1. Diffusion lungs:
Such type of lungs are found in most invertebrates including scorpions, pedipalps, spiders, chilopods and tropical snails.
These lungs as they are filled with air by simple diffusion are termed as “diffusion lungs”. These lungs are more primitive when compared with ventilation lungs, because of the absence of ventilating mechanism.
2. Ventilation lungs:
The lungs which are filled in with air by some sort of ventilating mechanisms, are referred to as “ventilation lungs”.
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These are found in most vertebrates and are highly elastic. Two different types of ventilation are found among vertebrates; a positive pressure types in which air is forced into the lungs by swallowing or buccal movements as in the frog and a negative pressure system in which air is drawn in by increasing the space around the lungs as in mammals. Birds have extensive air sacs.
3. Gas bladders and lungs of fishes:
In some teleost fishes a bag-like structure of variable shape is found above the oesophagus and below the vertebral column.
This bag-like, structure is called ”air bladder” or “gas bladder”. It is primarily a hydrostatic organ but in some fishes it serves as respiratory organ as it contains much oxygen which is usually used in hypoxic conditions. It is believed it is a forerunner of the vertebrate lung.
In fishes the gas bladders are of two types, open found in physostome teleosts and close found in the physoclistous fishes. Both types of gas bladders can secrete gas into the bladder.
4. Lungs of pulmonate gastropods:
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Pulmonate gastropods also possess lungs but these are in the form of cavities in the mantle whose walls are well supplied with blood vessels.
These animals are devoid of gills and depend solely upon pulmonary chamber to meet their respiratory requirements.
5. Alimentary mucosa:
A number of tropical fishes respire by means of the intestinal or gastric mucosa, because in these fishes the intestinal or gastric mucosa becomes richly vascularized.
The air is swallowed in and during its transit through the intestine exchange of gases takes place and the residual air is sent out through the anus.