Breathing holes on the surface of an animal's body are designed to allow the living organism to receive and release gases from the external environment. In the body of protozoan animals there are both primary and secondary respiratory organs. The first ones were developed in ancient single-celled organisms, the second ones are developed and complementary to simple ones. The primary protozoan respiratory organs are the olfactory cells.
The secondary respiratory organs of protozoa are primitive and have the simplest form. They correspond to the primary organs for multicellular organisms. In their early stages, structures with complex internal architecture are formed. It depends on the class of animals. For example, the epithelium in these organs of birds can be either incised or bordered. But they both must have common features inherent in the secondary respiratory organs: the presence of stigmas and their position.
Stigmas are characteristic of ciliates, so they are sometimes called their stigmatic openings. Secondary respiratory organs are much more complex than epithelial structures. These include funnels, lacunae and trachea. Funnels are characteristic of organisms that actively move through water. The trachea and tube are characteristic of animals living in water. The latter do this better, using gill plates to inhale water. The tube is a means of breathing for aquatic animals. Stigmozoans sometimes surround the central canal, as a result of which the resulting cavity fills with water and is subjected to strong compression. Air enters this organ through a special valve for gas exchange. Thus, all modern infusor species belong to the class of protozoa and have a complex set of differentiated organs