Scoliodon is Dioecious and shows sexual dimorphism. The excretory and reproductive systems are so closely related to each other in vertebrates that they are treated collectively as urinogenital system.
(1) Male urinogenital system
Excretory organs:
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Excretory organs of the dogfish consist of a pair, of long, flattened ribbon-like mesonephric kidneys.
They are attached to the dorsal abdominal wall, above the peritoneum one on either side of the median line.
They extend nearly the whole length of the body cavity from root of liver in front upto cloaca behind. The anterior part of the kidney is greatly reduced, non-excretory, narrower and genital in function, hence called epididymis.
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The posterior part is greatly developed, excretory, thicker and forms the functional adult kidney called opisthonephoros (Graham Kerr 1919).
Each opisthonephros is formed by several coiled, glandular uriniferous tubules with Bowman’s capsules enclosing glomeruli.
The tubules have a special urea-absorbing segment in them. All the collecting tubules lead into a common duct or ureter.
Posteriorly both the ureters open into a wide median urinogenital sinus, which itself opens into cloaca through its aperture placed at the tip of a short urinogenital papilla.
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(1) Reproductive organs:
These consist of a pair of very large elongated testes attached to the mid-dorsal abdominal body wall by a double fold of peritoneum (mesentery) called mesorchium.
From each testis several very fine tubules, the vasa efferentia arise and run in the mesorchium to the anterior end of vas deferens.
Germ cells produce spermatozoa in the seminiferous tubules of the testis which are carried to the vas deferens.
The vas deferens forms a very large narrow and extremely coiled duct along the entire ventral surface of the epididymis, which produces a nourishing fluid for the spermatozoa.
Vas deferens leads into a wide straight tube, the seminal vesicle which stores the sperms. Like ureters the seminal vesicles of both the sides open behind separately into the urinogenital sinus.
This gives off in front on either side an elongated club-shaped blind sac, the sperm sac of doubtful function.
The accessory male reproductive system comprises of the claspers and siphon. Claspers are present on the pelvic fins of male dogfish and act as intromittent organs in coition.
Each clasper bears a closed groove with anterior opening called apopyle lying near cloaca and posterior called hypopyle. Siphon is a muscular sac which is situated beneath the skin of posterior ventral pelvic region.
It has two channels closed anteriorly but open posteriorly into apopyle. During copulation, sea water is drawn into siphon and forced down the clasper groove flushing sperms into the female cloaca.
(2) Female urinogenital system
Excretory organs:
In females also kidneys show the same differentiation into anterior or non-excretory and posterior excretory portions.
But the kidneys have no connection to the genital organs. All the uriniferous tubules of posterior part open large median urinary sinus. The sinus, in turn, opens into cloaca at the tip of a short urinary papilla.
Genital organs of female:
These constitute one pair of small ovaries. These are lobulated bodies attached mid-dorsally to anterior abdominal wall by a fold of peritoneum, the mesovarium.
Their form and size varies with the age. A pair of long tubular epigonal organs extend between ovaries in front and the caecal or rectal gland behind.
Ovarial follicles contain developing ovum. Mature ova are shed into the abdominal cavity from where they enter the oviducts.
The two oviducts or Mullerian ducts are large tubes extending the whole length of body cavity. Anteriorly both unite mid-ventrally below oesophagus to open into coelom by a single longitudinal slit the ostium or oviduccal funnel.
The shell gland secretes a thin membrane over the descending eggs which get fertilized between oviducal funnel and shell gland.
Posteriorly oviducts lead into a wide chamber, the uterus, in which development of embryos takes place. The two uteri unitedly open into a common vagina which opens into cloaca by a large aperture.