502 



CYCLOSTOMATA. 



supports the lips of the mouth. There is a complex basket- 

 work around the gill-pouches, but it is not likely that its 



III. 



FIG. 240. The Lamprey (Petromyzon marinus}. 



I. The entire animal ; 'note the seven gill-slits of which the first is 

 marked g.s., the nostril n. , and the unpaired median fins. 

 II. Ventral aspect of the head, tt.t., upper teeth ; /./., lower 

 teeth ; /., the piston in the mouth. III. Upper surface of the 

 head ; ;/., the nostril with the pineal groove behind it ; c., the eye. 



elements correspond to visceral arches. Endoskeletal 

 cartilaginous rods, not comparable to the dermal fin-rays 



