The circulatory systems of animals, the main fusion of which is to transport substances from one bodily organ to another within the body, can be classified in the following way
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1. Intracellular transport:
This type of transport mechanism is the characteristic feature of the lowly-organised organisms such as protozoans; the protoplasm of these animals constantly shows streaming movement in a definite course which supplements not only the diffusion of the substance into and outside the body but also proper dispersal of them throughout the cell cytoplasm.
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This can be exampled by definite course of the movements of food vacuoles in the ciliates. Protoplasmic streaming also occurs in most, if not all, cells of the metazoans. It is most evident in amoeboid cells.
2. Transport through external medium:
In sponges and coelenterates water serves as circulatory medium. In these animals water along with dissolved nutrients and oxygen passes through definite channels and cilia, flagella and muscular activity of the body assists in maintaining the regular circulation of water.
3. Transport through fluid filled body spaces:
This type of transport mechanism is found in those animals which are having primary body cavities such as pseudocoeloms of nematodes, ectoprocts and rotifers.
The fluid contained in these primary cavities is made to propell by the muscular movements of the body.
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As the fluid moves, the nutritive substances and oxygen contained in it also move from one place to another and the cells of different organs pick up required substances and oxygen from the fluid.
Some transport occurs in the mesodermally lived coelom of echinoderms, annelids, ectoprocts and some chordates.
In arthropods and molluscs the coelom is very much reduced to gonads and kidneys, the circulatory medium in these animals is haemocoelomic fluid which runs along with haemocoel.
4. Open circulation of haemolymph:
In many invertebrates such as most arthropods, molluscs and ascidians, the presence of circulatory system though has come into existence but is of open type.
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In these cases the circulatory medium is haemolymph which is often called blood. It flows through a system of vessels in no more than a part of its circular path, in a part of the path the blood flows out into extensive tissue spaces.
Such a system is called an open system. There may be two types of open systems. In one the tissue spaces, called sinuses, are lined by a cellular membrane which keeps the blood separated from the tissue cells.
In the other type the blood spaces called lacunae are not lined by a cellular membrane and blood bathes the tissues directly.
Both sinuses and lacunae may exist in the same system. The haemolymph circulates mainly by virtue of muscular movements of the appendages.
Rudimentary heart is, of course, found in these animals but it does not play any role in the conduction of haemolymph.
Aquatic isopods have well-developed vessels and indistinct lacunae; in terrestial isopods the vessels are reduced and lacunae are well differentiated.
5. Transport through lymph channels:
In vertebrates tube like structures are found called lymphatics. These structures connect the intracellular spaces with the blood vascular system and coverage on veins and form a lined network which may be as extensive as the capillary bed.
In some amphibians and a few teleosts lymph hearts are also found. Fats and other substances are transported by the vessels.
6. Blood vascolar system:
The blood vascular system is a system of delicate branched tubes connected with each other. These tube are known as blood vessels.
This system is found in oligochaete many polychaetes, leeches, cephalopod molluscs, holothurian echinoderms and vertebrates. It is of closed type and appeared first in annelids.
The circulatory medium is blood which in this systems is pumped to the tissues through the aiteries where it comes into intimate association with the tissues by capillaries.
The blood returns to the heart by some sort of closed return vessels and thus blood is circulated again and again.