Section VI—Urogenital System

As various ducts of the excretory and reproductive systems become associated during embryonic develop­ment, it is convenient to discuss these systems together. The kidneys and gonads, noted earlier (page 49), are the main organs of the excretory and reproductive systems, respectively.

FIGURE 3.33 Pleuroperitoneal cavity in ventral view, showing the urogenital system of the male shark. Much of the viscera has been removed. The right testis is reflected to the left. The anterior end of the right siphon has been cut away. The bristle inserted into the siphon shows the path leading out of the siphon on the clasper.

FIGURE 3.34 Pleuroperitoneal cavity in ventral view, showing the urogenital system of the female shark. Much of the viscera has been removed. The left ovary is reflected to the right.

parietal peritoneum lining the pleuroperitoneal cavity, the kidneys are retroperitoneal. The paired, elongated gonads, testes (sing., testis) in the male and ovaries (sing., ovary) in the female, lie dorsal to the liver in the pleuroperitoneal cavity. Note the caudal ligament between the posterior ends of the kidneys. It arises from the vertebrae and passes to the tail. The posterior mesenteric artery pierces the ligament on its way from the dorsal aorta to the digitiform gland.

Male Urogenital System

The kidney of the male (Figure 3.33) has two distinct functional parts. The anterior part is related to sperm transport.

Each testis is supported by the mesorchium, a mesen­tery that suspends the organ from the middorsal wall of the body cavity. Within the anterior part of the mesor- chium are small tubules (efferent ductules, which may be observed as strand-like structures) that extend from the testis to tubules in the epididymis of the kidney, which in turn are connected to the archinephric duct. Sperm take this route from the testes to reach the archinephric duct (originally a duct of the kidney) and then to be transported posteriorly for eventual release from the body (see below). In a mature male the archinephric duct is tightly convoluted and embedded in the ventral surface of the kidney. In an immature male it is nearly straight and resembles the oviduct of the female (see below).

As the archinephric duct approaches the cloaca, it straightens and expands into the seminal vesicle. Trace it posteriorly into the sperm sac, a small, anteriorly blindly ending pouch that lies on the ventral surface of the seminal vesicle. Free the anterior end of the sperm sac from the seminal vesicle and slit the sperm sac ven­trally. Probe its roof and note the entrance of the seminal vesicle. Probe the posterior part of the sperm sac. Left and right sperm sacs unite to form the uro­genital sinus, a median space that continues posteriorly through the urogenital papilla, which is used for the passage of urine and sperm.

As noted above, urine is produced almost entirely (and probably exclusively in the adult male) by the posterior part of the kidney. However, it does not enter the archinephric duct or seminal vesicle. Instead, a thin, del­icate accessory urinary duct carries urine to the uro­genital sinus.

The remaining reproductive structures of the male are associated with the pelvic fin. The clasper is an intro­mittent organ inserted into the cloaca and oviduct of the female during copulation. A groove lies along the medial side of the clasper. Sperm is released from the urogeni­tal papilla and travels along the groove to the cloaca of the female. Associated with each clasper is the elon­gated, sac-like siphon, which lies just below the skin on the ventral surface of the pelvic fin. Expose the siphon by removing the skin, and then cut open the siphon. Using a thin probe, search its posteromedial region for a passageway. Follow it as it passes along the groove of the clasper. The siphon expels fluid through this route that contributes to seminal fluid.

Key Terms: Male Urogenital System

Female Reproductive System

The ovaries are suspended from the roof of the anterior pleuroperitoneal cavity by the mesovarium (Figure 3.34).

Trace the oviducts posteriorly and note that each even­tually enters the cloaca through an opening on either side of the urinary papilla. An oviduct has two enlarged regions. The first, appearing as a slight anterior swelling dorsal to the ovary, is the nidamental gland, which secretes a thin membrane around groups of eggs as they pass through the oviduct. It is also the location where eggs are fertilized. The second enlargement, the uterus, occurs posteriorly. The uterus is greatly enlarged in pregnant specimens because it contains the developing pups. If you have a gravid female, cut open one of the uteri to observe the developing embryos. If not, observe embryos from another specimen. Depending on the stage of gestation, the pups will vary in length. They are nourished mainly with yolk, much of it stored in the external yolk sac, which enters the embryo via a stalk. Cut open the pleuroperitoneal cavity and observe that the stalk is continuous with the internal yolk sac.

The anterior part of the female kidney is greatly reduced and, unlike in the male, the kidney is not involved in the reproductive system. However, an archinephric duct, which is formed from the kidney, is present. In the female it is uncoiled, much smaller than in the male, and transports urine. Accessory urinary ducts are not present in female Squalus, but may be present in females of other sharks.

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