320 HISTOLOGY 



The respiratory epithelium is one of the most generalized tissues found 

 in the animal body. On account of the negative nature of the duties to 

 be performed, almost any epithelium can execute them, and in conse- 

 quence, we find that the tissues devoted exclusively to respiration may 

 be developed on a great variety of locations of the integument or on the 

 inner surface regions of the body. Very many animals have no specific 

 surface for respiration. Some of these, too, are otherwise highly or- 

 ganized, as, for instance, the earthworm, which utilizes its general body 

 surface for that purpose. This use of the whole body surface might be 

 looked upon as a specialization in itself. 



The lack of cytological specialization also causes as great a diversity 

 in the form of the respiratory organs as of their distribution. This 

 diversity is well shown in the worms, for instance, where some of them 

 have no special respiratory organ, while others have them in a variety 

 of forms. 



As a rule, animals breathing water use an evagination of some sur- 

 face for a respiratory organ, while animals that derive their oxygen from 

 the atmosphere use an invaginated surface for the same purpose. In the 

 latter case the organ is designed to protect the delicate respiratory cells 

 from the effects of drying. Cells, strong and resistant enough to stand 

 drying, would not easily permit the gases to be exchanged and, besides, 

 the moist condition is the most favorable for the process. Water-breath- 

 ing animals may live in the atmosphere, and yet carry enough water in 

 their body to use it for respiratory purposes, as do the land Crustacea 

 and some fishes, while other organisms that breathe air may live a large 

 part of their lives under water, and at the same time carry with them 

 the air that they breathe. 



Notwithstanding the negative character of a respiratory epithelium, 

 it has, in many cases, retained some characteristics of the epithelium 

 from which it has been derived. These characteristics are of no real 

 use to it, usually, except the ciliated condition found in some forms. 

 Here the cilia, which only appear on a part of the cells, are used to drive 

 the currents of water over the respiratory surfaces, thus performing the 

 act of breathing which in other organisms is performed by arrangements 

 of cavities, valves, and muscles that belong to a morphological study of 

 the subject. Some other residual characteristics found in the respiratory 

 tissues are the chitinous covering found on the gills of the Crustacea, 

 worms, etc., and the mucous cells and pigmented cells that can be seen 

 on the gill surfaces in other forms. 



The exchanges of gases do not take place through the respiratory 

 cells alone. When the blood is held in a closed circuit of vessels, the 

 walls of these vessels are also interposed between the blood and the 

 respiratory medium. Thus the gases must pass through two layers of 



