. Outlines of zoology. Zoology. 5i6 PISCES—FISHES. The ventricle of the heart has an anterior auxiliary region— a contractile conus arteriosus. The males are provided with copulatory modifications of the hind-limb, known as claspers. Fertilisation is internal. The ova are few and large. Large egg-purses are common, but some Elasmobranchs are vivi- parous. The embryos have external gills. Subdivisions.—The shark and the skate are types of two distinct suborders : (i) The older Selachoidei, with approximately cylindrical bodies and lateral gill-open- ings, as in shark and dog-fish ; (2) th
. Outlines of zoology. Zoology. 5i6 PISCES—FISHES. The ventricle of the heart has an anterior auxiliary region— a contractile conus arteriosus. The males are provided with copulatory modifications of the hind-limb, known as claspers. Fertilisation is internal. The ova are few and large. Large egg-purses are common, but some Elasmobranchs are vivi- parous. The embryos have external gills. Subdivisions.—The shark and the skate are types of two distinct suborders : (i) The older Selachoidei, with approximately cylindrical bodies and lateral gill-open- ings, as in shark and dog-fish ; (2) the more modified Bato- idei, with flattened bodies, ventral gill - openings, and pectoral fins joined to the head, as in skates or rays. Special forms.—Mus- telus, Carcharias, Squalus, Torpedo, Acanthias, and others, are viviparous ; Raja, Scyllium, Cestracion, and others, are oviparous. In two species of the genera first named there is a placenta-like connection between the yolk- sac of the embryo and the uterus of the mother. Zygietia has a peculiar hammer-like head expansion ; Pristis has the snout prolonged in a tooth-bearing saw; Torpedo has a powerful electric organ. History.—The Elasmo- branchs appear in the Upper Silurian, are very abundant from the Carboniferous on- wards, but are now greatly out-numbered by the Bony Fishes. An increasing calcification of the axial skeleton is traceable through the ages, and in some of the ancient forms the exoskeleton was greatly developed, often including long spines or ichthyodorulites firmly fixed on the dorsal fins or on the neck. Among the most remarkable extinct genera is Pleuracanthus, from Carboniferous to lower Permian. It had a terminal mouth, a naked. Fig. 219.—Young skate.—From Beard. The yolk-sac has been cut off, the yolk-stalk is left, m., Mouth; , nostril; , exter- nal gills ; a., cloaca ; c, Please note that these images are extracted from scanned page images that may have been digital
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
