CHAPTER XII.. Organs of Reproduction.

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, and have been observed in the Cod-fish, some Pleuronectidæ, and in the Herring. Either the generative organ of one side was found to be male, 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 intromittent 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 discharge 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, which is sunk between two labia of the skin in Myxine, and produced into a long papilla in Petromyzon.

The ova of the Lampreys are small, 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 closed covering and without oviducts, as in Salmonidæ, Galaxiidæ, Notopteridæ, Murænidæ, 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 laminæ; 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. Frequently such ovaries coalesce into a single body, or one in which the division is effected internally only by a more or less complete septum. Fixed by a mesoarium, the ovaries occupy generally a position outwards of the intestine or air-bladder; their form varies as well as the thickness and firmness of their covering, which frequently is an extremely thin transparent membrane. The inner surface of the ovarian sac is transversely or longitudinally plaited or covered with fringes, on which the ova are developed, as in the open ovaries. In the viviparous Teleosteans the embryons are likewise developed within the ovary, notably in the Embiotocidæ, many Blenniidæ, and Cyprinodontidæ, Sebastes viviparus, etc. Among the Cyprinodonts the end of the oviduct is attached to the anterior anal rays, which are modified into supports of its termination. In Rhodeus the oviduct is periodically prolonged into a long oviferous tube, by means of which the female deposits her ova into the shells of living Bivalves.

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 microscopic. The small sized roe in the Herring, Lump-fish, Halibut, and Cod-fish, have been estimated at respectively 25,000, 155,000, 3,500,000, and 9,344,000. Larger in size and fewer in number are those of Antennarius, Salmo, Aspredo, Lophobranchs, etc. Comparatively largest are those of Gastrosteus; and the Siluroid genus Arius, the males of which take care of their progeny, produces ova from 5 to 10 millimeters in diameter. The ova of all Teleosteans are perfectly globular and soft-shelled. Teleosteans without 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 cords, in which the ova are aggregated.

Instances of the female taking care of her progeny are extremely scarce in fishes. At present only two examples are known, that of the Siluroid genus Aspredo, and of Solenostoma. In the former, during the time 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 the back. When the eggs are hatched the excrescence on the skin disappears, and the abdomen 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 of the rays than in the middle of their length, behind which they disappear entirely. They are also more developed in examples in which eggs are deposited in the sac than in those which have the sac empty. The filaments most developed have a length of half an inch, and are beset with mamilliform appendages. A slightly undulated canal runs along the interior of the filament.

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 Teleosteans the urogenital papilla is frequently enlarged, and clearly serves as an intromittent organ. In Clinus despicillatus the vas deferens widens within the abdomen into a cavity occupied by a complex network of loose fasciculi, rising from the mucous membrane. The cavity can be compressed by a special powerful muscle, the accumulated semen being thus expelled with considerable force through the narrow aperture of the penis. In many Cyprinodonts the vas deferens runs along the anterior anal rays, which may be thickened, and prolonged into a long slender organ.

Many Teleostei take care of their progeny, but with the exception of Aspredo and Solenostoma, mentioned above (p. 160), it is the male on which this duty devolves. In some, as in Cottus, Gastrosteus, Cyclopterus, Antennarius, Ophiocephalus, Callichthys, 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 Arius carries the ova (Fig. 71) 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 has aided this instinct by the development of a pouch on the abdomen or lower side of the tail. In the Syngnathidæ 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.

The genital organs of Ganoids show similar diversity of structure as those of 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 deferens is represented by a funnel-shaped prolongation of the peritoneum, which communicates with the wide ureter. The inner aperture of the funnel is on a level of the middle of the testicle or ovary, the outer within the ureter; and it is a noteworthy fact that only at certain periods of the life of the fish this outer aperture is 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 abdominal apertures in about the middle of the abdominal cavity, are developed; they coalesce with the ureters close to the common urogenital aperture.

In Ceratodus (Fig. 77), 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 oviduct begins with a funnel-shaped dilatation, and terminates in a wide pouch, which posteriorly communicates with that of the other side, both opening by a common aperture behind the urinary bladder.

The ova of Ganoids, as far as they are known at present, are small, but enveloped in a gelatinous substance. In the Sturgeon have been counted as many as 7,635,200. Those of Lepidosteus seem to be the largest, measuring 5 millimetres in diameter with their envelope, and 3 millimetres without it. They are deposited singly, like those of Newts.

a, Right ovary shown from the inner surface, which is covered by the peritoneum; a’, Left ovary, showing its outer surface; l, Portion of liver; o, Oviduct; p, the lower part of the oviduct is opened to show the folds of its inner membrane; q, Opening of the left oviduct into the right; r, Abdominal orifice of the oviduct.]

In Chondropterygians (and Holocephali) the organs of reproduction assume a more compact form, and are more free from a lengthened attachment to the back of the abdominal cavity. The ovaries of the majority are paired, single in the Carchariidæ and Scylliidæ, one remaining undeveloped. But the oviducts are always paired, beginning immediately behind the diaphragma with a common aperture. They consist of two divisions, separated by a circular valve; the upper is narrow, and provided within its coats with a gland which secretes the leathery envelope in which most of the Chondropterygian ova are enclosed; the lower forms the uterine dilatation, in which the embryoes of the viviparous species are developed. Generally the vitelline sac of the embryoes is free, and without connection with the uterus, which in these cases has merely the function of a protecting pouch; but in Carcharias and Mustelus lævis a placenta uterina is formed, the vascular walls of the vitelline sac forming plaits fitting into those of the membrane of the uterus. The ends of the uteri open by a common aperture behind the ureter into the cloaca.

The testicles are always paired, rounded, and situated in the anterior part of the abdominal cavity, covered by the liver. Vasa efferentia pass the semen into a much-convoluted epididymis, which is continued into the vas deferens; this, at the commencement of its course, is spirally wound, but becomes straight behind, and has its end dilated into a seminal reservoir. It opens with the urethra in a papilla within the cloaca.

The so-called claspers of Chondropterygians (Fig. 78) are characteristic of all male individuals. They are semi-ossified appendages of the pubic, with which they are movably joined, and special muscles serve to regulate their movements. Sometimes they are armed with hook-like osseous excrescences (Selache). They are irregularly longitudinally convoluted, and, when closely ad-pressed to each other, form a canal open at their extremity. A gland, abundantly discharging a secretion during the season of propagation, is situated at, and opens into, the base of the canal. It is still doubtful whether the generally-adopted opinion that their function consists in holding the female during copulation is correct, or whether they are not rather an intromittent organ, the canal of which not only conducts the secretion of their proper gland but also the impregnating fluid.

I. External view. II. Vertical section.

a, One spiral ridge; b, The other spiral ridge; c, Cavity for the ovum.]

The ova of the oviparous Chondropterygians are large and few in number; they are successively impregnated, and the impregnation must take place before they are invested with a tough leathery envelope which would be impenetrable to the semen, that is, before they enter the uterus; therefore, copulation must take place in all these fishes. The form of the egg-shell differs in the various genera; generally (Fig. 79) they are flattened, quadrangular, with each of the four corners produced, and frequently prolonged into long filaments which serve for the attachment of the ova to other fixed objects. In Notidanus the surfaces are crossed by numerous ridges. In Cestracion (Fig. 80) the egg is pyriform, with two broad ridges or plates, wound edgewise round it, the two ridges forming five spires. The eggs of Callorhynchus (Fig. 81) have received a protective resemblance to a broad-leaved fucus, forming a long depressed ellipse, with a plicated and fringed margin.