RESPIRATION IN WATER 31 



ventilation water-filled- lungs are useless for respiration, as 

 diffusion will be much too slow to provide any significant 

 amount of oxygen. 



Two soft-shelled river turtles, Amyda mutica and Aspidonotus 

 spirifer, both belonging to the family Trionychidce ; have been 

 observed by Simon and Susanna Gage (1886) to ventilate 

 their mouth and pharynx with water about 16 times per 

 minute when submerged and to absorb oxygen through villus- 

 like, richly vascularized processes covering the mucous mem- 

 brane of the pharynx. Special experiments will be required 

 to see whether any significant quantity of oxygen can be 

 obtained by this mechanism. 



On the whole it seems clear that respiration by ventilation 

 of lung-like organs with water is resorted to more or less 

 accidentally by a small number of animals only. 



Branchial respiration. The normal way for aquatic animals 

 to supplement the respiration through the general body surface 

 is by appendages which are in the zoological literature usually 

 called gills when no other major function can be assigned to 

 them. We find all possible transitions between gills which 

 provide only a small supplement to the exchange taking place 

 through the general surface, and highly specialized structures, 

 the sole function of which is the gas-exchange which they have 

 taken over almost completely. 



The more primitive gills are appendages on the outer sur- 

 face of the animal, the more complex and efficient are en- 

 closed in a gill cavity which has to be ventilated by a flow of 

 water. Only in some of these latter can the function be 

 studied by quantitative experimentation. 



Certain appendages are called gills, although a respiratory 

 function is doubtful or almost certainly absent. The "anal 

 gills" of Diptera larvae are normally salt-absorbing (Koch, 

 1938) and not respiratory, and it appears that the so-called 

 "ventral gills" of Chironomid larvae absorb oxygen at a slower 

 rate than the rest of the body surface (Harnisch, 1937). 



