1884.] The Segmental Sense-Organs of the Leech. 1109 
consolidated into the sucking disk. The somites at the anterior 
end are the first to arise and hence the first to exhibit special- 
ization. 
Among existing species three in Japan have departed less from 
the hypothetical ancestral form than has Hirudo. They agree 
with the medicinal leech in having twenty-six somites, but differ 
from it in having eighteen instead of sixteen complete somites. In 
one of these species there are plain evidences that abbreviation 
has already begun in one of the eighteen somites. 
The segmental papilla then enable us to give a tolerably com- 
plete analysis of the body, to read some chapters in its past his- 
tory, to predict some portions of its future, and to draw a few safe 
conclusions respecting the phylogenetic relationship of different 
species and genera. The discovery that these papillz are sense- 
organs might lead us to speculate on affinities of a more distant 
and uncertain nature; such as are supposed by the writer, in 
common with many others, to exist between annelid worms and 
vertebrates. At all events the existence of such organs in the 
leech furnishes a broader basis for the discussion of the question 
whether the vertebrates and annelids have been derived from a 
common form possessing metameric sense-organs.” 
Assuming that the sense-organs of the lateral line of the ver- 
tebrate and the segmental papillæ of the leech may be traced to 
a common origin in some remote ancestral form, it does not fol- 
low that they should now present close structural resemblances. 
It is far more important to show that they possess certain general 
features in common. The most important of their common fea- 
tures is undoubtedly their metameric origin. The nerve-supply 
forms another feature of fundamental importance, in which ac- 
cording to the interesting observations of Mr. Beard on “ the seg- 
mental sense organs of the lateral line” (Zool. Anz., vil, Nos. 161, 
162) of the vertebrate, there is essential agreement. The devel- 
, OPmental history of these lateral organs in the fish, where they 
Make their first appearance as segmental papilla in the strictest 
Sense of these words, cannot at present be explained on a more 
Satisfactory hypothesis. 
! Dr. Eisig of the Naples Station is the original expounder of this view. 
