The Retina and Optic Ganglia in Decapods, especially in Astacus. 69 
nl.'px Nucleus of proximal retinular v Ventral. 
cell., va.sng Blood vessel. 
n.opt Optic nerve. I. Punktsubstanz of first ganglion. 
p Posterior. 
II. 
» 
of second 
r Retina. 
III. 
» 
of third 
rhb Rhabdome. 
IV. 
)> 
of fourth 
rhb’ Rhabdomere. 
The numbers 1 to 8 are used to designate the proximal retinal cells, the same 
number referring always to corresponding cells. 
Plate 1. 
Fig. 1—6 are magnified 130 diameters; Figs. 7—22, 430 diameters; Figs. 23 — 25, 
640 diameters. The figures of all transverse sections are placed upon the 
plate so that their upper edge is dorsal, their lower ventral, their right side 
anterior, and their left posterior (cf. pag. 7). 
Figs. 1 and 2. Longitudinal sections of ommatidia showing the arrangement of 
pigment as influenced by light (Fig. 1) and by darkness (Fig. 2). The 
numbers at the right of Fig. 2 indicate the levels at which the sections 
for Figs. 3 — 6 were taken. 
Fig. 3. Transverse section through the corneal hypodermis; the tip of the cone 
appears in the centre of the section. 
Fig. 4. Transverse section through the cone cells at the level of their nuclei. 
Fig. 5. Transverse section through the cone and nuclei of the distal retinular 
cells. 
Fig. 6. Transverse section through the cone cells proximal to the cone. 
Fig. 7. Longitudinal section of a retinula; x cavity filled with fluid; the 
numbers at the left indicate the levels at which sections for Figs. 8 — 20 
were made. 
Figs. 8— 13 represent consecutive transverse sections through the region occu- 
pied by the nuclei of the eight proximal retinular cells. 
Fig. 8. The most distal section in the series , containing parts of the nuclei 
of cells 1, 3, and 7. 
Fig. 9. The second section, containing, in addition to parts of the three nuclei 
just named, part of the nucleus of cell 6. 
Fig. 10. The third section, containing the proximal portions of the nuclei 
in cells 6 and 7 and the distal part of the nucleus of cell 2. 
Fig. 11. The fourth section, containing, in addition to a part of the nucleus 
in cell 2, parts of the nuclei of cells 4 and 5. 
Fig. 12. The fifth section, containing the proximal ends of the nuclei in 
cells 4 and 5. 
Fig. 13. The sixth section, containing, in addition to cells 1 — 7, the nucleus 
of cell 8. 
Fig. 14. Transverse section of a retinula at the level of the fluid-filled cavity 
{x) distal to the rhabdome (compare Fig. 7), showing the two fibres 
[fbr.dst) from the distal retinular cells, the four fibres [fbr.con) from 
the cone cells, and a fibre [y) probably representing a proximal process 
from the eighth proximal retinular cell. Depigmented in potassic hydrate. 
Fig. 15. Transverse section from near the distal end of the rhabdome showing 
the transparent central axis leading to that structure. 
