. Elementary text-book of zoology. Zoology. 160 PISCES. The urinary organs of the Plagiostomi are paired kidneys, which sometimes retain the ciliated funnels (nephrostomata). The sexes can be easily distinguished by the form of the pelvic fins. A true copulation always takes place. The female genital organs consist of a large, single or double ovary and paired glandular oviducts, which are separate from the ovaries and begin with a common funnel-shaped ostium, and in their further course each of them possesses a uterus-like dilatation. The two oviducts open by a common aperture (in the Ckiince
. Elementary text-book of zoology. Zoology. 160 PISCES. The urinary organs of the Plagiostomi are paired kidneys, which sometimes retain the ciliated funnels (nephrostomata). The sexes can be easily distinguished by the form of the pelvic fins. A true copulation always takes place. The female genital organs consist of a large, single or double ovary and paired glandular oviducts, which are separate from the ovaries and begin with a common funnel-shaped ostium, and in their further course each of them possesses a uterus-like dilatation. The two oviducts open by a common aperture (in the Ckiinceridce only by separate orifices) into the cloaca. The ova have a large amount of food-yolk, and are enclosed by a mass of albumen, and sometimes by a thin membranous folded chorion, sometimes by a tough, pai-chrnent-like, flat shell, which is prolonged into four horns, or into twisted strings, which serve to attach it to marine plants. In the latter case the eggs are laid (the true Rays and Dogfish) : in the former (elec- tric Rays and viviparous Squa- lides), on the other hand, they develop in the uterus. In this case the eggs are closely applied to the walls of the uterus during the development, the folds of the chorion interlocking with the ridges of the uterine walls. Thus the addition of nutriment is rendered possible. Sometimes the connection between the mother and the embryo is more intimate, and is effected by means of a true umbilical placenta, which was known to Aristotle in Mustelus Icevis (fig. 604). As Joh. Miiller * has shown, the long-stalked yolk-sac of the embryos of Mustelis Icevis and species of Carcharias develops a great number of villi, which are covered by the delicate egg mem- brane, and like the cotyledons of Ruminants fit into corresponding depressions in the uterine mucous membrane. In other respects, * Compare Job. Miiller, " Ueber den glatteu Hai des ; Alhandl. der JBerlini-r Alitid., FIG. 604.—Embryo of Mnstelus Iceoi
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
