FISHES 133 



b. Describe the situation of the gills with reference to 

 the mouth cavity. 



3. Hold the preserved fish with its mouth upward and care- 



fully pour water into the mouth opening. Where 

 does the water come out ? 



4. Place a gill that has been removed from a fish (a salmon 



if possible) in a watch glass and cover it with water. 

 Find the following parts : (l)a soft part made up of 

 slender divisions called gill filaments. (2) a curved 

 part to which the gill filaments are attached, known as 

 the gill arch, and (3) projections on the side opposite 

 the gill filaments which are known as gill teeth or rakers. 



a. Is the gill arch relatively hard or soft compared to the 



filaments ? 



b. Are the gill teeth or rakers relatively hard or soft ? 



c. Look again at the perch and state whether the rakers 



are found on the side of the arch nearest to the 

 mouth cavitj^ or on the opposite side. 

 What is the use of the gill teeth when the fish takes 

 in a mouthful of water containing food, and does 

 not wish to swallow the water ? 



d. Make a sketch (about four inches long) of a gill to 



show the shape of the whole and the structure of a 

 small portion. Label gill arch, gill filaments, gill 

 rakers. 



5. The gill filaments contain thin-walled blood vessels 



(capillaries) which are separated from the water by 

 a thin membrane. The heart forces the blood 

 into certain arteries that carry it to the capil- 

 laries in the gills and thence blood passes back to 

 the body through another set of arteries. (Fig. 100.) 

 Bearing in mind that breathing is essentially the 

 same in animals as in plants (P. B., 82), — 



a. What gas will the blood bring from the body to be 



given off in the gills in the process of breathing? 



b. What gas is taken up by the blood in the gills to be 



carried around through the body ? 



c. How are the gill filaments (as stated above) fitted by 



structure to permit this interchange of gases ? 



