GILLS OR BRANCHI^ 253 



These visceral pouches develop in all vertebrates, but in the 

 mammals onlyTlew or even none of them break through to the ex- 

 terior. In the adult amniotes the pouches may disappear without 

 leaving a trace, aside from the Eustachian tube (p. 202) and the 

 various ductless glands mentioned below (p. 264). The largest 

 number of clefts in any true vertebrate (there are more in Amphioxus 

 and Balanoglossus) is fourteen pairs in some specimens of Bdellos- 

 toma. Other cyclostomes have seven (Dohrn thinks there are traces 

 of more in Petromyzon) , eight to seven in notidanid sharks, five or six 

 in teleostomes, amphibia and reptiles, and five in birds and mam- 

 mals. In this numbering the oral cleft is not included, though there 

 is some evidence that the mouth arose by the coalescence of a pair of 

 gill clefts (p. 221). 



There is some difference between the serial repetition of the gill clefts 

 (branchiomerism) and the other segmentation of the body, and possibly the 

 metamerism of the head is not of the same character as the branchiomerism. 



The appearance of branchial pouches or clefts and the relation of the aortic 

 and branchial arches in the amniotes, where gills are never developed, can best 

 be explained by the assumption that these animals have descended from 

 branchiate ancestors. 



Between each two successive gill clefts is an interbranchial 

 septum, covered externally with ectoderm, internally with entoderm, 

 and with an axis of mesoderm, the latter, in the early stages containing 

 a diverticulum of the coelom. Later, blood-vessels (aortic arches) and 

 skeletal elements (visceral arches, p. 69), are developed in each 

 septum, the visceral arches forming on the splanchnic side of the 

 coelom and hence are not comparable to girdles or ribs (p. no). 



In cyclostomes and fishes the gills proper are developed on the 

 anterior and posterior walls of the interbranchial septa. The gills 

 are either filamentous or lamellar outgrowths of epithelium, each con- 

 taining a loop of a blood-vessel, so that between the blood and the 

 water there are but two extremely thin layers, a matter of importance 

 in the exchange of gases. Each cleft is thus bounded in front and 

 behind by gill plates or filaments (fig. 274) those on a side forming a 

 de mibranc h, the two demibranchs of a septum constituting a gill, 

 while each cleft is bounded by demibranchs belonging to two gills. 

 In the young elasmobranch and in the young of a few teleosts before 

 hatching, the gill filaments protrude from the clefts as long filaments 

 (external gills), but they are absorbed later. These are partly 



