THE INNER TISSUES OF ANIMALS. 325 



munlcation between them, and the alimentary canal j and 

 there might thus eventually result closed chambers the 

 gaseous contents of which, instead of being obtained from 

 without, were secreted from their walls, as gases often are 

 from mucous membranes. Contrariwise, aquatic 



vertebrata in which the swallowing of air-bubbles, becoming 

 habitual, had led to the formation of sacs that lodged the 

 bubbles ; and which continued to inhabit waters not always 

 supplying them with sufficient oxygen ; might be expected 

 to have the sacs further developed, and the practice of chang- 

 ing the contained air made regular, if either of two advan- 

 tages resulted — either the advantage of being able to live in 

 old habitats that had become untenable without this modifi- 

 cation, or the advantage of being able to occupy new habitats. 

 Now it is just where these advantages are gained that we see 

 the pulmonic respiration coming in aid of the branchial 

 respiration, and in various degrees replacing it. Shallow 

 waters are liable to three changes which conspire to make 

 this supplementary respiration beneficial. The summer's sun 

 heats them, and raising the temperatures of the animals they 

 contain, accelerates the circulation in these animals, exalts 

 their functional activities, increases the production of car- 

 bonic acid, and thus makes aeration of the blood more needful 

 than usual. Meanwhile the heated water, instead of yielding 

 to the highly carbonized blood brought to the branchiae the 

 usual quantity of oxygen, yields less than usual ; for as the heat 

 of the water increases, the quantity of air it contains diminishes. 

 And this greater demand for oxygen joined with smaller 

 suppty, pushed to an extreme where the water is nearly all 

 evaporated, is at last still more intensely felt in consequence 

 of the excess of carbonic acid discharged by the numerous 

 creatures congregated in the muddy puddles that remain. 

 Here, then, it is, that the habit of taking in air-bubbles is 

 likely to become established, and the organs for utilizing them 

 developed ; and here it is, according!}', that we find all stages 

 of the transition to aerial respiration. The Loach before- 



