. A manual of zoology. Zoology. IV. VERTEBRATA: PISCES 505 filament. The skin is covered with placoid scales, usually close together, these being so small in some cases that the skin {shagreen) is used instead of sandpaper. More rarely the scales are larger, and the spines, which project from the skin, justify in size and form the term dermal teeth. Such strong spines occur especially at the front of the dorsal fins {ichthy- odolurites of paleontologists). The skeleton is cartilaginous, frequently calcified on the outside. The calcification can also extend into the vertebra;, producing star-li
. A manual of zoology. Zoology. IV. VERTEBRATA: PISCES 505 filament. The skin is covered with placoid scales, usually close together, these being so small in some cases that the skin {shagreen) is used instead of sandpaper. More rarely the scales are larger, and the spines, which project from the skin, justify in size and form the term dermal teeth. Such strong spines occur especially at the front of the dorsal fins {ichthy- odolurites of paleontologists). The skeleton is cartilaginous, frequently calcified on the outside. The calcification can also extend into the vertebra;, producing star-like figures (Asterospondyli) or concentric circles. Fig. 555.—Acanlhias vulgaris* dogfish ("from Claus). B, ventral fin; Br, pectoral fin; Ks, gill clefts; n, nostril; R^, R-^ dorsal fins;6', heterocercal caudal fin;5'/'/, spiracle. (Cyclospondyli). Since bone is lacking, the sharks have no upper jaws, but bite with the pterygoquadrate. The amphicoelous vertebne (lack- ing in Holocephali and several extinct groups,) have neural arches, small ribs, and intercalaria. The number of gill arches and clefts varies between five and seven, the first cleft lying between the hyoid and the first branchial arch. Besides, most elasmobranchs have a spiracle and pseudobranch (fig. 555,5/1/). Except in the Holocephali the gill clefts open separately, the hyoid arch being without an operculum. In the visceral anatomy these points are of importance as distinguish- ing elasmobranchs from Teleostomes. (i) The heart has a large conus, with several rows of valves (fig. 553, ^4), but lacks a bulbus. (2) The alimentary tract (fig 550, A) has a spiral valve, but lacks swim bladder and pyloric cx'ca. (3) The sexual products are carried to the exterior by the urogenital ducts except in Lccmargus which has abdominal pores. The eggs escape from the follicles of the ovary (occasionally unpaired) by dehiscence into the body cavity, and from thence by the unpaired ostium tubas and the paired Mi'illerian
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
