FINS 



157 



also essentially similar to one another, the latter being rather 

 the simpler of the two. From a segmented main ray or axis a 

 number of segmented secondary rays arise on either side in 

 Ceratodus: these are not, however, strictly symmetrical (Fig. 118). 

 Beyond them dermal rays are present (p. 137). A proximal 

 (basal) segment of the axis, which bears no rays, articulates with 

 the arch. In Protopterus and Lepidosiren the fins, with their 

 skeleton, have undergone a marked reduction, so that little more 

 than the segmented axis remains. 



Thus the fins of Dipnoans differ from those of most Elasmo- 

 branchs (as well as of Teleostomes) in being formed on a biserial 

 type, indications of which are, however, as already stated, 



FKJ. 119. RIGHT PELVIC FIN OF A Y/orxo Pol yodon folium. 

 From the dorsal side. 



F8, bony dermal rays ; M, metapterygium ; Pru, uncinate ("iliac") processes; 

 Ra, Ha}, radii of the first and second orders. 



seen in the embryos and adults of certain Elasmobranchs. 

 Physiologically, the Dipnoan fin, like that of the young Polypterus, 

 serves not merely as a swimming organ, but also to support the 

 body when the animal is resting on the bottom, as do the limbs of 

 a Urodele. 



Ganoids. The skeleton of the fin is much simpler and the 

 primary rays much fewer in number in Ganoids than in Elasmo- 

 branchs. This is, however, compensated for by the formation of 

 secondary dermal bony structures, as in the case of the pectoral 

 arch and skull : these arise on either side of the fin and may or 

 may not be segmented: they are always more strongly developed on 

 the anterior than on the posterior border of the fin. The most 



