. Elementary text-book of zoology. 436 CHORD ATA. The paired fins show similar modifications. The archi- pterygium is found in the Dipnoi and consists of a median axis with symmetrical lateral rays {cf. the protocercal tail). The ichthyopterygium consists of one or moje basal parts bearing secondary rays only on the outer border. In the pectoral fin there are commonly three primary basal pieces (cf. skate) and in the pelvic only one. Fishes are usually covered or protected with scales, of which there are usually distinguished four kinds. The placoid scale has a bony base and bears a spine, usu
. Elementary text-book of zoology. 436 CHORD ATA. The paired fins show similar modifications. The archi- pterygium is found in the Dipnoi and consists of a median axis with symmetrical lateral rays {cf. the protocercal tail). The ichthyopterygium consists of one or moje basal parts bearing secondary rays only on the outer border. In the pectoral fin there are commonly three primary basal pieces (cf. skate) and in the pelvic only one. Fishes are usually covered or protected with scales, of which there are usually distinguished four kinds. The placoid scale has a bony base and bears a spine, usually Fig. 311.—Fins of Fishes. A B A, Archipterygium. * B, Uniserial type of ichthypterygium, pelvic fin of Skate. found in Elasmobranchii; the cycloid is a flat circular plate arising in the dermis : with its ally, the ctenoid, which has a toothed edge, it is found chiefly in the Teleostei. Lastly, the ganoid scale is a hard rhomboidal plate closely apposed to its neighbours and occurring in the certain archaic fishes of the Teleostomi. All fishes have gills borne upon four or more gill-slits. The slits may be widely apart and opening separately, as in Elasmobranchii, or they may be close together and covered by a flap or operculum, as in the other 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 Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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