... called choanocytes line the internal chambers of a sponge; these choanocytes generate a water current that circulates throughout the sponge via the many pores that a sponge is built around. Thus the cells of a sponge can independently respire from a renewable source: the water the sponge inhabits. Cnidaria, like Porifera, lack any specialised respiratory system. Instead these hydras, jellies, sea anemones and corals have a gastro vascular cavity. This cavity has one opening that acts as mouth and anus, and deals with both digestion and gaseous exchange. Cnidaria, like sponges, employ a method of pumping water to keep fresh oxygen entering the gastro vascular cavity and so enabling continuous gaseous exchange. However, unlike sponges (which possess no true body tissues), Cnidaria have simple muscular structure. This allows them to contract and relax their bodies to create water currents for digestion and respiration. . Flatworms (Platyhelminthes) and Roundworms (Nematoda) employ a system ...

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