272 



ZOOLOGY 



The Senses. — Notice the position of the eyes from the front and in a 

 side view of the fish. Is the position of advantage and how? The eye is 

 globular in shape. Such an eye has been found to be very near lighted. 

 Thus it is unlikely that a fish is able to perceive objects at any great dis- 

 tance from it. The eye is unprotected by eyelids, but the position on the 

 sides of the body affords some protection. There is much opportunity for 

 a careful boy or girl to make simple experiments to determine how much 

 and how far the fish can see. 



Feed the fish; does it appear to see the food or to go to the food by a 

 sense of smell? The nostrils of a fish can be proved to end in little pits, 

 one under each nostril hole. Thus they differ from our own, which are 

 connected with the mouth cavity. In the catfish, for example, the barbels 

 or horns receive sensations of smell and taste. The sense of perceiving 

 odor is not as we understand the sense of smell, for a fish perceives only 

 substances that are dissolved in the water in which it lives. The senses of 

 taste and touch appear to be less developed than the other senses. A fish 

 rejects bits of food which it does not like. 



Breathing. — Notice that a fish, when swimming quietly or when at rest, 

 seems to be biting when no food is present. A reason for this act is to be 

 seen when we introduce a little finely powdered carmine into the water 

 near the head of the fish. It will be found that a current of water enters 

 the mouth at each of these movements and passes out through two slits 

 found on each side of the head of the fish. Investigation shows us that 



under the broad, flat plate or 

 operculum forming each side of 

 the head lie several long, feathery, 

 red structures, the gills. 



Gills. — If we examine the 

 gills of any large fish , we find that 

 a single gill is held in place by a 

 bony arch, made of several pieces 

 of bone which are hinged in such 

 a way as to give great flexibility 

 to the gill arch, as the support is 

 called. Covering the bony frame- 

 work, and extending into the 

 throat, are a series of delicate 

 filaments of flesh, covered with a 

 very delicate membrane or skin. 

 Into each of these filaments pass 

 two blood vessels, one downward 

 and one upward. Blood reaches 

 the gills and is carried away from 

 tht;se organs by means of two large 

 vessels which pass along the bony 

 arch previously mentioned. Blood passes into the gill filament, and there 

 comes in contact with the free oxygen of the water bathing the gills. An ex- 

 change of gases through the walls of the gill filaments results in the loss of 

 carbon dioxide and a gain of oxygen by the blood. Possibly other waste 

 products find their way into the water bathing the gill filaments, the wastes 

 being carried off by the current of water passing over the gills. 



Teeth. — Notice the arrangement and number of the teeth in the stickle- 

 back. Sticklebacks are carnivorous, preying upon the eggs and young of 

 other small fish inhabiting the brooks where they live. They also feed 

 upon decaying and live vegetable matter, especially algae. The teeth of 



Gills and heart of the fish, exposed by removal 

 of gill cover on left side; a, first of the four 

 bony arches which carry the gills ; b, gills ; 

 b', lower edges of gills on the right side; 

 h, heart. 



