No. 3.] SOME NEW FACTS ABOUT THE HIRUDINEA. 593 
namely, that the segmental sense-organs of annelids have formed 
the starting-point for the development of the organs of special 
sense tn the higher animals, not excepting even the eyes of ver- 
tebrates. The evidence is rapidly growing stronger in favor 
of the origin of the olfactory and auditory organs from lateral- 
line organs; and the gustatory organs are certainly destined 
to fall into the same line. Mr. Allis’ observations show 
how lateral-line organs may travel through gill-slits, and the 
mode of growth of the surface bulbs shows how they may 
spread to new areas. In regard to the vertebrate eye, we can 
never expect, of course, to determine its identity with lateral- 
line organs by such direct evidence as is available in tracing the 
origin of the leech eye. Assuming that vertebrates and anne- 
lids have had common ancestors with segmental sense-organs, 
the fact that such organs have been converted into eyes in 
at least one large group of annelids, raises the suspicion that 
nature may have practised the same economy in all branches 
from the common stock. And when we find strong grounds 
for thinking that the lateral-line organs have served as the point 
of departure for the formation of gustatory, olfactory, and audi- 
tory organs, our suspicion in regard to the eyes no longer 
appears incredible. I am not at all unmindful of what might 
now appear to be an almost insurmountable objection to regard- 
ing the vertebrate eye as a segmental organ. As one of my 
laboratory colleagues is soon to bring some new facts to bear 
on this subject, and as further discussion might lead to the 
expression of views that have certainly been corroborated by 
his observations, I drop the question for the present. I would, 
however, mention one observation of mine with reference to the 
eye of Wecturus. The basis for the eye is already discernible as 
a circular area—after treatment with osmic acid followed by 
Merkel’s fluid —long before the closure of the medullary folds 
of the brain, at a stage corresponding closely with Goette’s Fig. 
12, Pl. IIL, of Bombinator igneus. 
But we have an wxpaired vertebrate eye, of recent discovery, 
but possibly of very ancient origin. How is it possible to 
apply the hypothesis of segmental derivation here? As seg- 
mental sense-organs are always paired, it is not easy to account 
for the origin of an azygous organ from them except through 
the fusion of at least one pair. We are not able to point to 
