160 



PISCES. 



urinary organs of the Plagiostomi are paired kidneys, which 

 sometimes retain the ciliated funnels (nephrostomatot). 



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 Chimceridce only by separate orifices) into 

 i he 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, parchment-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 Kays 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 Musteliis 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. Muller, " Ueber den glatten Hai des Aristoteles." Abkandl. 

 der Berliner AJtad., 1840. 



FIG. 601. Embryo of Mnstelus laevis, connected with the uterus 

 by the umbilical placenta (Dp) (after Jrfh. Muller). 



