6o4 The American Naturalist. [July, 
little gills stand out on each side of the neck, and on these may 
be seen a few rudiments of lateral filaments. The fore-legs 
exist as the merest little buds. The head is rounded in front, 
and the mouth is below, features due to the yet persisting cranial 
flexure. It is doubtful if the mouth is yet perforated. The 
heart may be seen beating at a lively rate, and the blood cours- 
ing through the gills. During the earliest period of its free life, 
currents of water are directed over the gills and the body by the 
action of the cilia ; but soon currents may be seen to enter by 
the nostrils and to make their exit through the gill slits. After 
this the ciliary action becomes feebler, and at length ceases. 
When the larvae have attained a length of about 1 2 mm. [one- 
half inch] the lateral filaments of the gills have become distinct, 
and may be seen arranged in two rows on the under side of the 
main stem. There are four to six filaments in each row. The 
mouth is now nearly terminal, and microscopic sections reveal 
the existence of premaxillar>^ vomerine, dentary, and splenial 
teeth. Nothing was found in the stomach of this sectioned 
specimen, but it may have been an unsuccessful hunter. Toward 
the last of April, the larvae have reached a length of from 1 5 to 
18 mm. The anterior limbs are conspicuous, and show each two 
short toes. The posterior limbs are present as elongated pro- 
cesses. The so called "balancers" have shrunken somewhat, 
and give evidences that they will soon be lost. The tadpoles are 
more inclined to lie at the bottom of the water when resting than 
to cling to the sides of the vessel. 
From the time of hatching up to this stage the " balancers " 
are conspicuous organs. They are attached just behind the 
mouth on each side, and resemble a base ball bat. They are 
said by Professor S. F. Clark^ to function as supports for the 
larvae when they fall to the bottom of the pond during the 
period while the fore-legs are still undeveloped. I doubt if they 
are of much use in tlys way. In the aquarium they spend much 
of their time sticking to the walls, and it is by means of these 
organs that they suspend themselves. They are by no means 
" suckers," and it is doubtful if they secrete a sticky fluid, as the 
1 Studies from Biolog. Lab. Johns Hopkins University, No. II., 1880. 
