VI 



IIVLIDAE 187 



absent, and their sacral diapophyses are only slightly dilated. 

 On the other hand, it has to be emphasised that the possession 

 of adhesive discs on the fingers and toes does not necessarily 

 constitute a member of the Hylidae. That requires the further 

 comhination of an arciferous sternum, with dilated sacral dia- 

 pophyses and teeth in the npp)er ja%u. Finger -discs are easily 

 developed, and still more easily lost. Those of the typical 

 Hylidae are constructed as follows. The terminal phalanx is 

 elongated, claw-shaped, swollen at its base. Between it and the 

 penultimate phalanx lies an interphalangeal cartilaginous disc 

 which projects ventrally below the end-phalanx, thus assisting 

 the formation of tlie ventral pad, and the turning upwards of the 

 whole disc-like phalanx like the claw of a cat. This peculiar 

 motion can be well observed in Tree-frogs which are at rest 

 upon a horizontal leaf, or, better still, upon a rough stone, when 

 the creatm-es take good care to adjust their discs into a safe 

 and easy position. The pad or disc itself is furnished with 

 unstriped, smooth muscular fibres, the contraction of which pro- 

 duces one or more longitudinal furrow^s on the under side. When 

 the disc is in action or adhering, being flattened to a smooth 

 surface, the end-phalanx sinks into the cushion ; when not in 

 action, the cushion swells and the phalanx appears as a slight 

 dorsal ridge. The disc is rich in lymph-spaces, and its surface 

 contains mucilaginous glands. 



Various suggestions have been made to explain the function 

 of these discs. Suction, adhesion, and glueing-on have been 

 resorted to. Suction, through production of a vacuum, is quite 

 imaginary and does not exist. The question has been thoroughly 

 studied by Schuberg.^ Adhesion is due to the molecular attrac- 

 tion of two closely appressed bodies. The less air remains 

 between them the stronger it is. Consequently it can be in- 

 creased by the interference of a thin layer of fluid, which as 

 everyday observation shows, possesses both adhesion and cohesion. 

 The more sticky the fluid, the more effective it is, as shown 

 experimentally by Schuberg, who moistened the under surface 

 of a glass plate, and pressed it against a little disc of glass from 

 which was suspended a weight. A disc of 1 6 square millimetres, 

 a]i])roximately equal to the aggregate surface of the 18 discs of 

 a European tree-frog of 4 grammes in weight, carried with watei- 

 ^ Arbeitni Instil. Wdrzhurg, x. 189.5, y. 57. 



