TREE SNAKES 49 



Few free-living animals except terrestrial 

 snakes, are so exquisitely stereotropic as are 

 leeches ; and snakes offer a further analogy 

 with Annelids as a whole in the combination 

 of stereotropism and pleotropism in some fresh- 

 water species, stereotropism alone in burrowing 

 and in arboreal species, pleotropism alone in 

 many marine species. In some tree snakes, e.g., 

 Dendrophis, Dendrelaphis, and Chrysopelea, there 

 is a lateral keel or suture on each side of the 

 ventral shields. It has been suggested by Mr 

 R. Shelford 1 that this carination or hinging 01 

 the ventral shields, by allowing the body to 

 flatten itself out and become concave below, 

 enables the snake, when springing from a height, 

 to descend gently like a parachute. Against this 

 it has to be noted that burrowing snakes like 

 Cylindrophis, which have very small ventral 

 shields, are able to flatten the body in a 

 marvellous manner. I rather think that the 

 lateral hinge-lines are in the first instance an 

 accessory stereotropic contrivance enabling the 

 snakes to climb up rough tree-trunks with greater 

 facility; moreover, a lateral angulation of the 

 ventral shields is found in the terrestrial genera, 

 Lycodon and Hydrophobus. Nevertheless, the 

 possibility is not excluded that they might also 

 serve on occasion for the parachute flight ; but 



1 R. Shelford. A note on "Flying" Snakes, P. Zool. Soc.> 

 London, 1906, i., pp. 227-230. 



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