THE LARVA AND METAMORPHOSIS 



139 



The larva and metamorphosis 



By stage 22 (Fig. 80) the embryo has attained a length of 10 mm. and 

 is now termed a larva. The tail is broad and transparent. The gills have 

 grown out and branched. The eyes are well developed, with the cornea 

 transparent and the lens visible. The nasal pits are deep and the openings 

 constricted. A ventral view shows the suckers, which serve to attach the 

 embryo to water plants. These suckers are really mucous glands, which 

 secrete a sticky substance. The mouth parts are beginning to take on form, 

 and horny teeth develop. 



In the last stage of this series (23, Fig. 81) the larva takes on the general 

 "tadpole" appearance. The tail is very long in comparison with the body and 

 is very active in swimming movements. The suckers are being resorbed, and 

 the larva begins to feed actively. The mouth and some horny larval teeth 

 have formed. A fold of skin, the operculum, begins to creep backward over 

 the external gills. Eventually this fold will completely cover the gills and 

 will form a chamber which has one opening on the surface of the larva, 

 the spiracle, and which communicates with the pharynx. Water taken in by 

 the mouth then passes over the internal gills and out through the spiracle. 

 The ventral epidermis becomes transparent at about this time, and the in- 

 ternal organs may be observed directly. 



EYE 



NASAL OPENING 



GILL 



MOUTH 



NASAL OPENING 



Fig. 80. Dorsal, lateral, ventral, and frontal views of a stage-22 Rana sylvatica 

 larva. (Courtesy Professor Arthur W. Pollister, Columbia University.) 



