FISHES 



115 



Fig. 215. — Carp, with 

 right gill cover removed 

 to show gills. 



What is the advantage of the backbone plan of struc- 

 ture over the armor-plate plan ? You have seen the spool- 

 like body of the vertebra in canned salmon. Is it concave, 

 flat, or convex at the ends ? 



The gills are at the sides of the head (Fig. 215) under 

 the opercula, or gill covers. What is the color of the gills ? 

 Do the blood vessels appear to be 

 very near the surface of the gills, or 

 away from the surface ? What advan- 

 tage in this ? Are the gills smooth 

 or wrinkled? (Fig. 215.) What ad- 

 vantage ? The bony supports of the 

 gills, called the gill arches, are shown 

 in Fig. 216 {k x to k 4 ). How many 

 arches on each side? The gill arches have projections 

 on their front sides, called gill rakers, to prevent food 

 oS from being washed 



through the clefts 

 between the arches. 

 The fringes on the 

 rear of the gill 

 arches are called 

 the gill filaments (a, 

 Fig. 216). These 

 filaments support 

 the thin and much- 

 wrinkled borders of 

 the gills, for the 

 gills are constructed 

 on the plan of exposing the greatest possible surface to 

 the water. Compare the plan of the gills and the human 

 lungs. The gill opening on each side is guarded by 

 seven rays (kk, Fig. 216) along the hinder border of the 



Fig. 216. — Skeleton around Throat of Fish. 



