452 VERTEBRATE LIFE AND ORGANIZATION 



Figure 23.5. Adaptations of frogs that protect the larvae from aquatic predators. 

 A, The imid craters of the BraziHan tree frog, Hyla faber; B, the brood pouch of the 

 marsupial frog, Gastrotheca, cut open to show the eggs; C, the modified embi7o of 

 Eleuthewdactylus. {A, After Barbour; B, after Noble; C, after Lynn.) 



The chief adaptation to arboreal life has been the evolution of 

 digital pads upon the tips of the toes (Fig. 23.4). The surface epithelium 

 of the pads is rough and grips the substratum by friction. The gripping 

 action is enhanced by the discharge of a sticky mucus from numerous 

 glands within the pads. 



A particularly fascinating aspect of anuran biology is the evolution 

 of methods by which development can proceed elsewhere than in the 

 open water. This has occurred primarily among tropical frogs and prob- 

 ably as a protection against varying aquatic conditions and numerous 

 acjuatic enemies such as predaceous insect larvae. A Brazilian tree frog 

 (Hyla faber) protects its young by laying its eggs in mud craters which 

 it has built in the water (Fig. 23.5 A). A more striking means of protec- 

 tion is seen in a small Chilean frog, Rhinoderma darwinii. The male of 

 this species stuffs the fertilized eggs into his vocal sacs where they remain 

 iMitil metamorphosis is complete. Both of these frogs have fairly typical 

 anuran larvae that hatch from the egg and develop in a shelterecl en- 

 vironment. 



In certain species the vulnerable larval stage is omitted, and the 

 embryo develops directly into a miniature adult. Anurans with direct 

 development include the marsupial frog, Gastrotheca, and Eleuthero- 

 dactyl us— both of the New World tropics (Fig. 23.5 B). The former 

 carries her eggs in a dorsal brood pouch; the latter lays eggs in protected 

 damp places such as beneath stones or in the axil of leaves. The jelly 

 layers about the egg of Eleutherodactylus help prevent desiccation (Fig. 

 23.5 C); sufficient yolk is stored within the egg for the nutritive require- 

 ments of the embryo; such larval features as horny teeth, gills and 

 opercular fold are vestigial or absent; the fins of the larval tail are ex- 



