GILL CHARACTERS . iq 



by contrasting the highly modified gill apparatus of a bony 

 fish with the more archaic type of the shark. 



In the sharks (p. 73) the gill slits pierce separately 

 the throat wall, as in the lamprey, and thus retain their 

 primitive segmental arrangement (Fig. 11). Their number 

 is usually five on either side, but in an archaic form (Hep- 

 tanchus, p. Z%) may be increased to seven. Above and 

 in front of the line of gill slits occurs a small opening 

 leading into the gullet, the spiracle (SP). This, though 

 at present possessing but few gill lamellae, and therefore 

 of little respiratory value, was doubtless quite like its 

 neighbours before its gill-supporting tissue became of value 

 in suspending the lower jaw. It may now aid the mouth 

 opening in admitting water to the gills. At the left of 

 the figure (Fig. 11), the narrow slit-like openings of the 

 gill clefts are seen at GC: at the right, where the upper 

 portion of the head has been removed, the gill lamellae 

 are shown at GF; the tissue intervening between the 

 gill pouches is reduced to a thin tendinous septum, ST, 

 at whose inner rim is the cartilaginous gill arch or bar, 

 GB, supporting the branchial vessels, G V. 



In the gill region of a bony fish (Fig. 12) a number of 

 modified characters are now evident : the spiracle has 

 become obliterated; the number of gill bars reduced — 

 in one form but two on either side remaining. These 

 have become closely pressed together, and bent backward, 

 receding from the surface of the head : their gill lamellae 

 have become larger and more numerous, their intervening 

 septum, ST, reduced in size. The gills no longer open 

 separately at the surface, but into an outer branchial 

 chamber formed and protected by a large overlapping 

 scale, or opercle, OP. This shield-like organ is hinged 

 at its anterior margin and opens or shuts rhythmically as 



