THE LEECHES OF JAPAN. 331 
centripetal abbreviation, the maximum limit of abbre- 
viation or concentration appearing first of all in the most 
extreme somites, and advancing from these, step by step, to 
those that lie successively nearer the middle of the body. 
Allowing that abbreviation progresses centripetally, it is easy 
to see what ring, if any, is destined to disappear next. The 
next step for the Medicinal Leech is to drop the eyeless 4th 
ring ; and for the Land Leech, first the anterior and then the 
posterior eyeless ring now separating the fourth and fifth pairs 
of eyes. 
That every step thus far taken in this direction has been 
beneficial, appears evident enough from the fact that the eye- 
bearing rings have been retained and functionally improved in 
proportion to the number of the less important eyeless rings 
sacrificed. These rings have been still further advanced by 
transverse concentration, the more important elements being 
brought into closer order and strengthened at the expense of 
the parts eliminated. The 1st ring in the Land Leech, leaving 
out of consideration the thin, lighter-coloured margin, is repre- 
sented by two large eye-bearing plates; the 2nd by two slightly 
smaller eye-bearing areas and two still smaller median areas, 
bearing segmental papille, or incipient eye-spots; the 3rd, 
by two similar ocellated areas and eight small interposed areas ; 
the 4th, by two still smaller ocellated areas and six small 
intermediate areas; and the 7th, by a row of small areas 
in two of which are seen the posterior pair of eyes, which are 
considerably smaller than those of the preceding rings. The 
only incongruity in all this with the view here taken lies in the 
fact that the third ring has a larger number of intermediate 
plates or areas than the fourth, which is the reverse of what 
we might have expected. The arrangement of these areas, 
however, suggests an explanation of the difficulty. They form 
a single transverse row which becomes double at the two ends 
adjoining the ocellated areas. The most natural way of 
accounting for this duplicity is to assume that two of these 
areas are remnants of the ring that has disappeared between 
the third and the fourth pair of eyes, 
