GILLS 



355 



Fishes. In the true Fishes the gills come into close relation 

 with the visceral arches, and in Elasmobranchs they consist of 

 closely approximated transverse laminae, which are firmly attached 

 to both sides of the septa extending outwards from the arches and 

 separating the individual gill-sacs from one another, so that each 

 septum bears a half-gill, or Jiemibranch, on both its anterior and 

 posterior surface (Figs. 261, A, and 202). A gill, or holobranck, 



Fi<;. '261. Dissection of the head from the dorsal side of A, an Elasmobranch 

 (Zyywna maUtu*), and B, a Teleost (Gar.ln* (H/ttfinn*^, to show the branchial 

 apparatus. In both figures the branchial arches on the left side are shown 

 cut through horizontally. (From R. Hertwig's Zooloyy.} 



'/--. external branchial apertures ; 1>, branchial arch ; W 1 , W 2 , hemibranchs ; 

 h, branchial septum ; hm, hyomandibular ; /., internal branchial apertures 

 and gill-rakers ; m. oral cavity ; ma. maxilla ; o, oesophagus ; op, operculum ; 

 op#, opercular aperture ; pa. palatine : phi, inferior pharyngeal bone ; Pq, 

 palatoquadrate. and a. its connection with the cranium' anteriorly ; p,-)n, 

 premaxilla ; s, pectoral arch ; uk. lower jaw ; -, tongue. 



thus consists of the branchial septum and arch plus the posterior 

 hemibranch of the sac in front of it and the anterior hemibranch of 

 the following sac. The gill-sacs, of which there are commonly five 

 in Plagiostomes, 1 open separately to the exterior, and a vestigial 



1 There are six in Hexanchus and 

 Heptanchus in addition to the spiracle. 



Chlamvdoselaehus and seven in 



A A 1 



