ESTERLY : EUCALANUS. 23 



('88-94, p. 361), also, uses practically the same expression, though he 

 considers it as unsettled whether the nerves leave the cells from the 

 inner or the outer ends. And Hesse (:01 and :02) states many times, 

 that the median eyes of Crustacea are inverted. 



As far as my observations on Eucalanus elongatus go, I have been 

 able to confirm in every respect the statements of Grenacher ('79) 

 concerning the relation of the optic nerves to the retinal cells. 



As already stated, the bundle of fibres (axis cylinders) composing- 

 the optic nerve leaves the eye directly at its posterior border, dorsal 

 to the basal plate of the ventral eye and between the posterior basal 

 plates of the lateral eyes. This may be readily determined from whole 

 preparations (Fig. 1, n. opt.), and is seen in sagittal sections (Plate 1, 

 Fig. 6, u. opt; Plate 5, Figs. 44, 46, 48) and frontal sections, though 

 more clearly in the former. If the section coincides precisely with the 

 sagittal plane, the basal plates of the lateral eyes are not cut, but the 

 relation described above will appear in cross (Plate 5, Fig. 49) or 

 frontal sections (Plate 1, Figs. 2, 5). Immediately behind the eye 

 (Plate 4, Figs. 38, 39) the optic nerve is circular in cross section, 

 but within the eye it is separated into parts, which have come from 

 the two lateral portions and the single and ventral portion of the eye 

 (Plate 1. Figs. 2, 5, 6). 



g. Numerical Relation of Nerve Fibres and Visual Cells. — The 

 number of fibres in the optic nerve corresponds precisely with the num- 

 ber of retinal cells in the eye as a whole. This can be so readily seen 

 in cross sections of the nerve (Plate 4, Figs. 38-43) and has been ob- 

 served in so many cases that there can be no doubt on that point. In 

 the region of the optic nerve behind the eye before the frontal nerves 

 have joined it (Plate 1, Fig. 1 ; Plate 4, Fig. 38) the fibres are closely 

 massed into a single cylindrical bundle, but posterior to this region the 

 fibres gradually become more widely separated from one another 

 (Plate 4, Fig. 39; Plate 5, Fig. 47). About half way between the eye 

 and the brain and thence to the brain, the cross section of the nerve is 

 very much flattened dorso-ventrally and elongated laterally (Plate 4, 

 Figs. 40, 41). In any cross section of the nerve posterior to the point 

 at which the frontal nerves (n. f.) join it, the latter are always distin- 

 guishable by the presence of a fibre which has a delicate sheath stain- 

 ing black in vom Path's fluid (Plate 4). 



Near the eye, the fibres of the optic nerve are rounded in cross 

 section, and each is provided with a delicate sheath, which, though 

 distinguishable from the axial bundle, is very closely applied to it 

 (Plate 4, Figs. 38, 39). But farther from the eye, the sheath and the 



