RESPIRATORY ORGANS 103 



The essential feature of respiratory organs, tlie gill or the 

 lungs, is the thin, moist membrane which sc])arates on one 

 hand the oxygen of the air or the water from the respiratory 

 fluids of the body, generally "blood," and which permits of 

 a free interchange of gases, oxygen passing in and carbon 

 dioxide passing out. In many cases, such as leeches and 

 earthworms, this membrane is the outer layers of the skin, 

 and in certain leeches the minute capillary channels con- 

 taining the blood actually lie between the neighbouring cells 

 forming the epidermis or outermost layer of cells, a membrane 

 only one cell thick. Many marine worms have processes which 

 are regarded as gills. These may be extensions of the body 

 wall and take the form of folded flaps or feathery plumules. 

 Often a gill performs two functions. For instance, in the bi- 

 valve molluscs the bulk of the respiration is carried on in the 

 inside of the thin mantle lining the shell, whilst the highly 

 complicated so-called gills are in the main used for catching 

 and conveying food to the mouth. 



A gill or lung is obviously a delicate organ and is in most 

 cases well protected : the lateral processes of the body which 

 form the gills of many Crustacea are sheltered by a calcareous 

 outgrowth of the skin and a special arrangement exists for 

 pumping the water over these so-called branchiae. The gills 

 of fishes are vascular processes on certain slits opening into 

 the gullet. The oxygen-containing water is as a rule taken in 

 by the mouth and expelled through these gill-slits, or it may 

 be taken in and then expelled from the gill-slits without 

 passing through the mouth. In nearly every case the gills are 

 protected by a gill-cover or ojjerculum. Some animals breathe 

 by pumping water in and out of the hinder end of the ali- 

 mentary canal. This is true, for instance, of certain Crustacea, 

 of the sea-cucumbers, insect-larvae, and other aquatic animals. 



The insect provides a remarkable example of eflicieney in 

 respiration. Here the oxygen is conveyed to and the carbon 

 dioxide removed from the tissues, and even to and from the 

 individual cells, direct, a much more efficient process than in 

 other animals, where both the carbon dioxide and the oxygen 

 have to pass into solution in the blood and then pass out again 

 to the cefl. The gain in energy with a practically unlimited 



