﻿EXTEENAL 
  CHAEACTERS. 
  

  

  15 
  

  

  In 
  our 
  single 
  representative 
  of 
  the 
  arboreal 
  type, 
  tlie 
  

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

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

   which 
  assist 
  the 
  animal 
  in 
  climbino: 
  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, 
  aud 
  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. 
  5), 
  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 
  Thjla 
  arhorea, 
  

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

  

  