224 AMPHIBIA 



a specimen of the common tree-frog, Hyla arborea, is thrown 

 against a pane of glass it sticks to the glass almost like a piece 

 of putty, adhering by these discs. In the majority of cases, as 

 in the Hylidae and in Ceratophyla among the Cystignathidae, the 

 terminal phalanges which bear the discs are claw-shaped and 

 bent upwards, their bases being enlarged. Between the terminal 

 and the penultimate phalanx is a cartilaginous disc which pro- 

 jects ventrally and helps in the formation of the pad, which 

 however is chiefly carried by the terminal phalanx. When 

 not in use, as for example when the frog is sitting on a rough 

 stone, the pad is round and turned upwards, and the phalanx is 

 seen as a slight ridge on its upper surface ; when the pad is ad- 

 hering it is flattened and the phalanx is sunk into it. The pad 

 contains unstriated muscular fibres the contraction of which 

 produces one or more longitudinal grooves on the lower side. 

 The adhesion of the discs is due not to sucker-like action, but 

 merely to the attraction of two surfaces in close contact assisted 

 by the viscosity of the secretion of the glands present in the 

 skin of the discs. It has been shown by Schuberg that a glass 

 disc of sixteen square millimetres in area merely pressed against 

 the moistened under surface of a glass plate supported a weight 

 of fourteen grammes while the weight of a frog having a similar 

 amount of surface in all its discs together weighed only four 

 grammes. On the other hand when there is an excess of moisture 

 adhesion does not occur and a tree-frog cannot attach itself to 

 thoroughly wet glass or leaves. In Rhacophorus among the 

 subfamily Raninae the terminal phalanges which bear the ad- 

 hesive discs instead of being claw-like are bifurcated, and in the 

 subfamily Dendrobatinae, which are also Ranidse modified for 

 arboreal habits in South America and Madagascar, the terminal 

 phalanges are T-shaped. Within the limits of South American 

 Cystignathidag we find similar dfferences in the shape of the 

 terminal phalanges which carry the adhesive discs ; in Centro- 

 lene they are bifurcated, in Hylodes, Plectromantis and others 

 T-shaped. Possibly these differences may be due to the differ- 

 ent positions in which the toes are habitually held in the 

 different forms ; it would seem that the discs in Hylidae are 

 developed at the base of the terminal phalanges, where the 

 bones are bifurcated on the under sides and where the bones 

 are T-shaped on the tips of the toes. 



