1884. | The Segmental Sense-Organs of the Leech. 1105 
left give the number of somites, those on the right the number of 
the first ring of each somite. The position of the seventeen pairs 
of nephridial pores is shown by short dashes (zst p-r7th p). With 
the exception of the genital and nephridial pores, the diagram 
shows only what belongs to the dorsal side. It will be seen that 
there are twenty-six transverse rows of papillæ—one for each 
somite ; and that, owing to their uniform and symmetrical ar- 
rangement, they form also eight /ongitudinal rows. We have 
two median rows (m) formed of twenty-five successive pairs ; four 
lateral rows, an inner (il) and an outer (of) on each side of the 
median line; and zwo marginal rows (mg). The first two eyes 
hold the position of a pair of median papillz, while the remain- 
ing eyes replace as many inner lateral papille. About this cor- 
respondence in position there is not, in my opinion, any room for 
doubt. In the diagram the outer lateral and the marginal papil- 
læ are to be seen as far forward as the first eye-bearing ring, the 
Marginal ones alone being absent on this ring ; but in most species 
of Hirudo both of these rows of papilla are very indistinct or 
entirely absent on the first three eye-bearing rings. They are 
present in Aulostoma, and are very distinct in the large medicinal 
leeches of Saigon, Singapore, Java and Ceylon. 
The median rows of papillz, if their position is not misleading, 
must be regarded as the metameric equivalents of the frst pair 
of eyes; the inner lateral rows hold the same relation to the 
second, third, fourth and fifth pairs of eyes. There is a possibil- 
ity that the first pair of eyes are derivatives of the inner lateral 
Papillæ, the median papillz of this ring having been lost and the 
tyes brought nearer together so as to stand in line with the 
median papilla of the following somites. However, as all the 
papilla have the same structure, there is no objection on this 
Score to the opinion that the eyes are derived from both the 
median and the inner lateral papillz. 
The structure of the papilla confirms the homology above in- 
dicated, if one feature alone be excepted. The eye of the leech, 
as is well known, is a cylindrical mass of cells, three or four 
times as long as wide. The central or axial portion is made up 
of Peculiar large glassy cells, in general appearance entirely un- 
like the other cells of the body. What the peculiarities of these 
cells are cannot well be explained without illustrative drawings ; 
but for Present purposes it will be sufficient to say that each of 
