Chap. 33 VERTEBRATES LOWER CHORDATES AND FISHES 677 



By late October, the female prepares the nest. It is a basin a foot or two in 

 diameter, if possible placed near a spring, swept in the gravelly bottom by 

 vigorous brushing with the body and tail. The male takes no part in the 

 preparations but is always nearby. After some hours of courtship, the two 

 fishes vibrate their bodies above the basin and spawning occurs. The pair 

 then separates and the female swims a short distance upstream from the nest 

 and stirs up sand and gravel which the water carries over the eggs. The 

 average nest contains nearly 200 eggs about 70 per cent of which hatch in the 

 spring. 



The herring and the brook trout, sunfish and other nest makers represent 

 extremes in deposition of the sex cells and the numbers of eggs. There are 

 hundreds of variations in spawning habits. The eggs of fresh-water fishes are 

 relatively large, fewer, and usually sink to the bottom. The newly hatched 

 young (fry) of a one pound brook trout is half an inch long. The newly 

 hatched fry of a 300-pound swordfish is one quarter of an inch long. 



Nervous System and Special Senses. The discussion of the nervous sys- 

 tem is given in Chapter 16. Whatever is said about the special senses will 

 mean more if Chapter 17, Sense Organs, is read with it. 



Vision. Most of us know a fish's eye as the hard white ball in the head of a 

 baked fish. That ball is the crystalline lens, the gatherer of light rays that has 

 lost its translucence but is a perfect sphere as it was when the fish was alive. 

 The shape of the fish's lens cannot be changed like that of the human eye. It 

 can only be moved backward and forward to get a Uttle better focus (Fig. 

 33.13). Fishes have no true eyelids and no lachrymal glands. The living cells 

 of the cornea are washed by the waters of lakes or oceans, not by tears. With 

 some exceptions, e.g., shark, there appears to be practically no control of the 

 amount of light that enters the pupil. An iris is present but immovable. 



When the human eye is focused on objects that are close by, the lens is 

 nearest to spherical. The lens of the fish's eye is always spherical, always ad- 

 justed to close vision. Many fishes are naturally nearsighted. On the other 

 hand, sharks that pursue rapidly fleeing prey have lenses that are peculiarly 

 set for distance. The fish's eyes are on opposite sides of its face; they look in 

 opposite directions and see different things but only a little of what is in front 

 of them. If the headlights of automobiles were moved even a Uttle distance to 

 the sides of the hood the front view would be greatly foreshortened. Something 

 comparable to that has happened to the fish. 



When a light ray passes from air into water its direction is changed. This 

 occurs when rays pass from air into the watery interior of the eye. The human 

 eye is adjusted to this and vision is clear, but under water human vision is 

 blurred. The direction of light rays is not changed as they enter the eye of a 

 fish because they pass from water into wet cells and a watery interior. Such 

 facts have been learned by experimentation and repeated observations. On 



