346 0. O. WHITMAN. 
power of the Leech to remain out of water; but as these are 
not the only organs for supplying moisture to the skin, it 
would be rash to conclude that all those Leeches which are 
provided with very small vesicles, or with none at all, are 
incapable of leaving their native element. It is certainly 
going too far to assert that Nephelis and Clepsine never 
leave the water, and that they may therefore be kept inde- 
finitely in uncovered vessels. 
There are seventeen pairs of nephridia as in Hirudo. The 
number, position, and external appearance of the reproductive 
organs agree closely with the same in Hirudo. The histolo- 
gical features of the internal organs will be dealt with in a 
future paper. 
GENERAL REMARKS. 
Only a few general conclusions concerning the origin and 
distribution of the Land Leeches are here offered, as a fuller 
discussion of these questions may be best reserved for a paper 
which will deal with all the species at present known. 
There are certain peculiarities of structure common to all 
the Land Leeches I have examined; such as the absence of an 
eyeless ring between the two rings bearing the third and fourth 
pairs of eyes, the marginal position of the nephridial pores, 
the large size of the vesicles, and the peculiar lobes which cover 
the posterior pair of pores. These features point to a common 
origin of species that are now widely separated. It is quite 
certain that at some period of their genealogical history they 
exchanged aquatic for terrestrial life. Their nearest relatives 
are the Medicinal Leeches (Hirudo), all of which, as is well 
known, are confined to fresh water. At first thought, it would 
seem somewhat remarkable that an animal so thoroughly 
adapted to aquatic life as the Medicinal Leech should be able 
to accommodate itself exclusively to life on land; but when we 
compare its habits and conditions of life with those of the 
Land Leech, and look more closely into the nature of the change 
implied in the exchange of respiratory media, we find little in 
the transition to excite our wonder, The Medicinal Leech has 
