MORPHOLOGY OF ANIMALS 



which occurs in all living matter, it follows that respira- 

 tion is a universal function in organisms. In small and 

 simple animals this exchange of gases takes place directly 

 between the living cells and the surrounding medium and 

 occurs all over the surface of the body. In more complex 

 forms with body fluids the exchange takes place between 

 the cells and the body fluid or blood {internal respiration) 

 and then between this fluid and the external medium (^.x:- 

 ternal respiration). There are no special organs for in- 

 ternal respiration. External respiration may take place 

 through the general integument of the body without the 

 aid of any specific organs, as is the case in all small animals 

 and in many larger ones, — for example flat worms, round 

 worms, rotifers, small annelids, and even some vertebrates, 

 such as the lungless salamanders. However, in most ani- 

 mals of any considerable size, special organs exist to 

 facilitate this exchange. 



Branchiae^ Gills, In aquatic animals vascular processes 

 are present which serve to bring the blood into close rela- 

 tion with the water. These processes, which are called 

 branchiae or gills^ are covered by a thin epithelium 

 through which an interchange of gases contained in the 

 blood and in the water can readily take place. To facili- 

 tate this interchange the gills are usually much folded 

 or branched so as to afford a large surface, and they are 

 frequently covered by cilia which serve to keep the water 

 in motion, while at the same time the blood is circulated 

 through them. The simplest type of gill is a ciliated ten- 

 tacle, which may also serve other functions, as in Mollus- 

 coida and some MoUusca (Fig. 13, A); such gills may 



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