152 A. A. W. HUBRECHT. 
For Cyclostomes the same reasoning holds good, although 
there are certain indications that in this group we have before 
us animals in which degeneration and regression with con- 
siderable modification has gone on to such an extent that it 
would perhaps not be impossible to link them on later to 
higher vertebrate ancestral forms as yet unknown. 
And so the question presents itself :—Are we justified in 
displacing the dividing line which in vertebrate classification 
is almost generally adhered to! and which separates Ichthyop- 
sida from Sauropsida and Mammalia? Or is it necessary to 
accept a primary division which brings together on one side 
the Cyclostomata and the Elasmobranchii, and on the other 
the Teleostomes, Dipnoi, Amphibians, Sauropsida, and 
Mammals ? 
IT am well aware that I would not be justified to propose 
such a radical change only on the strength of the arguments 
which I have brought forward in this paper, and by which 
I have attempted to show that the second group is character- 
ised by the more or less distinct presence of an additional 
larval layer, the trophoblast, whereas in the first group no 
traces of this have up to now been found. 
But if we penetrate somewhat more deeply into the 
question by considering whether there are yet additional 
characteristics by which this dividing line might be strength- 
ened, because also on other points the two groups are equally 
distinct from each other, then we may arrive at a firmer 
foundation in support of such a radical change. 
In my opinion there are even two different lines of argu- 
ment alone which to advocate the new dividing line here 
proposed. 
The first is offered by that series of organs which are so 
intimately connected with respiratory processes, and which 
we call the lungs and the air-bladder (swimming-bladder). 
After Spengel’s (04) and Goette’s (’04) lucid articles there 
1 T must make an exception for Ray Lankester’s article on Vertebrata in 
the ‘Encyclopedia Britannica,’ in which, with prophetic insight, he entirely 
ignores this subdivision. 
