Unicellular organisms: Protozoans (Ammonotelic) – Excretory organ are absent, hence excretion occurs through general body surface by diffusion. Contractile vacuoles may be present. 2. Sponges (Ammonotelic): Diffusion by body surface into canals, spongocoel and outside. 3. Coelenterates (Ammonotelic): Diffusion through general body surface. 4.
Platyhelminthes: Excretory system made up of flame cells, joining to form excretory capillaries which in turn join to form excretory ducts that open to outside by excretory pores, (Eg., – Planaria, Fasciola, Taenia). The term solenocytes with blood supply is also used for flame cells. Eg-, Amphioxus. 5. Annelids: Well developed nephridia. Each nephridia has an opening inside the body called nephrostome, which leads to ciliated ducts. They in turn open to outside by nephridiopores.
If the waste product is discharged outside the body the nephridia are called exonephridia e.g. integumentary nephridia of earthworm (Pheretima).
If it is discharged inside they are called enteronephridia e.g. pharyngeal nephridia (pharynx) and septal nephridia (intestine). Based on size nephridia are micro-, meso and meganephridia. 6. Arthropoda: Crustaceans – Excretory organs are green glands situated at the base or coxa segments of second antennae (hence coxal or antennary glands) and a median renal sac that lies over the stomach and reaching upto gonads. Eg. prawn.
Insects – Excretory organ is malpighian tubule. These are fine numerous, fine threads in groups closed at free ends, attached and opening into alimentary canal at the junction of midgut and hindgut. The tubules pick up waste products from hemolymph and form urine. Eg. Centipedes and Millipedes.
7. Nematodes: A system of intracellular canaliculi and extracellular canals is present for collecting and taking excretory fluid to excretory pores. 8. Mollusca: Excretory organ is Keber’s organ or renal gland or organ of Bojanus. E.g. Unio.
9. Echinoderms: Specialized excretory organs are absent in echinoderms. Excretory products, chiefly ammonia, are eliminated by diffusion through dermal branchae and tube teets. 10. Hemichordates: Excretory organ is proboscis gland (glomerulus) Eg.
Balanoglossus. 11. Urochordates: Excretory organ is neural gland Eg. Herdmania. 12. Cephalochordates: They have nephridia with solenocytes. Eg.