By Jared Baker and Mary Ellen Prince (1999)
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The dogfish shark (Squalus) is a member of the class Chondrichtyes, subclass Elasmobranchii. This subclass includes the sharks or cartilaginous fishes. This subclass shows a variety of primitive traits characteristic of vertebrates. The reproductive system includes a variety of structures that are evolutionary stepping stones to higher vertebrate structures.
The first, most obvious primitive structure, is the cloaca, shown in (repo1). The cloaca is a common urogenital opening that leads to the reproductive organs as well as the excretory and digestive organs. In the male, the cloaca is not divided, but the excretory and reproductive products are released more dorsally and caudally than the digestive products. In females, however, the urogenital portion of the cloaca is separated from the digestive portion by a horizontal fold. As in the male, the female excretory and reproductive products are released dorsally. Further development of the horizontal fold found in the female Squalus is seen in higher vertebrate taxa. The reproductive and excretory openings are completely separated from the digestive openings.
(repo2) points out other significant portions of the male urogenital system. The male testes are located at the cranial end of the pleuroperitoneal cavity and are suspended in the cavity by the mesorchium. In the mesorchium, several small tubules called the ductus efferentes carry the sperm from the testes to the archinephric duct. The archinephric duct transports sperm to the cloaca. This duct is also used to drain the kidneys. Later in vertebrate evolution, the opisthonephros (cranial portion of the archinephric duct) is specialized to form the epididymis in amniotes. This specialization allows sperm and fluid from the kidneys to travel in different ducts. The Leydig’s gland, shown in (repo2), secretes fluid into the archinephric duct to protect the sperm. The caudal end of the archinephric duct enlarges and forms the seminal vesicle, which leads to the sperm sac. The joining of these two structures produces the urogenital sinus. The urogenital sinuses of the left and right sides connect to form the urogenital papilla discussed earlier. The male also possesses external structures called claspers (repo2) that aid in copulation.
Like the male testes, the female ovaries are located in the cranial portion of the pleuroperitoneal cavity. The ovaries are suspended by a mesovarium, shown in (repo1), which contains eggs in different stages of maturity. Once the eggs reach maturity, they are released from the ovary and mesovarium and travel to the oviducts via the ostium tubae. Unlike the male Squalus, the female reproductive tubes are separate from the archinephric duct. The oviduct, shown in (repo1), transports the egg from the ovary to the uterus. The enlargement in the oviduct caudal to the ovary is the nidamental gland. This gland stores sperm to fertilize released eggs. The sperm can remain viable and delay fertilization for several weeks. This allows the female to mate when males are available and delay fertilization and pregnancy until conditions are suitable for development of young. The second enlargement of the oviduct is the uterus (repo1). In mature females, the uterus is the caudal one-third to one-half of the oviduct. Following the uterus, the two oviducts join in the dorsal portion of the cloaca.