332 Dr. T. Margé on the Classification 
e. The composition of all the nerves of exclusively white 
elements. 
f. The direct course of the nerve? optici to the two eyes 
without any crossing. 
g- A remarkably large and wide stomadeum, or the 
remarkable size of the anterior portion of the ali- 
mentary canal representing the stomadeeum. 
h. The occurrence of a hypopharyngeal groove, always 
developed at least during the larval period. 
7. The smoothness and softness of the integuments, as 
well as the absence of any formation of scales. 
k. Absence of the lateral organs which are developed 
elsewhere in all fishes and Amphibia (in the latter 
in the larval period). 
If the Cyclostomi be compared with Amphiorus and the 
Tunicata, we find, further, that of all these the Cyclostomi 
alone are possessed of a primitive cartilaginous skull, in which 
a prechordal part is already developed. If, however, we 
examine this Cyclostome skull more closely, we arrive at 
once at the conviction that, as regards its external form and 
internal structure, it presents essential differences from the 
cartilaginous skull of fishes (Selachii, Dipnoi, Ganoidei). 
The blood is red, like that of other fishes, but the blood- 
corpuscles are of a circular and not of an elliptical form. 
The segmentation of the ovum, as shown by recent investi- 
gations, is not regular, as in Amphiowus and the Tunicata, 
but irregular and amphiblastic (as in Acipenser and the Batra- 
chia). But if we consider that the mode of segmentation 
may sometimes be quite different even in nearly allied ani- 
mals, and that, moreover, the conditions of existence of the 
animal may exert a great influence, if not directly, yet in- 
directly, upon the process of segmentation, we must regard 
the different mode of segmentation as by no means a safe 
criterion in judging of the relationships of the Cyclostomi. 
On the other hand, it seems to us to be of much greater im- 
portance that the earhest larval form of Petromyzon is very 
like Amphioxus, and then subsequently becomes transformed 
first into the Ammocetes-torm, and finally into the definitive 
form. 
If, then, we take all these peculiarities into consideration, 
and weigh them all carefully, it appears clearly that the 
number of bonds which unite the Cyclostomi with Amphiowus 
and the Tunicata is greater than the number of characters 
which those animals have in common with the Selachii and 
the other fishes. 
