The excretory system is closely related to the reproductive system. Males and females differ significantly in several key ways. A pair of opisthonephric kidneys, which are long, thin, highly vascularized strips of tissue that filter nitrogenous wastes from the blood, chiefly urea, make up the excretory system. The archinephric ducts are responsible for kidney drainage. The more caudal section of the archinephric duct also carries urea produced by the kidneys in males, whereas the cranial portion of the duct serves primarily a reproductive purpose. The female has smaller and shorter archinephric ducts. The two tiny auxiliary urinary ducts in males that drain into the urogenital sinus receive urine from the archinephric ducts. The male auxiliary urinary ducts and the archinephric ducts discharge into the urinary papilla, a little projection into the cloaca. In males, the urogenital papilla is another name for the urinary papilla. Materials from the digestive, reproductive, and excretory systems are delivered to the cloaca. Excretory waste leaves the cloaca and travels outside. A duct connects the rectal or digitiform gland to the big intestine. This tubular organ helps the dogfish maintain a healthy osmotic equilibrium in its bodily fluids by excreting extra sodium chloride.
