PHYLUM CHORDATA 



89 



forming the anterior and posterior walls of the pouches is raised 

 up into a number of horizontal ridges, the hranchial filaments 

 {hr. /.), -which are abundantly supplied with blood. A current of 

 water entering at the mouth passes into the pharynx, thence by 

 the internal gill-slits into the gill-pouches, and finally makes its 

 way out by the external gill-slits, bathing the branchial filaments 

 as it goes. The exchange of carbonic acid for oxygen takes place 

 in the blood-vessels of the branchial filaments, which are, therefore, 

 the actual organs of respiration. It will be noticed that the re- 

 spiratory epithelium is endodermal, being derived from that of the 

 pharynx, which, as we have seen, is a portion of the mesenteron. 



As already mentioned, the walls of the pharynx are supported 

 by the visceral arches, which surround it like a series of incom- 

 plete hoops, each half-arch or visceral bar being embedded 

 in the inner or pharyngeal side of an interbranchial septum. 

 Thus the visceral arches {v. b.) alternate with the gill-pouches, 

 each being related to the 

 posterior set of filaments 

 of one pouch and the an- 

 terior set of the next.- In 

 the higher Fishes, such 

 as the Trout or Cod, the 

 interbranchial septa be- 

 come reduced to narrow 

 bars enclosing the visceral 

 arches (right side of Fig. 

 771), with the result that 

 a double set of free 

 branchial filaments springs 

 from each visceral bar and 

 constitutes what is called 

 a single gill. Thus an 

 entire gill or holobranch 

 {hi. hr.) is the morphologi- 

 cal equivalent of two half-gills — hemibranchs {hm. hr.), or sets of 

 branchial filaments belonging to the adjacent sides of two con- 

 secutive gill-pouches. On the other hand, a gill-pouch is 

 equivalent to the posterior hemibranch of one gill and the 

 anterior hemibranch of its immediate successor. 



In some Amphibia water-breathing organs of a different kind 

 are found. These are the external gills : they are developed 

 as branched outgrowths of the body-wall in immediate rela- 

 tion with the gill-slits, and differ from the internal gills just 

 described in having an ectodermal epithelium. They are 

 probably, however, of the same essential character as the endo- 

 dermal gills. 



Iiungs (Fig. 760, A, Ig) are found in all Craniata from the Dipnoi 



iT-^ 



l.hr.s 



Fig. 771. — Diagrammatic liorizontal section of the 

 pharyngeal region of a Craniate : on the left are 

 shown three gill-pouches {g. p.) with fixed branchial 

 filaments (Jjr. f.) and separated by inter-branchial 

 septa (i. hr. s.) ; on the right one hemibranch (hm, 

 br.) and two holobranchs (Id. hr.) with free fila- 

 ments, covered by an operculum (op). Ectoderm 

 dotted, endoderm striated, mesoderm evenly 

 shaded, visceral bars (v. b.) black. 



