xin PHYLUM CHORDATA 87 



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 incomplete 

 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 anterior set 

 of the next. In the higher Fishes, such as the Trout or Cod, the 

 interbranchial septa become reduced to narrow bars enclosing the 

 visceral arches (right side of Fig. 780), 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 holo- 

 branch (hi. br.) is the morphological equivalent of two half -gills 

 hemibranchs (hm. br.), or sets of branchial filaments belonging to 

 the adjacent sides of two consecutive 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 relation 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 endoderrnal gills (cf. also under Pisces). 



Lungs (Fig. 769, A, Ig) are found in all Craniata from the Dipnoi 

 upwards. They are developed as a hollow outpushing from the 

 ventral wall of the embryonic fore-gut or anterior part of the 

 enteric canal ; this passes backwards and upwards, usually 

 dividing into right and left divisions, and finally coming to lie 

 in the dorsal region of the ccelome. The inner surface of the single 

 or double lung thus formed is raised into a more or less complex 

 network of ridges so as to increase the surface of blood exposed 

 to the action of the air ; and, in the higher forms, the ridges, 

 increasing in number and complexity, and uniting with one another 

 across the lumen of the lung, convert it into a sponge-like structure. 

 The respiratory epithelium is, of course, endoderrnal. Since the 

 lungs are blind sacs, some contrivance is necessary for renewing 

 the air contained in them : this is done either by a process analogous 

 to swallowing, or by the contraction and relaxation of the muscles 

 of the trunk. 



In some Fishes there occurs, in the position occupied in air- 

 breathers by the lungs, a structure called the air-bladder, which 

 contains gas, and serves as an organ of flotation. Like the lungs, 

 it is developed as an outgrowth of the fore-gut, but, except in 

 four instances, from its dorsal instead of its ventral side. In 

 many cases the air-bladder loses its connection with the pharynx 

 and becomes a closed sac. 



VOL n G* 



