256 PISCES. 



or laminated projections, shaped like a frill, as in the Eel and Petro- 

 myzon, take their rise, and in these the ova are developed. In such 

 cases the oviducts are absent, and the eggs, having fallen into the 

 ventral cavity, are expelled the body through a single or double slit 

 lying between the anus and urinary opening, and more rarely through 

 an aperture communicating with the ureter: the latter arrange- 

 ment appears to occur generally in the Sturgeons, e. g. Acipenser 

 Huso, stellatus and Ruthenus, but not in A. sturio, where we find 

 the anal slits to be present ; these, however, being absent in the 

 other species, the ova, after having been freed from the ovaries, pass 

 into two membranous infundibuliform tubes, which are united with 

 and open into the two wide ureters about the middle of the kidneys ; 

 behind these apertures is a valve, to prevent the escape of the urine 

 into the ventral cavity ; the ova thus pass out of the female through 

 the ureters. 



The structure of the female sexual organs in the Plagiostomi and 

 Chimaerae is more perfect, and analogous to that in Reptiles and 

 Birds. The ovaria are here generally double, situated far forward, 

 and each presents the form of a more or less considerable plate, 

 rarely that of a sac, upon which the ova ripen in succession, the vi- 

 telline spheres gradually attaining a size equal to those of Birds. In 

 some Sharks only a single ovarium is present. A common and wide 

 abdominal opening conducts into the double oviduct, which is con- 

 structed like that of Birds, being wide, thick-walled, and lined with 

 folded mucous membrane. The oviduct at its commencement is 

 narrow, but dilates above the middle, and is generally surrounded in 

 the Rays, Sharks, and Chimseras by a cordiform or reniform gland, 

 which is sometimes, as in those Sharks, e. g. Mustelus and Galeus, 

 that possess a nictitating membrane, and also a single ovarium, of a 

 spiral form ; it is in the majority of examples very compact, and 

 formed of filamentary follicles, like the caudal gland of Birds. Fur- 

 ther backward the oviduct expands considerably into a kind of 

 uterus, to make room for the large ova that are provided with horny 

 shells ; within this the embryos are attached and developed, as will 

 be described further on. The external sexual opening is situated be- 

 hind the anus, and we find there a papilla, or rudimentary clitoris. In 

 many Fishes the ovaries and oviducts are secured in their place by 

 mesentery, in others they are free. 



Two ovaria with free oviducts, thus essentially repeating the type 

 of structure in the Plagiostomi, have been found in the Lepidosiren ; 

 the oviducts join the ureters and enter the cloaca ; the ovaries are in 



