Organs in animal - Excretion
In all the other animal groups, there are discrete organs which function in osmotic regulation, ionic regulation and nitrogen excretion. These are the contractile vacuoles of protozoans and poriferans, the nephridia of flatworms, roundworms and annelids, the green glands of crustaceans, the coxal glands of arachnids, the malpighian tubules of insects and the metanephric kidney of vertebrates. Depending on whether the nephridial tubule is closed at the inner end or opens into the coelom by funnel shaped structure called nephrostome, nephridia are respectively termed as protonephrldia or metanephridia.
The excretory organs of molluscs, crustaceans and arachnids are actually modified coelomoducts and hence are not true nephridia. With the exception of the contractile vacuoles of protozoans and sponges, the excretory organs are either tubules or aggregates of numerous tubules.