It is of the following types:
(i) Water Circulation:
(a) In sponges, Canal system is filled with water which brings food and oxygen into the body and carries waste products and reproductive bodies outside. (b) In Hydra, incoming water current brings food and oxygen into the coelenteron for different cells and outgoing water current carries waste products outside. (ii) Parenchymal Circulation: In flat worms like liver fluke, Parenchyma (a peculiar tissue) transports food to different body parts. (iii) Pseudocoelomic fluid circulation. In roundworms (e.g., Ascaris) pseudocoelomic fluid of the pseudocoel transports food to different body parts, (z’v) Blood Circulation: Blood transport various materials in annelids, molluscs, hemichordates and chordates.
(a) Open Circulatory System:
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When the blood does not remain confined to the blood vessels and flows into spaces. It is termed as the open circulatory system. Invertebrate animals that have open circulatory system are the arthropods (such as prawn crabs, lobsters, and spiders) and most of the molluscs (Such as snails, oysters and clams). In insects like cockroach, the body cavity is filled with blood, the haemolymph (colourless blood), that is why, it is called haemocoel.
(b) Closed Circulatory System:
When the blood remains confined to the blood vessels it is called the closed circulatory system. Invertebrate animals with a closed circulatory system are some annelids (such as the earthworm) and some molluscs (such as squid). All vertebrate animals have closed circulatory system. The circulation of blood in the closed circulatory system was first discovered and demonstrated by William Harvey (1578-1657).