Introduction 



be a perennibranchiate Urodele (Hylaeobatrachus). Probably Ba- 

 trachia were abundant throughout the Reptilian Age (Mesozoic), 

 but were represented by delicate, marsh-living animals not suited 

 to fossilization. The modern types of the Salientia appear in 

 the Tertiary with about as many genera and species as now. 



Not only do the Salientia show their development from the 

 Fishes through fossils, but their structure still tells the story of 

 their descent. In addition to this, some points of the proof are 

 shown in their development through a larval form adapted to 

 water life. 



The adult structure which constitutes the main likeness is 

 the three-chambered heart with its valved conus arteriosus and 

 the symmetrical arrangement of arterial arches. Two of the 

 telling points in the development are the presence of lateral line 

 sense organs previous to the metamorphosis (p. 6), and the 

 existence of four or five gill arches in the larva. It is thought 

 that the gills of the tadpole are not phylogenetically related to 

 the gills of true Fishes, but that they are so related to the ex- 

 ternal gills possessed by some of the Dipnoi and Crossoptergii, 

 or that they have developed in response to the needs of the tad- 

 pole in its adaptation to a water environment. This is true 

 even of the so-called internal gills, which replace the first external 

 gills and are covered by the opercular membrane. They have 

 been called internal because they were under this opercular mem- 

 brane, not because they were homologous with the internal gills 

 of Fishes. 



The lowest of the Salientia and the most nearly related to 

 the primitive Batrachians are the Discoglossidae (outside of the 

 Aglossa, which are not represented in North America). These 

 have not only the primitive characteristics of the skeleton (ver- 

 tebrae opisthocoelous, distinct short ribs, etc.), but they have other 

 structures also in primitive condition. Such is a tongue adherent 

 to the floor of the mouth, so that they must capture food with 

 the jaws, as do the Urodela. The Pelobatidae are perhaps inter- 

 mediate in position between the Discoglossidae and the Bufonidae 

 and lead also to the Cystignathidae. These last, in their turn, 

 point towards the Hylidae. The Ranidae are thought to be the 

 highest among the Salientia. 



Thus, historically, Batrachians are more interesting than any 

 other vertebrate Class. They bridge the greatest gap in verte- 



14 



