ADAPTATIONS 383 



idosteus) are the living representatives, show in their fossil forms 

 a complete transition to the Order Teleostei, which includes the 

 great majority of existing fishes, and are undoubtedly their im- 

 mediate predecessors and progenitors. Both Lepidosteus and 

 Amia are confined to fresh water, and in both of them the air- 

 bladder is single and opens into the gullet dorsally : at the 

 communication there is a special chamber called the vestibule 

 which resembles a larynx, the organ containing the opening to 

 the lungs in higher Vertebrates, and communicates with the 

 gullet by a narrow slit, the glottis. Moreover, in these two 

 fishes but especially in Lepidosteus, the inner surface of the 

 bladder is much increased by being produced into lateral recesses 

 called alveoli, and each recess is again subdivided or sacculated 

 much in the same way as in a true lung. In both of these fishes 

 it has been shown by the American naturalist Wilder that the 

 bladder is actually used for respiratory purposes. 



In the Chondrostei or Sturgeons, the respiratory character 

 of the bladder is not so apparent, the lining membrane is smooth, 

 and the bladder opens without a vestibule into the dorsal wall of 

 the pharynx. The fossil forms related to them are very old, 

 Palajoniscidae being represented in the Devonian, but most 

 abundant in the Carboniferous and Permian strata. Of the 

 living forms, the spoon-bill sturgeons are confined to large rivers, 

 but the common sturgeons, though they spawn in fresh water, 

 descend to the sea between the breeding seasons. 



Thus we come to the fringe-finned ganoids (Crossopterygii) 

 and lung-fishes (Dipnoi), which, as we trace them back to the 

 beginning of their pal aeon tological history, become difficult to 

 distinguish from each other, and lead to the earliest bony fishes 

 from which all the other groups have descended. The Cross- 

 opterygii as already mentioned are represented by only two 

 living genera, Calamoichthys and Polypterus, inhabiting the great 

 rivers of Africa, the Nile and the Niger. In Polypterus the air- 

 bladder is divided, but the two parts are not symmetrical, that 

 on the right side being long and tubular, while that on the left 

 is much shorter, only about one-fourth the length of the other. 

 The two sacs unite together anteriorly, and open by a single 

 aperture into the ventral side of the gullet, like a pair of lungs. 

 Mr. Budgett observed in specimens kept alive in captivity that 

 air was inspired and expired through the spiracles, which are 



