330 C. O. WHITMAN. 
as four instead of two. This gives an increase of two rings, 
whcih, added to the four cephalic rings and the post-anal, gives 
a total of 102. Iu a close comparison like the one we are 
about to make, it will soon be seen that both the dorsal and 
ventral aspects of the rings must be considered, and that it is 
advisable to include in our count the rings of the cephalic lobe 
which are not seen from the ventral side. We are now pre- 
pared to take up the comparison of rings with a view to ascer- 
taining precisely which rings have been lost by the Land Leech. 
There is a universal tendency among Leeches to a reduction 
of the number of rings in the somites at both extremities of 
the body. A glance at the arrangement of the eyes and the 
segmental papille in fig. 10 makes it perfectly evident that 
the metameric division extends to the very end of the cephalic 
lobe. The Ist and 2nd somites are each represented by a 
single ring bearing a single pair of eyes; the 3rd somite has 
two rings, the first of which bears the third pair of eyes ; the 
Ath, 5th, and 6th somites include each three rings, the 
fourth and fifth pairs of eyes being borne on the first rings of 
the 4th and 5th somites. Behind there are three somites 
of two rings each and then a somite of three rings. The re- 
maining somites have each five rings. In fig. 6, in which the 
relation of the eyes to the segmental papille is very satis- 
factorily shown, we find exactly the same number of abbre- 
viated somites as in the corresponding portion of the Medicinal 
Leech; and the reduction in the number of rings is precisely 
the same, except that one ring is missing in the 3rd somite, 
in consequence of which the third and fourth pairs of eyes are 
on contiguous rings instead of being separated by a single 
intervening ring as in fig. 10. The cephalic lobe has simply 
lost a single ring, which bore no eyes, and which could there- 
fore be dropped to the advantage of those possessing a higher 
functional value, since the fourth pair of eyes could thus be 
added to the semicircle of the more important eyes. In 
dropping this ring the Land Leech has advanced one step in the 
well-trodden path of development pursued by its aquatic pro- 
genitors. The course of progress may be briefly defined as 
