The organs responsible for the elimination of
metabolic waste products are called excretory organs. They help in the
elimination of nitrogenous wastes, water balance and maintains constant ionic
composition of the extracellular fluids in the body. A variety of excretory
organs are present in the animal kingdom. In most invertebrates, these
structures are simple tubular forms. However, in man and other vertebrates they
are complex tubular structures called kidneys. Some of the excretory structures
in invertebrates are mentioned here.
❖ Protonephridium is a network of dead-end
tubules with cellular units called flame bulbs. Each flame bulb has a tuft of
cilia projecting into the tubule. Protonephridia are found in platyhelminths,
rotifers, some annelids, molluscan larvae, and lancelets (with solenocytes).
Protonephridia are primarily concerned with ionic and fluid volume regulation
i.e osmoregulation.
❖ Metanephridia are tubular excretory
structures which are immersed in the coelomic fluid and enveloped by a
capillary network. They are found in most annelids such as the earthworms. They
help to remove nitrogenous wastes and maintain fluid and ionic balance.
❖ Malpighian tubules are-'blind' tubular structures
floating in the haemolymph and opening into the digestive tract. They are found
in insects and other terrestrial arthropods. Besides excretion, they have a
role in conserving water and salts effectively, as they send wastes into the
gut where water is reabsorbed.
❖ Antennary glands or green glands are paired
structures which lie at the bases of the antennae and open to the exterior.
These structures draw waste materials from the haemolymph. They are found in
crustaceans.
❖ Coxal glands are the excretory structures
in the arachnids.
❖
Kidneys (metanephridia/renal organs) and pericardial glands are the excretory structures
found in molluscs.
