758 



COMPARATIVE ANATOMY 



disappear without leaving any trace whatever except a Eustachian tube 

 and the various ductless glands already mentioned. In the true verte- 

 brates fourteen pairs of these clefts is the largest number found although 

 there are more than this in Amphioxus and Balanoglossus. In the 

 cyclostomes there are usually seven (eight to seven in notidanid sharks, 

 five or six in teleostomes, and five in birds and mammals). In this num- 

 bering the oral cleft is not included though there is some evidence that 

 the mouth arose by the coalescence of a pair of gill clefts. 



The gill clefts do not form a serial repetition in the same manner 

 as does segmentation in other parts of the body, and it may even be .that 

 the metamerism of the head is not of the same character as the meta- 

 merism of the gill clefts. In the amniotes where gills are never devel- 

 oped the branchial pouches or clefts, however, appear and bear prac- 

 tically the same relation to the aortic and branchial arches as in the lower 

 forms. From this, it is often assumed that all of these higher forms 

 having this relation have had ancestors with gills. 



There is an interbranchial septum covered externally with ectoderm 

 and internally with entoderm between every two successive gill clefts. 

 The inner portion of this septum is composed of mesoderm which in its 

 earlier stages contains a diverticulum of the coelom. Later, blood-ves- 

 sels (aortic arches) and skeletal elements (visceral arches) 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. 



In Cyclostomes and fishes the gills are either filamentous or lamellar 

 outgrowths of epithelium, which have developed on both anterior and 

 posterior walls of the interbranchial septa. Each gill contains a loop 

 of blood-vessel. There are two very thin layers between the blood and 

 the surrounding water, which thus permit an exchange of gases. 



The filaments (sometimes called 

 gill-plates), (Fig. 440), which bound 

 each gill anteriorly and posteriorly 

 on one side, form a demibranch, and 

 it is the two demibranchs of a sep- 

 tum which then constitute a gill. 

 This means that each cleft is 

 bounded by demibranchs belonging 

 to two gills. 



Some forms have external gill- 

 filaments in the very young which 

 are later absorbed. 



In sharks that have more than 

 B five gill clefts, as well as in the 



Cyclostomes, the first cleft bears 

 gills, but in many elasmobranchs, as 

 veil as in the ganoids (sturgeon and 



A. 



Diagram of a gill, a, gill-arteries ; br, 

 branchial ray ; d, demibranchs ; kb, cross sec- 

 tion of bone of branchial arch ; s, septum ; v, 

 veins. (B, after Cuvier.) 



