1108 The Segmental Sense-Organs of the Leech. (November, 
the dorsal side they are still distinct, but not so deeply marked — 
off from the fifth and eighth rings as from each other. The sam — 
process of consolidation is seen in Hzemopis, Aulostoma and — 
Hemadipsa, but in different stages. In Macrobdella all fou | 
‘rings are distinct on both sides, but the consolidation has already 
begun, as the grooves separating the fifth from the sixth, and the — 
seventh from the eighth, are not so deep as the groove between 
the sixth and seventh, or that between any two of the succeeding _ 
rings. 
If the abbreviation is centripetal, we should expect the fourth — 
ring to disappear before the sixth and seventh. This course of 
events has already been completed in the land leeches, in none of 
which is there a ring intervening between the third and fourth | 
pairs of eyes. 
Two non-papillate rings may also unite with each other; at 
- instance is seen in the twenty-third somite of Macrobdella, where 
the second and third rings are well nigh consolidated. ° 
The syncopation of rings has been carried farther in the acelab 
ulum than in either of the posterior somites. In some spect | 
the segmental papilla are quite distinct on the disk, and in thes | 
their arrangement shows that papillate rings alone have been pè 
served in this region. l 
_ The sacrifice of rings has been greatest in those parts z 
have been compelled to do the most work, namely, the aa 
tremities. In the anterior somite the papillate rings have i 
preserved and functionally improved in proportion to the 
of the less important non-papillate rings eliminated. In the s l 
terior disk the loss of rings is not correlated with an i 
ment of the sense-organs, but with an increased development a 
muscles. The habits of the land leech have favored ae 
opment of a still greater muscular power in the disk, and ge 
been attended with a loss of five rings, leaving only four" pat 
its four terminal somites to offset nine rings on the aqua 
If the historical development of the leech has been ee | 
sive course of abbreviation such as we have described, sont 
dent that an ancestral form must have existed in which sc dert 
were more nearly alike from end to end. The embryon! 7 
opment confirms this view, for in its earlier phases peer 
oe form a chain of very nearly equal parts. It is somewne is off a 
ee "~ (7-8) of the posterior somites become constric™ —— 
