THE RESPIRATORY SYSTEM 



331 



The mechanism of breathing differs considerably in fishes which, Hke 

 the elasmobranchs, have modified the first gill shts into spiracles, and 

 those which have not. In all fishes, through the action of antagonistic 

 pharyngeal muscles, the cavity of the pharynx is alternately expanded 

 and contracted, so that water is sucked in through the mouth or the 



Fig. 275. — Diagram of the relations nt external and internal gills in the anuran 

 tadpole, ab, eb, afferent and efferent branchial arteries; h, heart; o, ear cavity; ph, 

 pharynx; ra, radix aortae. (From Kingsley's "Comparative Anatomy of Vertebrates," 

 after Maurer.) 



spiracles and forced out through the gill slits. In forms with an opercu- 

 lum, this functions as a valve, and prevents the entrance of water through 

 the gill slits. Gaseous exchange takes place through the thin mucous 

 epithelium which covers the gill lamellae. The gills of fishes function 

 also as excretory organs, excreting nitrogenous waste as do the kidneys. 



PIENCEPHALON 

 LENS 



COELOM 



ESOPHAGUS 



ECTODERM 



SPINAL CORC 



PRONEPHROS 



NOTOCHORD 

 MYOTOMES 

 DORSAL AORTA 



PETROMYZON. 16-DAY EMBRYO -FRONTAL SECTION. 



Fig. 276. — Frontal (horizontal) section of a 16-day petromyzon embryo, showing 

 seven pairs of gill-pouches (1-7) formed as lateral diverticula of the pharynx. Slight 

 invaginations of the ectoderm to meet the gill-pouches are seen. Between the successive 

 gill-pouches the mesoderm is divided into a series of branchiomeric segments, from 

 which the muscles and skeletal arches of the gills develop. 



External Gills. External gills are of two sorts, external gill filaments 

 such as occur in elasmobranch embryos as prolongations of the posterior 

 gill lamellae, and external gills which characterize some adult urodeles 

 and the larvae of some fishes and amphibians. The evidence on the 

 whole supports the opinion that they are secondary derivatives of the 



