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 What is the advantage of the backbone plan of structure 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?

—, with right gill cover removed to show gills.

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 advantage in this? Are the gills smooth or wrinkled? (Fig. 215.) What advantage? The bony supports of the gills, called the gill arches, are shown in Fig. 216 (k_{1} to k_{4}). How many arches on each side? The gill arches have projections on their front sides, called gill rakers, to prevent food 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 (kh, Fig. 216) along the hinder border of the