256 TRANSACTIONS OF THE [JUNE 4, 
states that they are absent in the Aglossa,* but I think they may 
possibly be represented by the long tentacles of Dactylethra. 
In the larve which I had under observation, the balancers 
made their appearance at the same time as the external gills, as 
lateral buds on the head, similar to those representing the gills, 
and some distance in front of them. 
From the first, the development of the ‘‘ balancers” was 
strikingly like that of the gills, and at the time of hatching, the 
principal difference in appearance was due to the tips of the 
gills being slightly bifurcated ; this being the beginning of the 
lateral branching. 
The resemblance alluded to was so striking that I placed 
the larve under the microscope, to see if any trace of circulation 
could be found in the *‘ balancers.”’ 
Save that it is somewhat intermittent, the circulation is as 
perfect in the ‘‘ balancers” as in the external gills, the blood- 
corpuscles passing outward by an artery on the posterior aspect 
of the ‘ balancer” and returning by one on the anterior aspect. 
When re-entering the body, the corpuscles appear to flow forward 
and downward in front of the mandibular arch. On the sixth 
day after hatching, the circulation was still observable, though 
very slow and irregular. On the eighth day I could no longer 
detect it, and on the tenth the ‘‘ balancers” had disappeared. 
That the ‘‘balancers” of Amblystoma are true external 
branchie can scarcely be considered doubtful, and the suctorial 
processes of other Urodela, together with the sucking discs of 
the Anura, must be placed in the same category, and regarded 
as representing former respiratory organs, which have become 
functionally so modified as to leave but little trace of their 
original nature. 
An interesting question arises, as to whether the modification 
of the external gills in the amphibia can be regarded as afford- 
ing any explanation of the preoral suctorial discs of the larve 
of Lepidosteus and Acipenser. 
Concerning these, Balfour makes the following remarks.* In 
the Sturgeon (Acipenser): ‘‘In front of the mouth two pairs 
of papille grow out, which appear to be of the same nature as 
the papille on the suctorial disc of Lepidosteus. They are very 
short . . . but soon grow in length, and it is probable that they 
become the barbels, since they occupy a precisely similar posi- 
tion.” 
In a note on the same page he says: ‘‘If these identifications 
1 See Balfour, loc. cit., pp. 110 and 115. 
2 Balfour, ‘‘Comp. Emb.,” p. 89. 
