XIII 



PHYLUM CHORDATA 



83 



i.br.s 



Fir,. 726. Diagrammatic horizontal section of the 

 pharyngeal region of a Craniate : on the left are 

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

 filaments (/>/./.) and separated by inter-branchial 

 septa 0'. be. s.) ; on the right one hemibranch (km. 

 In:) and two holobrauchs (hi. b,:) with free fila- 

 ments, covered by an operculum (op.). Ectoderm 

 dotted, endoderm striated, mesoderm evenly 

 shaded, visceral bars (c. l>.) black. 



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 em- 

 bedded in the inner or 

 pharyngeal side of an 

 inter - branchial 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 a Trout or Cod, 

 the inter-branchial septa 

 become reduced to narrow 

 bars enclosing the visceral 

 arches (right side of Fig. 

 726), 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 holobrancTi (hi. fo\) 

 is the morphological equivalent of two half-gills, hemibranchs, 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 type 

 are found. These are the external gills (Fig. 886, Ms) : 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, there- 

 fore, comparable with the gills of Chaatopods or Crustacea. 



Lungs (Fig. 715, A, Ig) are found in all Craniata, from the Dipnoi 

 upwards. They are developed as a hollow outpushing from the 

 ventral wall of the pharynx, which passes backwards and upwards, 

 usually dividing into right and left divisions, and finally coming 

 to lie in the dorsal region of the coelome. 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 



G 2 



