10 
circumstance would seem to show, that the branchiae, 
as the animal advances in age and size, do not be- 
come of less consequence in the performance of the 
respiratory function. 
It is only when the body of the Siren is more at 
rest in the water, that he employs his branchiae to 
the greatest advantage, or at least with the greatest 
vigour, as organs of respiration. Then he moves 
them somewhat in the manner of the gills of a tish, 
but much less frequently: and it is at this time that 
the beautiful structure (for to the most insensible na- 
turalist the structure would appear beautiful) of these 
organs is most distinctly seen. — When he swims (as 
he often does) rapidly through the water, the bran- 
chiae, if I do not greatly mistake, are but of little use 
as organs of respiration. They now serve the pur- 
pose, as do his feet also, of remiges, or fins. 
Now and then, the animal opens its mouth above 
the surface of the water, at which time it-, doubtless, 
throws out a vitiated air. The number of these pul- 
monary respirations is very various, in a given time, 
at different periods. In warm weather, I have fre- 
quently observed them to occur about once in every 
five minutes: but in colder weather, they are, (some- 
times, at least,) not more frequent than in the Lacerta 
lacustris; or once in every fifteen minutes. 
But the Siren is capable of suspending all respira- 
tion, through the medium of his mouth, for a much 
greater length of time than I have mentioned. My 
own ebservations have shown me, that the animal 
sometimes continues under the water, without once 
coming to the surface, for the long term of sixty-five 
