BIOGRAPHICAL SKETCH lv 
of the homology of the segmental organs with the lateral line 
organs to include the ear and even the eye of vertebrates, which 
he believed to have had their origin in organs similar to the sen- 
sillae of leeches. — 
Three years later (‘Description of Clepsine plana,’) we find 
Whitman with all his enthusiasm applying the same criteria of 
metamerism and the same methods of analysis to the external 
morphology of the Glossiphonidae. This he designated the type 
somite and derived from it both abbreviated somites having the 
number of rings reduced to less than three and supplemented 
somites with the number multiplied to more than three. 
In support of these views he also appealed to the facts of em- 
bryology and instanced many cases of ring multiplication, incip- 
ient or advanced, in various genera of leeches. Each somite 
of the leeches’ body, to a considerable degree, undergoes an inde- 
pendent individual development, the nature and extent of which 
is correlated with the physiological demands to which it is sub- 
jected. But never did Whitman carry out his view to its logical 
consequences and see, as others have, that the uniannulate and 
biannulate somites existing at the ends of the body of nearly all 
leeches are steps toward the elaboration of the triannulate as 
the latter is toward the quinque-annulate somite. 
The relation of sensillae and eyes became clearer and the proof 
of their homology was buttressed by many facts. Cases of dual 
sense organs, composed partly of superficial cells bearing tactile 
hairs and partly of clear visual cells situated more deeply along 
the course of the optic nerve, and all gradations from typical 
sensillae with which one or two visual cells and perhaps a little 
pigment may be associated, to complete eyes with only a trace of 
tactile cells were described, the latter leading by easy gradations 
to the strictly visual organs of Hirudo. In some cases every step 
in the transition was found in the successive somites of a single 
leech, as in Clepsine hollensis. 
Strong embryological evidence was brought to bear, especially 
in a paper in the Zoologischer Jahrbiicher for 1893, as showing the 
common origin of both kinds of sense cells from common prolifera- 
tions of the ectoderm, also that the segmental sense organs are 
