Different types of circulatory media have been observed in different animals which are as follows:
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1. Hydrolymph:
This type of circulatory medium is found in lower category animals such as nematodes, ectoprocts and rotifers.
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It is a watery fluid having no respiratory pigment. It carries nutritive substances to organs and tissues and is also connected with the removal of waste products.
2. Haemolymph:
It is a common circulatory medium which is generally found in arthropods and many molluscs.
It is comparatively less watery than the hydrolymph. It is rich in proteins and serves as blood in higher animals.
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It not only transports nutritive substances to the tissues and remove waste products but it also serves respiratory function as it has haeme, a respiratory pigment, in its compsition.
3. Lymph:
It is a colourless liquid contained in a system of branching tubes called the lymphatics or lymph vessels.
It is similar to plasma in the composition as it, like blood plasma, contains proteins, lymphocytes and some granulocytes, but the concentration of these substance is much lower than the blood.
It contains no red cells. One of the most important functions of lymph is to transport materials to and fro in the tissue.
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It is generally found in almost all the animals including invertebrates and vertebrates.
4. Blood:
It is the chief circulating medium of the animals including the invertebrates and the vertebrates, with the exception of Amphioxus and Leptocephalus.
It transports different substances from one organ to another within the body. It ensures humoral intercommunication of all organs and serves also to maintain uniformity of temperature in the body.
In all its capacities it functions in the regulation and maintenance of the constancy of the normal environment of the body.
Although, various chemical substances continuously enter or leave the blood during its circulation the overall composition of the blood remains remarkably constant.