EXTERNAL CHARACTEKS. 16 



In our single representative of the arboreal type, the 

 tree-frog — Hyla arborea — the extremities of the fingers 

 and toes are expanded into adhesive disks (Fig. 6, b), 

 which assist the animal in climbing on vertical smooth 

 surfaces. These disks do not act as suckers, as was 

 once believed, but adhere by rapid and intense pressure 

 of the distal phalanx and special muscles upon the 

 lower surface, which is at the same time provided 

 with numerous glands producing a viscous secretion. 

 The upward rotation of the terminal phalanx, the 

 swollen base of which is hinged upon an interarticular 

 cartilage situated under the extremity of the penulti- 

 mate phalanx, and can be raised or lowered like the 

 claw of a cat, is easily to be observed on a living 

 specimen. When the disk does not adhere (Fig. 6), its 

 upper surface shows a short ridge produced by the claw- 

 like terminal phalanx, and the lower surface is convex 

 with some longitudinal grooves. During adhesion, 

 on the other hand, the claw-like phalanx is no longer 

 visible on the upper surface, but is indicated by a 

 groove, whilst the lower surface is flat and expanded. 



Fig. 5. 



Section through the extremity of the third toe of Hyla arborea, 

 the bones being indicated by oblique bars. pK', ph^. Second 

 and third phalanges, ia. Interarticular cartilage, d. Ad- 

 hesive disk. st. Subarticular tubercle. 



The adhesion of tree-frogs to smooth vertical sur- 

 faces is, however, not effected entirely through the 

 above-mentioned organs, their function being supple- 

 mented by the suctorial action of the belly, such as 

 is developed, though to a lesser degree, in the young 

 of most Batrachians which, devoid of digital expan- 



