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.
[Illustration: Fig. 69.--Ovum of Myxine glutinosa, enlarged.]
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.
[Illustration: Fig. 70.--Ditrema 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 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.
[Illustration: Fig. 71. Ovum of Arius boakii (Ceylon), showing
embryo. Nat. size.]
[Illustration: Fig. 72.--Abdomen of _Aspredo batrachus_,
with the ova attached; at _a_, the ova are removed, to show
the spongy structure of the skin, and the processes filling the
interspaces between the ova. (Natural size.)]
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.
[Illustration: Fig. 73.--Solenostoma cyanopterum ♂ (Indian
Ocean).]
[Illustration: Fig. 74.--Pouch with ova, formed by the ventral
fins of _Solenostoma_. Lower aspect; the edges of the fins
have been pushed aside to allow of a view of the inside of the
pouch. (Natural size.)]
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.
[Illustration: Fig. 75.--Syngnathus acus ♂, with
sub-caudal pouch.]
[Illustration: Fig. 76.--Sub-caudal pouch of Syngnathus acus,
with the young, ready to leave the pouch. One side of the
membrane of the pouch is pushed aside to admit of a view of its
interior. (Natural size.)]
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.
[Illustration: Fig. 77.--Ovaries of Ceratodus.
_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.
[Illustration: Fig. 78.--Ventral fins and claspers of
Chiloscyllium trispeculare.]
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.
[Illustration: Fig. 79.--Egg of a Scyllium from Magelhan’s
Straits (? Sc. chilense). Natural size.]
[Illustration: Fig. 80.--Egg-shell of Cestracion philippi, half
natural size, linear.
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.
[Illustration: Fig. 81.--Egg of Callorhynchus antarcticus.
_a_, Cavity for the embryo.]