xx] 



RESPIRATORY ORGANS 



503 



and in their respiratory organs. Thus the tail fin is a symmetrical 



fringe round the end of the body, and resembles the tail fin of the 



Teleostean larva. Such a primitive type 



of fin is called diphycercal. The dorsal 



fin is broken up into a series of finlets, 



each beginning with a stout spine (whence 



the name Polypterus, lit. many fins), 



and the pectoral fin has a median scaly 



lobe round which the dermal fin rays (lepi- 



dotrichia) form a fringe, whence the name 



Crossopterygii (lit. fringe-fmned, Gr. Kpov- 



a-oL, a fringe), which Huxley bestowed on 



the order. The air-bladder is deeply 



bilobed, the left lobe being much larger 



than the right one, and the two lobes 



unite into a median duct, which opens on 



the ventral side of the pharynx an 



arrangement which recalls that of the 



lungs of land animals and the Poly- 



pterus, like A mia and Lepidosteus, swallows 



air, which it uses for respiratory purposes, 



the used air escaping by the spiracle, 



which persists and has a little gill-cover 



of its own. The larva possesses a long 



feather-shaped external gill attached to 



the hyoid arch, resembling the gills borne 



by Amphibian larvae. Here again we see 



features which seem to be derived from 



the common ancestor of fish and land 



animals. 



The body is covered with rhomboidal 

 shining ganoid scales, beset with placoid 

 denticles, like those of Lepidosteus. In 

 the skull the jaws are articulated as in 

 Teleostei ; there is a hyomandibular bone, 

 but no symplectic, and in the lower jaw a 

 splenial bone is developed, and on the 

 under side of the throat a pair of gular 

 plates. The vomers are paired, and in 

 the dermal pectoral girdle clavicles are 

 present, as in Chondrostei. The pectoral is uniseriate with three 



FlG - 245 - 



