190 PROFESSOR E. R.VY L.VNKESTER AXD A. G. BOURNE. 



The nerve-end cells are arranged in the central eye in definite 

 groups of five, more clearly marked than in the lateral eyes, 

 though not so obviously segregated as in the multicorneal 

 (polymeniscous) eyes called ''compound eyes" in Insects and 

 Crustacea. 



Each group of five cells is entitled, as in the latter case, to 

 the name proposed by Grenacher of " E-etinula." Each reti- 

 nula is provided with a five-fluted rhabdom, formed by the 

 union of the five rhabdomeres, which are produced laterally 

 each by one nerve-end cell. The nature of these dispositions is 

 exhibited in the diagrams drawn in PI. XI, figs. 14, 16, 17. 

 A single nerve-end cell with its rhabdomere is shown in fisf. 

 12, where, however, it is drawn of insufficient proportionate 

 length. 



In a view of a horizontal plane (at right angles to the long 

 axes of the retinulse) it is possible to observe the five-fluted 

 rhabdoms in optical section -when they have the appearance of 

 five-rayed stars (Pi. XI, fig. 15). This appearance was ob- 

 served, and described and figured by Von Graber, who appre- 

 ciated its significance. We thus owe to him, in spite of his 

 other interpretations which are erroneous, the important dis- 

 covery that the nerve-end cells of the central eye of Scorpions 

 are grouped in retinulse and possess a compound rhabdom. This 

 discovery is of great importance, since in the unicorneal 

 (monomeniscous) eyes described by Grenacher, whether of 

 Arachnida or of Insecta Hexapoda, no such segregation of the 

 nerve-end cells was detected, although the existence of such an 

 arrangement serves more directly than anything else to con- 

 nect the structure of so-called simple (monomeniscous) eyes 

 with that of so-called compound (polymeniscous) eyes. 



The iNTRA-CAPSULAR NERVE FILAMENTS which ate given ofF 

 from the filamentary extremities of the nerve-end cells are in 

 fig. 8 seen to run parallel with the plane of section and issue 

 from the capsule in groups (nerves) which are placed to the 

 outer side (n) of the eye. In fig. 9 the section is taken in a 

 plane which cuts these nerve filaments at right angles to their 

 long axes, and accordingly they are seen as irregular masses. 



