. The chordates. Chordata. 508 Comparative Morphology of Chordates defined areas, pterylae, of the body. The intervening areas, apteria, are bare, or occupied only by sparsely distributed feathers of the smaller varieties (Fig. 398). However, the length and slope of the contour feathers are such that the bird is externally completely covered by them. The feather pattern, or pterylosis, is characteristic for a species. In some birds (penguins, screamers, toucans, and ostriches), pterylae are not distinguishable, the contour feathers being uniformly distributed over the body, or the apteria are,
. The chordates. Chordata. 508 Comparative Morphology of Chordates defined areas, pterylae, of the body. The intervening areas, apteria, are bare, or occupied only by sparsely distributed feathers of the smaller varieties (Fig. 398). However, the length and slope of the contour feathers are such that the bird is externally completely covered by them. The feather pattern, or pterylosis, is characteristic for a species. In some birds (penguins, screamers, toucans, and ostriches), pterylae are not distinguishable, the contour feathers being uniformly distributed over the body, or the apteria are, at most, restricted to very small areas or merely temporarily evident in the very young bird (ostriches). The color of feathers is due partly to pigments and partly to physical action of the horny substance. There are commonly yellow, orange, red, black, and, rarely, green pigments; the bright blue and purple, and usually green, and the iridescence of some feathers, are spectral effects resulting from the disruption of white light as it en- counters surfaces of the more or less transparent horny substance— "structural ; White results from absence of pigment. Feathers, like most horny structures, are not permanent organs. While in birds there is no periodic shedding of the stratum corneum, there is usually a seasonal molting during which extensive loss of feathers occurs. The attachment of the quill of the old feather at the. Fig. 398. Feather tracts of a cuckoo, Geococcyx californianus. (After Shufeldt. Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.). Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphia : Blakiston
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