GENERAL ANIMAL FUNCTIONS. 8 1 



The skin is the sole respiratory organ in some animals, 

 as the earth worm; and it is always of considerable im- 

 portance as an aid to respiration in most animals, par- 

 ticularly those that live in water. 



There are two special types of respiratory organs: 

 (i) those that extend outward from some part of the body 

 wall usually found in animals living in water. They 



ex. 



FIG. 10. FIG. ii. 



FIG. 10. Diagram illustrating gills, or branchiae (br.). b.c., body cavity in which 

 the body fluids circulate; ex., external medium in which the oxygen is dissolved; w, 

 body wall. 



FIG. ii. Diagram illustratig lungs or trachea (/); other letters as in Fig. 10. 



Questions on Figures 10 and n. What is the medium in which 

 the oxygen is found in each case? Show in what respects each 

 organ (gill or lung) is better suited to its medium than to the other. 

 How is the organ in each case related to the body wall? What is 

 gained over the plan of taking the oxygen through the general 

 body wall? 



go under the general name of gills (sometimes branchice) . 

 The blood goes out into these pouches and is thus brought 

 close to the water (Fig. 10, br}. (2) Ingrowths from 

 the body wall (or some other wall) into which the O 

 passes. These are hidden in the interior of the body. 

 Such are called lungs, tracheae, etc. (Fig. n, /.) The 

 blood in this case is on the outside of the organ proper. 



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