248 



CHORDATE ANATOMY 



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

 the gill-shts. 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. 



Fig. 229. — Diagram of the reUitiuns of 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.) 



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 gill system, developed 



DIENCEPHALON 

 LENS 



ESOPHAGUS 



ECTODERM 



SPINAL CORD 



PRONEPHROS 



NOTOCHORD 

 MYOTOMES 

 DORSAL AORTA 



PETROMYZON. 16-DAY EMBRYO -FRONTAL SECTION. 



Fig. 230.- — 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. By the rupture of the 

 double (ectoderm-endoderm) membrane each gill pouch is converted into a gill cleft. 

 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. 



in adaptation to special conditions. They have no genetic relation to 

 any human structure. 



Development of Gills. Gill-slits develop from a series of paired endo- 

 dermic diverticula of the pharynx which meet corresponding invaginations 

 of the ectoderm. (Fig. 230) By the disappearance of the double mem- 



