MORPHOLOGICAL STUDIES. 
61 
organ in question. The only two points I could observe were 
that there is a large white spot, as stated above, and that a deep 
depression in the cranium just beneath it is readily made out. 
Now, as this depression in P. fluviatilis is always associated 
with a fair development of the parietal eye in the individual, 
I think we run little danger in assuming that the organ will 
probably be found to be well developed in the marine form, 
all the more as the marine form is certainly less degenerated 
than the fresh-water one. 
In adult fresh-water Petromyzon one finds the same varia- 
tion in the presence or absence of pigment which we met with 
in the Ammoccetes, a fact which partially accounts for the 
non-finding of black pigment by earlier observers and especi- 
ally by Ahlborn. 
Relatively to the brain the organ in the adult lies further 
forwards (fig. 1 ,P.E.), and is connected throughout life with the 
brain by a somewhat long stalk. Its position and relations to 
the left ganglion habenulee have been already described by 
Ahlborn (No. 1, p. 233), and he has also recorded its division 
as in the Ammocoetes into an upper and a lower vesicle, dorsal 
and ventral. 
As in the Ammocoetes the dorsal one alone concerns us 
directly, for the ventral vesicle never presents any advance on 
the development as described in the Ammocoetes. 
The parietal eye in the adult usually lies in a deep depression 
of the skull (figs. 1 and 8, s.f.), but if no pigment be present 
in the eye, that is if the organ be ill developed, as happens 
in some individuals, the corresponding depression in the skull 
is also almost or entirely absent (fig. 9). This is a very 
curious fact. The pigment in the skin (p. s.) does not reach 
over the eye. And in longitudinal vertical section of the head 
one sees that the pigment stops short (fig. 8), some distance 
before and behind the organ. 
Further, the amount of pigment deposited in the eye varies 
in different specimens. In some the pigment is so thick as 
entirely to conceal the structure of the retina (fig. 1 ,p.). In 
others it is more modei’atcly developed (fig. 8,^.), and allows an 
