No. 3.]| SOME NEW FACTS ABOUT THE HIRUDINEA. 587 
tered species of both land and fresh-water forms, the eyes 
represent enlarged and more or less modified segmental sense- 
organs. The question remains to be answered, whether the 
same holds true of other leeches. If this be conceded, then I 
see no easy escape from the conclusion that the metameric 
sense-organs are earlier in origin than the non-metameric ones. 
If the scattered organs were first in order of development, we 
ought at least to find some cases in which the eyes are not seg- 
mentally arranged. There are some cases in which the serial 
homology of the eyes with the segmental sense-organs is not at 
first sight apparent. This is true of some species of Clepszne. 
The development of the eyes in these cases, as will be shown in 
one of my papers, settles, beyond dispute, the fact that the eyes 
are segmental in origin, and strictly homologous with the seg- 
mental sense-organs. 
4. In the case of Clepsine parasitica and C. chelydre (n.sp.), I 
- have satisfied myself that the segmental sense-organs appear 
very early in the embryo, before the time of hatching, while the 
scattered organs arise later. Another evidence in support of 
the above-stated conclusion. 
5. In Hementeria (commonly, but incorrectly, Hementaria), 
and in several species of C/epseve from Japan and America, there 
is one somite clearly marked by segmental sense-organs in front 
of the eyes. This makes twenty-six somites in front of the 
acetabulum, not counting the narrow tip of the cephalic lobe. 
Mr. Apathy insists on counting this tip as the first somite, but 
he has not yet produced any satisfactory grounds for so doing. 
The difference between us is, not that I deny the possibility of 
finding one, or even more than one somite in the “ prostomial,”’ 
but that I question the propriety of calling it oe, while it still 
remains undetermined whether it represents one, two, three, or 
none at all. The argument from the number of segments in 
the brain will be worth considering when we know precisely 
how many somites are there represented. The question is beset 
with some difficulties, which are not likely to be removed by 
anything less than thorough embryological research. The argu- 
ment from the labial sense-organs, as will be seen in the next 
paragraph, is likewise premature. 
6. The labial sense-organs — prostomial sense-organs of 
Apathy — are serially homologous with vextval segmental sense- 
