Urogenital system of Necturus

By Todd Baker and Jennifer Sobecki (1999)

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The urogenital system of Necturus is a primitive system as compared to mammals in general, and is homologous to the system of ancestral fishes. Evidence of this can be seen in the presence of the cloaca and absence of an intromittent organ, such as the penis. However, the excretory system has an incipient feature not found in most ancient fishes. That feature is the urinary bladder.

A bladder is a derived feature of the cloaca that, along with the opisthonephric kidney, allows the Necturus to collect urine. This feature is important because it allows the Necturus to better excrete wastes, reabsorb much of the fluid it ingests, become more terrestrial, and become less dependent on aquatic habitats. This is believed to be an important feature in the evolution of aquatic organisms to terrestrial organisms. It is also significant to point out that Necturus is only found in freshwater habitats, which is also believed to be the environment in which terrestrial organisms originated (Walker and Homberger 1992).

Their mode of reproduction is also primitive to most vertebrates. The male Necturus has testes that are located inside the pleuroperitoneal cavity and lacks a more specialized intromittent organ found in more "advanced" vertebrates. When sperm are made in the testes they pass through several ductuli efferentes to the cloaca through the archinephric duct. The sperm are deposited into the water in clumps called spermatophores, to be picked up later by the female (Walker and Homberger 1992).

In the female Necturus, ovaries release eggs into the ostium tubae which funnels them to the oviducts. The oviducts produce a gelatinous material that surrounds the eggs, and will later protect the egg when deposited through the cloaca. The female Necturus will use its cloacal lips to pick up a male’s spermatophores in the fall and store them until spring for fertilization and egg-laying. This process is referred to as delayed fertilization.