556 MULLER ON THE STRUCTURE OF THE GANOIDS, 
in an equation. Against the renewed comparison and conjunc- 
tion of the Stluroidei, especially the Loricarie with the Stur- 
geons and Scaphirhynchus, I need not indeed do anything more 
than refer to generally recognised facts in anatomy; and I shall 
merely remark that Scaphirhynchus, which I examined ana- 
tomically, perfectly resembles the Sturgeons, not possessing the 
slightest similarity to the Loricarie, either in the skeleton or in 
the intestines; and that even their resemblance in external form 
is metaphorical, because, on accurately viewing the compared 
parts, as the mouth or tail, they are completely different. There 
are as few points of resemblance here as between a pike and a 
shark. Loricaria and Scaphirhynchus are as widely separated 
from one another. 
The anatomical characters of the large sections must certainly 
be absolute, é. e. without exception, and they are so, only they 
have hitherto been too little attended to. How many zoologists 
and anatomists would have concluded before the present period 
that all the naked amphibia possess an aortic heart, and that it 
is absent in all the scaly ones! We know also that an amphi- 
bium which possesses an aortic heart undergoes metamorphosis, 
breathes with gills in the young state, and that an amphibium 
which undergoes metamorphosis has also an aortic heart. Ifa 
reptile has no aortic heart, we at once know that it does not 
undergo metamorphosis, and vice versd. That in the Ganoids 
it does not depend upon the rows of valves only is self-evident ; 
the remarkable differences in the valves are here simultaneous 
with the profound difference in the structure of the heart, and 
in the existence or the want of perfect septum to the heart. That 
which obtains among the amphibia, does not necessarily occur 
among fishes. It is, however, worthy of consideration, that 
among fishes, those which undergo a remarkable metamorphosis 
are also furnished with a heart of the arterial trunk. I allude 
to the Plagiostomi, the foetal larvee of which are furnished with 
external gills. We know as yet nothing of the early state of 
the Ganoids. Among the Sirenoids, Protopterus (Lepidosiren 
annectens) retains the external gills discovered by Peters. 
I distinguish absolute from relative anatomical characters. 
Organs which in certain families, genera or species are wanting, 
as the swimming-bladder, cannot be made use of in forming the 
great sections or subclasses, but have a relative value in the sub- 
ordinate sections; 2. e. the organ, when present, must be con- 
