14 



after hatching the mouth appears, with horny jaws which have 

 a sharp biting beak. The upper jaw has a free edge, upon 

 which are minute, horny teeth ; the edge of the lower jaw has 

 small, fleshy papillae. Between these edges and the beak the 

 upper jaw has three incomplete rows of teeth and the lower 

 jaw four complete rows. At the time of hatching the aliment- 

 ary canal is perfectly straight, but it soon becomes a long, 

 spirally-coiled intestine. At this stage the tadpole begins to 

 feed. The mouth opens, internal gills soon appear upon fleshy 

 processes of the branchial arches * and the external gills soon 

 shrivel and disappear. A hood (operculum) grows backward 

 from the arches and soon incloses the gills and fore limbs, 

 which have appeared about this time. 2 The operculum fuses 

 at the right side and along the lower surface, but an opening 

 remains on the left side through which the external gills often 

 protrude for a time before' shrinking. The tadpole then be- 

 gins to breathe like a fish, taking water into the mouth and 

 passing it through the gills, and finally out of the opercular 

 spout (left-hand aperture). The lungs, which had existed at 

 the time of hatching as diverticula of the oesophagus, now 

 extend along the sides of the bod3 T -cavity and begin to be 

 used, respiration for a time being by both lungs and gills. The 

 tadpole comes to the surface for air. The final metamorphosis 

 takes place while the tail is still very long, and is of great in- 

 terest on account of the changes. The outer layer of skin is 

 cast, the horny jaws are thrown off, the mouth widens and the 

 tongue becomes large, the eyes more prominent, the right fore- 

 leg forces its way through the operculum and the left one 

 appears out of the spout described above. The long intestine 

 takes on a condition of active inflammation and rapidly shrinks 

 to a straight alimentary canal. During this period no food is 

 taken and nutrition is carried on at the expense of the tail, 



1 In fishes the gills are upon the arches themselves. There are no 

 internal gills in salamanders. 



2 In development the anterior limbs appear first, but are concealed by 

 the operculum. In the salamanders the development is the same, but 

 there is no concealing hood. By this distinguishing character the novice 

 may ascertain whether he has a frog or salamander tadpole. 



