. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Fig. 171 scale of showing growth. (AfterHesse. Louis Agassiz distinguished two kinds of scales in teleosts, which he named, cycloid and ctenoid. Cycloid scales, as the name indicates, are rounded in shape (Fig. 171) and thicker in the center, thinning out towards the margin. They overlap like shingles and if spread over a surface like ganoid scales edge to edge would much more than cover the body. They are, however, embedded in pockets of the corium with o
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Fig. 171 scale of showing growth. (AfterHesse. Louis Agassiz distinguished two kinds of scales in teleosts, which he named, cycloid and ctenoid. Cycloid scales, as the name indicates, are rounded in shape (Fig. 171) and thicker in the center, thinning out towards the margin. They overlap like shingles and if spread over a surface like ganoid scales edge to edge would much more than cover the body. They are, however, embedded in pockets of the corium with only a part of the outer smooth margin exposed. Since they project diagon- ally at an acute angle with the surface of the skin, and overlap their neighbors, the entire body is protected by at least a double thickness of scaly armor at every point. Present-day bony dipnoans as well as Amia and some teleosts have cycloid scales. Ctenoid ("comblike") scales, also rounded in form, have in addition projecting teeth on the surface of the exposed areas (Fig. 172). All intermediate types be- tween the cycloid and the ctenoid stages are to be found. In some fishes, as for example certain flounders, the scales on the upper side are ctenoid, while those on the under side are cycloid. Of these two types the cycloid scale is the more primitive, occurring first in the fishes of the Jurassic Period, while the ctenoid type did not appear until Cretaceous times. Both cycloid and ctenoid scales are entirely dermal in their origin. The scleroblasts, or scale-forming cells in the corium, lay down two layers of different substance in the formation of a scale. The outer layer is homogeneous and bony, while the under side is fibrillar and contains calcareous deposits. Such scales in- crease in thickness and area by the activity of the sclero- blasts, successive additions being indicated by concentric lines of growth like similar rings of growth exhibited in the cross section of a tree trunk.
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Keywords: ., bookcentury1900, booksubjectanatomycomparative, booksubjectverte
