76 HIGH SCHOOL ZOOLOGY. 



28. Reference was made above to the fact that of all Ganoids 

 Polyodon is most nearly allied to the Sharks. This is not 

 merely a superficial resemblance depending on the position of 

 the mouth but it is seen also in other organs. The gill-archos, 

 for example, bear between the two rows of filaments, a mem- 

 branous partition which is hardly present in any of the other 

 forms but which in the Sharks is much more developed, and 

 bears the gill filaments. Thus in the Sharks the gill-arches are 

 not separated by mere gill-slits, but by pouches, the anterior 

 and posterior walls of which are formed of the aforesaid parti- 

 tions. The pouches are, at least outwardly, always five in 

 number (sometimes seven), and they open by a series of slits 

 uncovered by any operculum (except in the genus Chimaera). 

 This disposition of the respiratory organs has conferred on the 

 Sub-Class the name Elasmobranchii. 



It embi'aces marine forms familiarly known as Sharks and 

 Rays ; the former have elongated bodies with the gill-slits on 

 the sides of the head, the latter are flattened from above down- 

 wards, and as broad as they are long, from the enormous develop- 

 ment of the pectoral fins, which form the greater part of the 

 body. Their gill-slits are to be found on the lower surface of 

 the head. Certain forms are intermediate, in respect to the 

 size of the pectoral fins, between the Sharks and the Rays ; 



Fig. 52.— The Saw Fish. Pristis pectinatus. '^. 

 (U. S. F. C.) 



the Sawfishes e.g., (Pristis) (Fig. 52), which are further dis- 

 tinguished by the enormous development of the rostrum oi- 

 snout and the formidable lateral teeth of that organ. Again 



