in the larva or tadpole of one of the South Indian frogs (fig. 1), which, 

 when hatched, has a long compressed tail, ' and breathes by means of 



Fig. 1. 



external gills. As development advances, the lung sacs take on func- 

 tional activity, and the posterior extremities make their appearance, 

 being followed later on by the anterior extremities, which have mean- 

 while been hidden and developing beneath the skin. The tadpole, 

 which is now a four-legged air-breathing animal, has only to lose its 

 swimming tail to assume its perfect form and become adapted for a 

 terrestrial as well as an aquatic life. 



" The alimentary canal of the tadpole is at first long and coiled up 

 in a close spire, like a watch-spring, in the abdomen, but its length 

 becomes relatively less as age advances. At the same time, the diet 

 changes from vegetable to animal, the young tadpole being chiefly 

 herbivorous, the adult insectivorous." (Huxley.) 



The males of many species of Batrachian, e.g., the South Indian 

 bull frog {Rana tigrina) and Cacopus sgstoma (pi. ix) have a single 

 sac or a pair of sacs on the throat, called the sub-gular vocal sacs, which 

 open into the mouth cavity, and act as resonants to the waves of sound 

 set up by the air which is passing from the lungs, and help to increase 

 the intensity of the familiar Batrachian chorus by intensifying the 

 sound produced by the vocal cords. 



In his observations on sexual differences between the males and 

 females of Batrachian species, Darwin says ^ : — " With respect to sexual 

 differences of colour. Dr. Griinther does not know of any striking instance 

 either with frogs or toads, yet he can often distinguish the male from 

 the female by the tints of the former being a little more intense. Nor 

 does he know of any striking difference in external structure between 

 the sexes, excepting the prominences which become developed during 

 the breeding season on the front legs of the male, by which he is 



1 The Descent of Man, 2nd ed., 1875, p. 349. 



