364 C. O. WHITMAN. 
and at the same time as a most important guide to the genea- 
logical relationship of the various species and genera. 
The Genus Hirudo.—Every Hirudo has twenty- 
six somites, counting from the first pair of eyes to 
the acetabulum: ten of these—the first six and the 
last four—are abbreviated by the suppression of 
from two to four rings in each; and sixteen, lying 
between the first and the last pair of nephridial 
pores, have each five rings. The six anterior so- 
mites include thirteen rings,—the Ist and 2nd 
being represented each by a single ring, the 8rd 
by two rings, and the 4th, 5th, and 6th, each by 
three rings. The four posterior somites embrace 
nine rings (94—102), the 23rd somite including 
three rings, and the 24th, 25th, and 26th, each two 
rings. 
The first ring of each somite is marked, ante- 
riorly, by a pair of eyes; and, from the 11th ring 
onward, by the segmental papille, of which there 
are normally from six to eight on the dorsal half 
of the ring and six on the ventral half. 
The serial homology of the segmental papille 
and the eyes is apparent from their arrangement; 
for the first pair of eyes replace a pair of median 
papille; and the remaining four pairs of eyes re- 
place as many pairs of the inner lateral papille. 
The eye-bearing rings are the Ist, 2nd, 3rd, 5th, 
and 8th. 
The buccals are the 5th and 6th, which are united 
on the ventral side. The post-buccals are the 7th 
and 8th, also united ventrally. 
The first pair of nephridial pores is situated in 
the 18th ring; and the last (17th) pair in the 93rd 
ring. 
The male orifice lies between the 30th and the 
3lst ring, the second and third of the 10th somite. 
The female orifice is five rings behind the male, and 
