1884. ] The Segmental Sense-Organs of the Leech. 1107 
vision, the rest either remaining in their early indifferent condi- 
tion, or becoming specialized in some other direction. 
In order to get a clearer and fuller idea of the fact that the eyes 
are metameric organs representing merely structurally improved 
forms of the segmental papillæ, let us look more closely at the 
ringsand the somites composing the body. As a glance at the 
diagram will show, there are in all 102 rings between the first 
pair of eyes and the posterior sucker; the last two or three are 
more or less imperfect. The papillate rings show us precisely 
how many somites there are. Of the żwenty-six somites, sixteen 
(7-22 inclusive) have each five rings, while the remaining ten 
have from one to three rings. Of these abbreviated somites siz 
form the anterior and four the posterior end. We notice that the 
abbreviation is greatest at the extreme ends, from which it is plain 
that it began at these points and progressed towards the center of 
the body. The first two somites have each a single ring, the 
third is represented by two rings, and the fourth, fifth and sixth 
each by three rings. The first six somites then include only thir- 
teen rings, less than half the number contained in six complete 
somites, such as are seen in the middle region of the body. The 
twenty-third somite embraces three rings, and the twenty-fourth, 
twenty-fifth and twenty-sixth each two rings. The last four, 
Somites contain only nine rings. The abbreviation not only ex- 
tends to a larger number of somites at the anterior end than at 
the posterior, but it has been carried farther in the individual 
somites, at least in the first and second. At both ends the papil- 
late rings have been preserved, while the less important non- 
Papillate rings have been in part or wholly suppressed. The sup- 
Pression of rings takes place by consolidation, two successive 
"ings gradually coalescing. The papillate ring may coalesce 
either with the preceding or the following ring. In the medicinal 
h the fifth ring, bearing the fourth pair of eyes, is now in pro- 
“sss of uniting with the sixth ; while the eighth is absorbing the 
seventh. The evidence that these two rings are being swallowed 
secs is seen first of all in the rings themselves, and secondly, in the 
different conditions which they exhibit in different species and 
In Hirudo and several allied genera the sixth and Aides 
nth rings are comparatively narrow, and the grooves ee 
them from the fifth and eighth rings are entirely obliterated on 
the ventral side, so that here the four rings appear as two, On 
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