2o8 PHYSIOLOGY OF THE INVERTEBRATA. 



of the gaseous products of decay. In fact we may define 

 respiration as "the elimination of the gaseous products of 

 tissue-combustion, and the introduction of the oxygen neces- 

 sary for that combustion." 



The lower forms of the animal kingdom respire directly by 

 changes between the general surface of the body and the 

 medium in which they live; but in the higher forms, respira- 

 tion is a twofold process : («) internal respiration, or the 

 interchanges between the gases of the blood and the tissues ; 

 and (&) external respiration, or the interchanges between the 

 gases of the blood and the gases in the air-cells of the lungs. 

 These interchanges, however, are not always confined to the 

 lungs ; thus there is a true cutaneous respiration in the 

 skin, an intestinal respiration in the intestines, and most 

 probably interchanges of a like nature take place in other 

 organs ; for it may be remarked that many organs of the 

 Invcrtchratci contain various pigments, which have a res- 

 piratory function. 



The respiratory apparatus is always in intimate relation 

 with the organs of circulation. 



The Protozoa. 



In most of the Protozoa, respiration takes place all over 

 the general surface of the body ; but these animals differ 

 somewhat in the mechanism of respiration. In the 

 Gregarmida the interchange of gases takes place all over the 

 body. In the majority of the Iiifasoria and Rhizopoda there 

 is a differentiation of the function of respiration, for even in 

 these low forms the interchange of oxygen and carbonic 

 anhydride takes place at certain specialised regions (con- 

 tractile vacuoles), but the air is not brought into direct con- 

 tact with the circulating fluid. The oxygen or air for res- 

 piration is dissolved in water. The contractile vacuoles of 

 these organisms perform several functions, among these 

 being that of respiration. The contractile vacuoles contain 



