Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/678

660 660 ICHTHYOLOGY [ORGANS OP genital ; and if a papilla is developed, its extremity is perforated by the urethra, the genital opening being situated nearer the base. A few Teleosteans show an arrangement similar to that of Chondropterygians and Dipnoi, the urogenital openings being in the posterior wall of the rectum (Symbranchidce, Pediculati, and some Plectognathi). ORGANS OF REPRODUCTION. All fishes are dioecious, or of distinct sex. Instances of so-called hermaphroditism are, with the exception of Serranus, abnormal individual peculiarities; such have been observed in the cod-fish, in some Pleuronectidce, and in the herring. Either the generative organ of one side was found to be male and that of the other female, or the organ of one or both sides was observed to have been developed partly into an ovary, partly into a testicle. In the European species of Serranus a testicle-like body is attached to the lower part of the ovary ; but many specimens of this genus are undoubtedly males, having normally developed testicles only. The majority of fishes are oviparous (comparatively few viviparous), the embryos being developed either in the ovarium or in some dilated portion of the oviduct. In viviparous fishes actual copulation takes place, and the males of most of them are provided with copulatory or intromit- tent organs. In oviparous fishes the generative products are, during sexual excitement, discharged into the water, a very small quantity of semen being sufficient for effectual impregnation of a number of ova dispersed in a considerable quantity of water, circumstances which render artificial impregnation more practicable than in any other class of animals. In Branchiostoma the generative organs occupy the ventral side of the abdominal cavity, into which they dis charge their contents. No ducts are developed in either sex. In the Cyclostomes the generative organ is single, and fixed to or suspended from the median line of the back of the visceral cavity by a duplicature of the peritoneum (mesoarium), the testicle and ovary being distinguishable by their contents only. These escape by dehiscence of the cells or capsules and rupture of the peritoneal covering into the abdominal cavity, and are expelled, by reciprocal pressure of the intertwined sexes, through the porus genitalis. The ova of the lampreys are small and globular, like those of Teleosteans. Those of Myxine have a very peculiar shape when mature; they are of an oval form, about 15 millimetres long and 8 millimetres broad, enveloped in a horny case, which at each end is provided with a bundle of short threads, each thread ending in a triple hook. Whilst in the mesoarial fold, the eggs are attached to one another by means of these hooks, and after being expelled they probably fix themselves by the same means to other objects. As in all fishes producing ova of large size, the number of ova matured in one season is but small. In Teleosteans the generative organs are comparatively large. In some families the ovaries are without a closed covering and without oviducts, as in /Salmonidce, Galaxiidte, Notopteridce, Murcenidoe, and others. The surface of such an open ovary as, for instance, that of the salmon is transversely plaited, the ova being developed in capsules in the stroma of the laminae ; after rupture of the capsules the mature ova drop into the abdominal cavity, and are expelled by the porus genitalis. The ovaries of the other Teleosteans are closed sacs, continued into oviducts. Fre quently such ovaries coalesce into a single body, or one in which the division is effected internally only by a more or less complete septum. In the viviparous Teleosteans the embryos are developed within the ovary, notably in the Embiotocidce, many Blenniidce, and Cyprinodo?itidce l Sebastes viviparus, &c. FIG. 5S.Sitrema argenteum, with fully developed young, ready for expulsion by the genital orifice, o; a, folds of the ovarian sac ; v, vent. The ova of Teleosteous fishes are extremely variable in size, quite independently of the size of the parent species. The ova of large and small individuals of the same species, of course, do not differ in size; but, on the whole, larger individuals produce a greater number of ova than smaller ones of the same species. The larger the size of the ova is in a species, the smaller is the number produced during one season. The ova of the eel are almost micro scopic. The numbers of ova in the small-sized roe of the herring, lump-fish, halibut, and cod fish have been estimated respectively at 25,000, 155,000, 3,500,000, and 9,344,000. Larger in size and fewer in number are those of Anten- ovumof!ir*w narius, Salmo, Aspredo, Lophobranchs, &c. t&amp;gt;oaku (Cey- Those of Gastrostnis are comparatively the lar- j,g embryo! gest; and the Siluroid genus Arius, the males Nat. size. of which take care of their progeny, produces ova from 5 to 10 millimetres in diameter. The ova of all Teleosteans are perfectly globular and soft-shelled. Teleosteans with out oviduct deposit them separated from one another ; whilst in many Teleosteans with an oviduct, the ova are enveloped in a glutinous substance, secreted by its glands, swelling in the water and forming lumps or chords, in which the ova are aggregated. Instances of the female taking care of her progeny are ex tremely rare in fishes. At present only two examples are. known, that of the Siluroid genus Aspredo, and the Soleno- stoma. In the former (fig. 55), during thetime of propagation, the integuments of the lower side of the flat trunk of the female assume a soft and spongy texture. After having deposited the eggs, the female attaches them to and presses them into the spongy integument, by merely lying over them. She carries them on her belly, as the Surinam toad (Pipa) carries her ova on her back. When the eggs are hatched the excrescence on the skin disappears, and the ab domen becomes as smooth as before. In Solenostoma the inner side of the long and broad ventral fins coalesces with the integuments of the body, a large pouch being formed for the reception of the eggs. There is a peculiar provision for the retention of the eggs in the sac, and probably for the attachment of the embryo. The inner walls of the sac are lined with long filaments, arranged in series along the ventral rays, and more numerous and longer at the base ot the rays than in the middle of their length, behind which they disappear entirely. The testicles of the Teleosteans are always paired, and occupy the same position as the ovaries. Their size varies extraordinarily at the different seasons of the year. Vasa deferentia are constant. In the males of viviparous Teleo steans the urogenital papilla is frequently enlarged, and clearly serves as an intromittent organ. Many Teleostei take care of their progeny, but with the exception of Aspredo and Solenostoma, as mentioned above,