GILLS. 



249 



In the Holocephali, however, an opercular membrane is present, 

 and traces of a similar structure are seen in Chlamydoselache. 



FIG. 202. HORIZONTAL SECTION- THROUGH THE VENTRAL SIDE OF THE HEAD OF 

 A SELACHIAN. (Semidiagrammatic.) The floor of the mouth is exposed. 



KM, muscles of jaw ; Z, tongue ; Hy, hyoid arch cut through, behind which are 

 seen the five branchial arches, also cut through ; BM, mucous membrane of the 

 mouth ; Oe, oesophagus ; S, S, pectoral arch, cut through ; LH, body-cavity ; KD, 

 body-walls ; the arrows indicate the external apertures of the five branchial sacs. 



FIG. 203. HORIZONTAL SECTION THROUGH THE VENTRAL SIDE OF THE HEAD OF 



Siluris glanis. (Semidiagrammatic. ) 

 T, T, tentacles ; Zp, Zp 1 , teeth of the lower jaw ; BM, mucous membrane of the 



floor of the mouth ; Oe, oesophagus ; KM, muscles of jaw ; KD, gill-cover, behind 



which (at the arrow) the common branchial cavity opens. 



In Ganoids and Teleosteans there are no longer chambered gill- 

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



