RESPIRATION. 



141 



tial to life, than in the lower orders. A fish soon ex- 

 pires if taken out of the water, but an oyster will live 

 for weeks, with only that which it retains in the shell. 



420. Respiration in Fishes. One side of the gilte of 

 a fish is distinctly represented by Fig. 89, which also 

 shows the heart and artery by which pig. 65. 



the blood is sent to these parts. Of the 

 heart, d, in the auricle ; e, the ventri- 

 cle ; 6, the enlargement of the artery, 

 called the bulbous arteriosus, which is 

 shown distinctly in Fig. 85, f, the 

 branchial artery ; g, g, the gills. We 

 here have an opportunity of observing 

 how near the heart of the fish is to 

 the lungs, and consequently of infer- 

 ring the importance of well supplying 

 these parts with blood. This arises 

 from the circumstance that the heart 

 throws the blood only to the gills, and 

 not to the other parts of the body as 

 in the other animals, hence a large 

 proportion of the blood of the whole 

 system is constantly in the gills, to be well purified by the 

 air before it circulates through the other parts, and this 

 is the reason why these parts are highly colored with 

 blood, while the other parts of the fish are white. 



421. The gills consist of filaments arranged somewhat 

 like the feathers of a quill. When these filaments are 

 closely examined, they are found to be covered with mi- 

 nute processes crowded close together, and on which may 

 be observed millions of capillary" blood-vessels, spread like 

 a net-work, over the whole surface. It is through the thin 

 coats of these vessels that the air acts upon the blood they 

 contain. 



422. In the osseous, or bony fishes, there is a large 

 flap, called the operculum, which covers the gills from 

 injury, and below which there is an opening for the escape 

 of the water, after it has performed its office. The pro- 



How is this function carried on in the fish? 



