THE INNER TISSUES OP ANIMALS. 337 



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 need- 

 ful 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 supply, 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, ac- 

 cordingly, that we find all stages of the transition to aerial 

 respiration. The Loach before-mentioned, which swallows 

 air, frequents small waters liable to be considerably warmed. 

 The Amphipnous Cuchia, an anomalous eel-shaped fish, which 

 has vascular air-sacs opening out at the back of the mouth, 

 " is generally found lurking in holes and crevices, on the 

 muddy banks of marshes or slow-moving rivers " ; and 



