No. I.] EYES OF ARTHROPODS. BIE 
equally essential elements. In Pecten, Acilius and Cephalo- 
pods, both the clear and the refractive parts of the rods are pres- 
ent. In the latter group, each retinal cell is probably double and 
has two rods separated by a clear space containing an axial nerve 
fibre. The rods are arranged in such a manner that four of 
them, each one from a different retinophora, come together to 
form compact groups, which Grenacher calls rhabdoms. Such 
rhabdoms, however, are different from those of the compound 
eye. They are more like the pairs of rodsin Acilius. Jz nezther 
Acilius or Cephalopods have these groups of rods any morphological 
significance, for they are incidental results of the arrangement of 
rods tn pairs, and this arrangement varies greatly in the different 
eyes of the same individual. The tmportant fact to be borne in 
mind 1s that the patrs of rods belonging to one double cell are the 
anits composing the layer of rods, and that the clear spaces contatn- 
ing the axial nerves form the centres of these units. 
In the rods of the convex eyes of Avca, in the crystalline cones 
of Arthropods, and in the rods of the Vertebrate eye, if I am 
right in my interpretation of these structures, there is no clear 
space between the two rods of the same retinophora. On the 
other hand, in J/antis and probably in most Dzpzera, the hard 
refractive part of the rods has disappeared, and the retinidial 
fibrillaze are suspended in a clear fluid. I think it is fair to con- 
clude from these facts that the essential element of the rods ts the 
system of cross fibrille, or the retinidium, which may be suspended, 
in whole, or in part, etther in a clear fluid or tn a refractive and 
cuticula-like substance. 
In Acilius, the clear space between the rods is not covered by 
a cap of cuticular substance, as in Pecten, although in some 
cases I have clearly seen that it is covered by an extremely deli- 
cate, arched membrane, through which one of the axial nerve 
fibres projects, and, bending at right angles, unites with a simi 
lar fibre from an adjacent pair of rods (Figs. 57a and 59). Thus 
axtal-nerve loops are formed much like those described in Pec- 
ten. In the latter case, the direction of the loops is more 
uniform than in Acilius, where they may be bent in any way, 
although there is a tendency to turn in some directions more 
than in others. 
In Acilius, as in Molluscs, the retinophorz are supplied with 
numerous pigmented nerve fibres that extend along the surface 
