1. The nephridium (pl.: nephridia) is an invertebrate organ, found in pairs and performing a function similar to the vertebrate kidneys (which originated from the chordate nephridia). Nephridia remove metabolic wastes from an animal's body and come in two basic categories: metanephridia and protonephridia. In some cases, nephridia may fuse or become functionally integrated with coelomoducts—mesodermal structures that open from the coelom to the exterior—to form a more complex structure known as a nephromixium. All animals possessing nephridia or kidneys belong to the clade Nephrozoa. == Metanephridia == A metanephridium (meta = "after") is a type of excretory gland found in many types of invertebrates such as annelids, arthropods and mollusca. (In mollusca, it is known as the Bojanus organ.) A metanephridium typically consists of the nephrostome (a ciliated funnel) opening into the body cavity, connected to a duct which may be variously glandularized, folded or expanded (vesiculate) and which... from wikipedia.org