136 ANIMAL BIOLOGY 



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 nos- 

 trils, 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, 



