and the liberation of energy. We have just seen that when the goldfish takes in a mouthful of water and then closes its mouth, the water is forced over the gills, thus bringing oxygen to the filaments. The capillaries in the filaments absorb the oxygen, and the blood then passes on into other arteries which carry it all over the body of the fish. In the capillaries at the ends of the smallest arteries the oxygen passes into the cells as does the food. Now what becomes of the oxygen?

As in plants (P. B., 80), the oxygen unites with elements in the foods and in the protoplasm of the cells and produces oxidation and liberation of energy, which gives the fish the power to contract its muscles and so to push against the water with its tail and tail fin, thus propelling the animal in any direction, or to open its jaws and shut them on another fish, thus securing food. In fact, all the work that the fish performs is made possible through the burning of its foods or protoplasm by the oxygen.

Since the proteins, fats, carbohydrates, and protoplasm all contain carbon, when these are oxidized, carbon dioxid (CO2) is formed as one of the waste substances. All the waste substances pass out of the cells, through the walls of the capillaries, into the blood, which passes on into the veins and back to the heart. The heart contracts and drives the

blood loaded with carbon dioxid out into the arteries, which carry it to the capillaries of the gills (Fig. 100). Here the waste matters pass into the water, which is then forced out under the gill covers by the closing of the mouth.

102. Adaptations for sensation. (Optional.)

1. Study the eye of a goldfish.

a. Describe its position, shape, and size relative to that of the head.

b. Notice that the eye consists of a black center (the pupil) through which light enters the eye, and a colored iris. Add these features to the drawing of the goldfish (91, 5), and label each.

2. The nostrils lie in front of the eyes, and as they are small, a preserved fish head may help in locating them. (In the perch there are two on each side.)

a. Show in your drawing the position, shape, and size of the nostril of one side and label.

b. Gently probe the nostril of a preserved fish with a stiff

bristle.

(1) Do the nostrils open into the mouth or not?

(2) Could the nostrils be used in breathing? Give reason

for your answer.

(3) Bearing in mind the common uses of nostrils of higher animals, state which of these is the probable function of the nostrils of a fish.

103. Senses of fishes. Fishes are said to possess keen sight. The eyes, however, except in rare cases, are only fitted for seeing while in the water. These organs have no eyelids, so the fish always seems to be wide awake. The sense of smell is located in the nostrils, and since these do not open into the mouth cavity, this is the only function of the nostrils. The taste sense is said to be located in the outer skin. The fish has no external ears; it has, however, internal ears, but these are supposed to serve as balancing organs,

rather than as organs of hearing. Fishes from which these internal ears have been removed are unable to maintain their equilibrium.

Some fishes have special organs that serve as tactile organs such as are found on the under side of the head of a cod (Fig. 108) and also on the head of bullheads (Fig. 101). Along each side of the body and tail of fishes is a series of little openings or pores which form what is known as the lateral line (Fig. 108). These organs are supposed to be principally organs of touch.

104. Reproduction and life history. The flowers of seed plants are devoted to the production of seeds which, in turn, produce new plants of the same kind (P. B., 83). Likewise in fishes there are special organs the sole function of which is the production of new individuals. The organs of fishes which may be said to correspond in function to the stamens and pistils of flowers are the ovaries (Fig. 98) and spermaries. In the ovaries are produced many egg-cells, and the mass of eggs in the ovary of a fish is often called the roe. In order that an egg may develop it must first be fertilized by a sperm-cell from the spermary of a male fish. This process usually occurs in the water after the ripe eggs and sperm-cells have been extruded from the ovaries and spermaries of the parent fishes.

You will recall the fact that the pollen tube containing a sperm-nucleus makes its way into an ovule and that the

sperm-nucleus is forced into the ovule and unites with the egg-nucleus; this is the process known as fertilization (P. B., 91). In the case of fishes the sperm-cells swim to the eggs, and then force their way into the egg (Fig. 102, A).

The nucleus of the sperm- and egg-cells then unite just as in plants (Fig. 102, B, C, D). The egg nucleus thus fertilized first divides, and then the cell body, and thus are formed two cells. Each of these cells in turn divides, and so four cells are produced (Fig. 103, A, B). The process of