RESPIRATION. 563 



external membrane, or some extension of such membrane, outward pro- 

 longations constituting gills, inward prolongations constituting the lungs ; 

 so that all animals may be classed as air-breathers or water-breathers, 

 the latter breathing the air dissolved in water. To the former class 

 belong myriapods, spiders, insects, reptiles, birds, and mammals, while 

 all other animals, with few exceptions, are water-breathers ; in the first 

 class the organs of respiration are internal ; in the latter class more or 

 less external, but in all are to be regarded as modifications of the external 

 integument (Fig. 237). 



In the lowest forms of animal life, all of which are inhabitants of 

 water, there are no prolongations of this membranous surface — aeration 

 of the fluids being accomplished \>y their exposure to the surrounding- 

 medium containing oxygen in solution. No distinct respiratoiy organs 

 are present in these forms of life, unless the contractile vesicles described 

 as being constantly found in such organisms are of this character. In 

 animals belonging to the group of protozoa the surface of the body is 



FlQ. 237.— DIAGRAM ILLUSTRATING DIFFERENT FORMS OF RESPIRATORY 



Apparatus. (Carpenter.) 



A, simple leaf-like gill ; B, simple respiratory sac : C. divided gill ; D, divided sac ; E, pulmonary 

 branchia of spider. 



usually more or less provided with cilia, -which serve by their vibrations 

 to continually change the stratum of water immediately in contact with 

 the external surface. The presence of a fluid containing oxygen in 

 solution in contact with the respiratory surface is thus always insured. 



In sponges, as in infusoria and polyps, we find that the respiratory 

 surface exists in the form of tubulary passages through the body, pro- 

 vided at certain points with cilia, the air being absorbed from the currents 

 of water passing through them. 



In the ccelenterata, which are all aquatic, no circulatory organs are 

 present, and, as a consequence, no respiratory apparatus ; for we find 

 that while organs of circulation are dependent upon the complexity of 

 the alimentary apparatus, so the presence of distinct circulatoiy organs 

 governs the presence of organs of respiration. 



In the ccelenterata any part of the body surface appears to be capable 

 of accomplishing the interchange of oxygen and carbon dioxide. In some 

 the body cavity also, doubtless, fulfills this function. This would seem to 



