2i6 RESPIRATION 



hypothesis is supported by the existence in the arterial capillaries 

 of a very peculiar thickened endothelium figured clearly by Wood- 

 land (Figure 64). 



Another very interesting case of gas secretion occurs in Arcella 

 discoides, which is a microscopic unicellular organism found in 

 rivers and ponds. It has a more or less transparent shell, shaped 

 something like the top of a mushroom, with an opening where the 

 stalk should come. Through this opening it protrudes delicate 

 pseudopodia, by means of which it can creep about (Figure 65). 



Figure 65. 

 Arcella raising itself by developing bubbles. Two bubbles 

 visible through shell, and pseudopodia projecting through 

 lower opening. 



When a living and healthy arcella is examined in a drop of water 

 under the microscope, the presence of one or more gas bubbles 

 inside its protoplasm can at times be observed, particularly if by 

 accident or design the animal has been turned on its back, with 

 the opening of its shell upwards. The bubbles of course make the 

 animal lighter, so that it rises towards the surface of the water, 

 and also comes right-side up, after which they rapidly disappear 

 again. The occurrence of these phenomena was described many 

 years ago by Engelmann. Quite recently Dr. Bles took up the 

 subject again at my suggestion, as it looked as if oxygen want 

 was in some indirect way the real stimulus to the formation of the 

 bubbles, just as it is (as we shall presently see) the stimulus to 

 oxygen secretion in the lungs. He elicited the very interesting 

 fact that a quite slight fall in the normal oxygen pressure of the 

 surrounding water is sufficient to cause the immediate formation 

 of gas bubbles in the arcella, and thus cause it to rise to where 

 presumably there is more oxygen. It seems probable, also, from 

 other observations made by him later, that the bubbles which are 

 apt to develop when the animal is placed on its back are a conse- 

 quence of stimuli produced by internal want of oxygen owing to 



