RESPIRATORY ORGANS. 237 



the adult amniotes the pouches may completely disappear without 

 leaving a trace, aside from the Eustachian tube (p. 187) and the thymus 

 glands to be mentioned below. The number of clefts or pouches 

 varies between considerable limits. The largest number in any true 

 vertebrate (there are more in Amphioxus and the enteropneusts) is 

 fourteen pairs in some specimens of Bdellostoma. In other cyclo- 

 stomes there are seven, eight to seven in the notidanid sharks, six in 

 other elasmobranchs, five or six in teleostomes, amphibia and reptiles 

 and five in mammals. In this numbering the oral cleft is not included, 

 although there is some evidence that the mouth has arisen by the 

 coalesence of a pair of gill clefts (p. 206). 



The serial repetition of the visceral clefts does not strictly correspond to the 

 other segmentation of the body, their number and position being at variance with 

 those of the myotomes. There is a branchiomerism or serial repetition of the 

 gill clefts, apparently distinct from the true metamerism of the head. The ap- 

 pearance of these clefts or pouches and the relation of aortic and branchial arches 

 in the amniotes, where gills are never developed, can best be explained by the 

 assumption that these forms have descended from branchiate ancestors. 



Between each two successive gill clefts there is an interbranchial 

 septum, covered externally with ectoderm, internally with entoderm, 

 and with an axis of mesoderm, the latter in the earlier stages carrying 

 with it a diverticulum of the ccelom (fig. 243, c}. Later blood-vessels 

 (aortic arches) and skeletal elements (visceral arches, p. 63), are devel- 

 oped in each septum, the visceral arches appearing on the splanchnic 

 side of the ccelom and hence not being comparable to ribs or girdles. 



In the cyclostomes and fishes the gills are developed from the an- 

 terior and posterior walls of the typical interbranchial septa. They 

 were long regarded as of entodermal origin, but in recent years con- 

 siderable doubt has been thrown on this, at least for the fishes, and 

 there is some evidence for their ectodermal origin. The matter cannot 

 yet be regarded as settled. These gills are either filamentous or la- 

 mellate outgrowths of epithelium, each carrying a loop of a blood- 

 vessel. Thus each typical cleft is bounded in front and behind by gill 

 plates or filaments (fig. 246), those on a side constituting a demibranch, 

 the two demibranchs of a septum constituting a gill, while a cleft is 

 bounded by demibranchs belonging to two gills. In the young 

 elasmobranchs and in the young of a few teleosts (before birth) the 

 gill filaments protrude from the clefts as long threads, but later they 

 are withdrawn. 



