HOBPHOLOGICAL STUDIES. 
57 
account of their, in many points, exceedingly primitive charac- 
ters. I hoped from their investigation to get insight into the 
phylogeny of the parietal eye, for of that we know nothing. 
Spencer’s otherwise beautiful researches do not appear to throw 
any reasonable light on the former history of that organ, and 
the only point about its ancestry that one can accept as the 
outcome of Spencer’s work is his conclusion (No. 14, p. 233) 
that “ the pineal eye may probably be most rightly considered 
as peculiarly a sense organ of pre-Tertiary periods.” 
As it was hardly to be expected that the organ had originally 
developed in the group of Reptiles, the fishes were naturally 
turned to as affording the most probability of finding the solu- 
tion of the problems it presents. The result of an examination 
of the Cyclostomata has been to alter Spencer’s statement, true 
as it was when he wrote it, that “ there is not sufficient evi- 
dence to prove or disprove the existence of the organ within 
the group Pisces ” (No. 14, p, 233). 
In addition to Myxiuoid fishes I have examined several 
Teleostei, among them Callicthys, but with negative results 
as to the presence of an eye-like structure. 
Callicthys was brought under my notice by both Professors 
Wiedersheim and Howes on account of the curious "parietal 
foramen” it presents in the median line above the snout, but 
in front of the brain. 
This membranous portion of the skin has, however, nothing 
to do with the epiphysis ; what its meaning is I am unable to 
say, for I have only investigated its possible relations to a 
parietal eye. In Myxiue, some Ammoccetes, and nearly all 
adult Petromyzou examined by me the presence of a fairly 
well-organised parietal eye could be determined. 
I begin with the account of the structure of the parietal 
eye in 
The Ammoc(etes of Petkomyzon planeri. 
1 have had at my disposal a very large number of Ammo- 
ccetes obtained in Preiburg, and near Kirnhalde, in the Schwarz- 
wald. In addition, Herr Schwarz, a pupil of Professor 
Weismann’s, lent me sections of three Ammocoetes, to be 
