GILLS 



277 



gill-cleft known as the spiracle (p. 75), is as a rule present more 

 anteriorly, between the mandibular and hyoid arches. In the Holo- 

 cephali, however, the spiracle becomes reduced, there are only three 

 holobranchs in addition to hemibranchs on the hyoid and fourth 



FIG. 222. Dissection of the head from the ventral side of A, an Elasmobranch 

 (Zygfuna malleus), and B, a Teleost (Gadus aeglefinus), to show the branchial 

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

 cut through horizontally. (From R. Hertwig's Zoology.) 



Pq, palatoquadrate, and a, its connection with the cranium anteriorly; uk, 

 lower jaw ; m, oral cavity ; prm, prernaxilla ; ma, maxilla ; pa, palatine ; 

 Jim, hyomandibular ; is, internal branchial apertures ; as, external branchial 

 apertures ; ops, opercular aperture ; h, branchial septum; ft/ 1 , anterior, and ft/ 2 , 

 posterior hemibranch of a gill-pouch ; op, operculum ; s, pectoral arch ; 2, 

 tongue ; phi, inferior pharyngeal bone ; o, oesophagus. 



branchial arch, and an opercular membrane is present, covering the 

 external branchial apertures and opening by a slit posteriorly ; 

 traces of a similar structure are seen in Chlamydoselache. 



In Ganoids and Teleosts there are no longer chambered gill- 

 sacs. The septa on which the gili-laminse are borne become 

 greatly reduced, so that the apices of the latter extend freely out- 

 wards ; the whole branchial region is, moreover, covered over by the 

 operculum and branchiostegal membrane (cornp. pp. 75 and 79), 



