78 PROCEEDINGS OF THE AMERICAN ACADEMY 



The eyes are represented by five pairs of large black dots ; the pa- 

 pillae, by smaller dots. The numerals on the left of the diagram 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 (li^t p to 17th jo). With the excep- 

 tion of the genital and nephridial pores, the diagram shows only what 

 belonjis to the dorsal side. 



It will be seen that there are twenty-six transverse rows of papillae, 

 — one for each somite; and that, owing to their uniform and sym- 

 metrical arrangement, they form also eight longitudinal rows. AVe 

 have tioo median rows (w) formed of twenty-five successive pairs ; 

 four lateral rows, an inner (il) and an outer (ol) on each side of the 

 median line ; ^nd two marf/inal rows {mg). The first two eyes hold 

 tlie position of a pair of median papillae, while the remaining eyes re- 

 place as many inner lateral papillae. About this correspondence in 

 position there is not, in my opinion, any room for doubt. In the dia- 

 gram the outer lateral and marginal papillaj are to be seen as far for- 

 ward as the first eye-bearing ring, the marginal ones alone being 

 absent on this ring ; but in most species of Ilirudo both of these rows 

 of [)apill;c 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 papillae, if their position is not misleading, must 

 be regarded as tlie metameric equivalents o^ iYm Jirst pair of eyes ; the 

 inner lateral rows hold the same relation to the 2d, 3d, 4th, and .Jth 

 pairs of eyes. There is a possibility that the first pair of eyes are 

 derivatives of the inner lateral papilke, the median papillaa of this 

 ring having been lost, and the eyes brought nearer together so as to 

 stand in line with the median papilhe of the following somites. How- 

 ever, as all the papillos have the same structnre, there is no objection 

 on this score to the opinion that the eyes are derived from both the 

 median and the inner lateral papillaj. 



The structure of the papilhe confirms the homology above indicated, 

 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 unlike the other cells of 

 the body. What tlie peculiarities of these cells are cannot well be 

 explained without illustrative drawings; l)ut, for present ])urposes, it 

 will be sulficient to say that each of these cells has a vacuole-like 

 central space, which is probably filled with some kind offiiiid. What- 



