Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/679

661 REPRODUCTION.] ICHTHYOLOGY 661 it is the male on which this duty devolves. In some, as in Cottus, Gastrosteus, Cyclopterus, Antennarius, Ophiocepha- lus, Calliehthys, the male constructs with more or less skill a nest, and jealously guards the ova deposited in it by the female. The male of some species of Ji raw carries the ova (fig. 54) about with him in his capacious pharynx. The species of Chromis inhabiting the sea of Galilee are said to take care of their ova in the same manner. And, finally, in the Lophobranchs, nature lias aided this instinct by the development of a pouch or the abdomen or lower side of the tail. In the Syngnathidce this pouch is formed by a fold of the skin developed from each side of the trunk and tail, the free margins of the fold being firmly united in the median line, whilst the eggs are being hatched in the inside of the pouch. In Hippocampus the pouch is completely closed, with a narrow anterior opening. Flo. 55._Abdomen of Aspre a batrachus, with the OVH attached ; at a the ova removed, to show the spongy structure of the skin, and the processes filling tne interspaces between the ova. (Natural size.) ^ The genital organs of Ganoids show diversity of structure similar to that found in Teleosteans, but on the whole they approach the Batrachian type. The ovaries are not closed, except in Lepidosiren ; all Ganoids possess oviducts. In the sturgeons the oviduct as well as the vas dcf erens is repre sented by a funnel-shaped prolongation of the peritoneum, which communicates with the wide ureter. The inner aperture of the funnel is on a level with the middle of the testicle or ovary, while the outer is within the ureter; and it is a noteworthy fact that only at certain periods of the life of the fish is this outer aperture found to be open, at other times the peritoneal funnel appears as a closed blind sac within the ureter. The mode of passage of the semen into the funnel is not known. In Polypterus and Amia proper oviducts, with abdomi nal apertures in about the middle of the abdominal cavity, are developed they coalesce with the ureters close to the common urogenital aperture. In Ceratodus a long convoluted oviduct extends to the foremost limit of the abdominal cavity, where it opens by a slit at a considerable distance from the front end of the long ovary ; this aperture is closed in sexually immature specimens. The oviducts unite close to their common opening in the cloaca. During their passage through the oviduct the ova receive a gelatinous covering secreted by its mucous membrane. This is probably also the case in Lepidosiren, which possesses a convoluted oviduct with secretory glands in the middle of its length. The ova of Ganoids, so far as they are known at present, are small, and enveloped in a gelatinous substance. In the sturgeon as many as 7,635.200 have been counted. Those of Lepidosteus seem to be the largest, measuring 5 milli metres in diameter with their envelope, and 3 millimetres without it. They are deposited singly, like those of newts. In Chondropterygians (and Ilolocephali] the organs of re production assume a more compact form, and are more free owing to a lengthened attachment to the back of the abdo minal cavity. The ovaries of the majority are paired (single in the Carchariida; and Scylliidte, one remaining undevel oped). But the oviducts are always paired, with a common aperture beginning immediately behind the diaphragma. They consist of two divisions, separated by a circular valve ; the upper is narrow, and is provided within its coats with a gland which secretes the leathery envelope in which FIG. 56. Ventral fins and claspcrs of ChiloicyUium trifpeculare. most of the Chondropterygian ova are enclosed ; the lower forms the uterine dilatation in which the embryos of the viviparous species are developed. Generally the vitelline sac of the embryos is free, and has no connexion with the uterus, which in these cases has mereJy the function of a protecting pouch ; but in Carcharias and Mustelus lavis a placenta uterina is formed, the vascular walls of the vitel- liue sac forming plaits fitting into those of the membrane of the uterus. The ends of the uteri open into the cloaca by a common aperture behind the ureter. The testicles are always paired, rounded, and situated in the anterior part of the abdominal cavity, covered by the liver. The vas deferens opens with the urethra in a papilla within the cloaca. The so-called claspers of Chonclropterygians (fig. 56) are characteristic of all male individuals. They are semi- ossified appendages of the pubic, with which they are