A Sketch of the Structuie and Development of the Eye of Clepsine. (^21 



the mass of visual cells. The common nerve runs up in front of the 

 visual cells, dwindling gradually in size as its fibres i^ass to the cells, 

 and at length it is lost in the tactile cells." 



Had Mr. Maier seen the following (1. c. p. 589) in due season, 

 he might have spared himself the trouble of refuting an earlier opinion 

 of mine in regard to the eye of Hirudo: 



„The visual cells are the "large clear cells" of Leydig, the 

 socalled "G 1 a s k ö r p e r". The proof of this lies in a variety of facts, 

 only a few of w^hich can here be presented in a summary way. The 

 leading points are as follows: 1) These cells always make up the bulk 

 of the eye, and in the Hirudo pattern they are the only cells 

 supplied by the optic nerve. 2) The main axis of these cells 

 — that passing through the centre of the cell and the eccentric 

 nucleus — is generally, though not invariably, parallel with the axis 

 of the eye. This is most clearly seen in some species of Clepsine^ 

 and is very evident in BrancJielUopsis. 3) In these genera, the nucleus 

 lies on the side exposed to the light, the clear rod-like part of the 

 cell being directed towards the pigment. The cells are practi- 

 cally inverted, the nerve-fibres entering at the nucleated pole. 

 4) A comparison of the diflerent patterns of eye represented in Hirudo, 

 Nephelis, Clepsine and BrancheUiopsis, with the typical segmental 

 sense-organ, shows that the chief distinction between the 

 two classes of organs lies in the relative abundance 

 of the clear cells." 



Again p. 590: 



„In Hirudo,' the visual cells are symmetrically pla- 

 ced around the axial nerve fibres, and no tactile cells 

 are developed; in Clepsine and Haementeria, the visual 

 cells are developed only on the posterior side of the 

 nerve, while the tactile cells are grouped above and in 

 front. In Nephelis the nerve is again axial; in Bran- 

 chelliopsis it is eccentric, as in Clepsine, and there 

 are comparatively few visual cells." 



DcTelopment of the First Pair of Eyes. 



The development of the rudimentary eyes (e. l) has been described 

 in Leuckart's Festschrift (p. 391), but without figures, and hence 

 I repeat it. here. 



„This rudimentary eye has an interesting history, which I will 



Zool. Jahrb. VI. Abth. f. Morph. aq 



