CHAPTER XVIII 

 THE GAS-SECRETING TISSUES OF ANIMALS 



AMONG the many substances that cells can produce by the process 

 of secretion is free gas. Of course we have seen in the preceding part that 

 very many tissues can allow gas to be transmitted through their substance 

 under physical and chemical impulse (pressure and chemical affinity). 

 But the cells to be discussed in this part are able to take the gas materials 

 from the blood and to secrete and discharge them into a chamber that is 

 under a pressure greater than that of the surrounding medium in which 

 the animal is placed. As several gases are so handled as a mixture, 

 there are many unknown chemical changes involved. The gas appears 

 as tiny solid granules in the cytoplasm of the cell and swells into a fluid 

 droplet, and then into the gaseous state, when it is discharged from the 

 cell. 



We shall examine two of the very few forms of animals in which this 

 occurs, a teleost fish whose swim-bladder is filled with a mixture of gases 

 and a siphonophore medusa, one of whose zooids is developed into a 

 hollow float that is also filled with about the same mixture of gases 

 that we found in the swim-bladder of the fish. This mixture is CO 2 in 

 2-6 /&; Oxygen, 12-18 ^>; Nitrogen 79-80^. 



Gas secretion in the siphonophore medusa, Physalia, and others. 

 The float member, of the collection of individuals that a Physalia repre- 

 sents, is developed by an invagination, into a large hollow, double- 

 walled sac or bladder with a pore at one point that controls the exit, 

 and the consequent pressure of the gases by a sphincter muscle. When 

 gas is lost, by letting it out or when the supply is decreased by the growth 

 and enlargement of the float or by the loss of gas by osmosis through 

 the walls, a new supply is provided by a portion of the epithelium that 

 is situated on the inner membrane near the base of the float (Fig. 297). 



This epithelium, which lines the membrane and faces the hollow of 

 the float, consists of a single layer of long cells with a swollen distal por- 

 tion that narrows down into a rod-like base near the basement membrane, 

 where it branches into several root-like processes that are implanted in 

 the jelly tissue of the mesoglcea. The nucleus is of large size and placed 



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