DEVELOPMENT OF THE SACCUS ENDOLYMPHATICUS 263 
much further than in Neoceratodus. We also find in Protop- 
terus two pulmonary sacs, whereas Neoceratodus possesses 
only one. Parker writes of Protopterus: 
‘Although the lungs are usually said to be paired in their 
origin, the outgrowth which forms them, though bifurcating 
close to its origin, is in fact so far as I can gather unpaired at — 
the first.” 
This fact might also support the view that Neoceratodus is 
the more primitive animal. The uniserial fin of Protopterus 
has been regarded as a form developed from the biserial fin of 
Neoceratodus. Thus it seems as if the relationship between 
Neoceratodus and Protopterus indicated by the structure of the 
recessus labyrinthi is in reality a true one. 
As may be expected from their systematic relationship, all 
the higher vertebrates, reptiles, birds, and mammals, have a 
similarly constructed saccus endolymphaticus. The exceptional 
position of the Ascalabotae appears therefore very strange. In 
these animals, as we have seen, the saccus leaves the cranial 
cavity and runs between the muscles of the neck into the shoul- 
der region, where it ends as a large closed vesicle. As there can 
be no thought of a close relationship between Ascalabotae and 
Anura, this similarity (at least as far as size is concerned) of the 
extension of the saccus is probably due to convergence. There 
is no question here of a convergence caused by a similar habitus, 
as the Ascalabotae are entirely terrestrial animals, many of 
them even living in deserts. Such a convergence could only be 
explained as having arisen, in spite of a different mode of life, 
through the fact that the stimulus which causes the great exten- 
sion of the saccus is the same from a mechanical point of view. 
This question can only be answered by a knowledge of the func- 
tion of the organ. As far as I know, no experiments to ascer- 
tain the function of this structure have been made. 
Several theories as to its probable function have been advanced. 
Hasse (’73) thought this organ served to regulate the pressure 
in the labyrinth by sucking up the endolymph out of the saccu- 
lus, when the pressure there was too high. Wiedersheim (’76), 
Keibel (’15) and Streeter (’16) have adopted this theory. Hasse 
