COMPOUND EYES OF AUTII HOPODS. :j03 



lit, PI. XXX). Along tlie axial side of the rctinnla cell there 

 exists a definite loniritudinnl tract which is free from the pigment 

 grannies. On the outside of this colorless region there exists a 

 thin layer of chitin. This chitinous covering on the outside of 

 the rctinnla constitutes the rod or the rhabdomere (/rt, Fig. 10, 

 PI. XXX). When the non-pigmented portion of the retinula 

 cell is examined with a high power, we find tlic whole structure 

 to be transversely striated. This striated appeanuice seems to be 

 produced by the existence of an enormous number of transverse 

 fibrils lying in the inside of the chitinous cuticle. That these 

 striae are due to the existence of independent fibrils is further 

 shown by the fact that by careful focusing np and down of tlie 

 microscope some striae can bo directly traced out beyond the 

 limit of the chitinous covering, in the form of independent fibrils 

 {Tr.fh, Fig. 10, PI. XXX). 



"When we make a transverse section of tlie retinulae, each cell 

 will present a wedge-shaped outline, witii its pointed end turned 

 toward the axis of the ommatidium ; the non-pigmented part of 

 the cell in its relation to the cuticular secretion on the outside 

 will be seen from the diagram (Fig. 6a, h, lih, PI. XXIX). 



The number of retinula cells entering into a single ommati- 

 dium is quite variable. Thus, Grenacher* counts fifteen; Lan- 

 kester and Bourne- count ten ; I have found some with eleven, 

 others with nine ; in one ommatidium I counted as many as 

 nineteen. 



I am not able to say under what conditions this numerical 

 variation takes place. Whatever significance there may exist in 

 this numerical variation, it does not alter in the least, as in the 

 case of different elements in other ommatidia we have already 

 considered, the general idea that the ommatidium of the Arthro- 

 pod is morphologically a depression in the skin. 



Each ommatidium contains a single ganglion cell. Only in 

 one case did I see two ganglion cells in a single ommatidium. 

 The ganglion cell is situated morp])ologically"in the centre of 

 the sensory part of the ommatidium. The retinula cells com- 

 pletely envelop this ganglion cell from the outside, in a similar 

 way as the rods encircle the cone in the centre in the retinae of 

 some vertebrates. 



Hoc. cit. Pig. 125, Tar. XI. Uoc. cil. Fig. 10, PI. XL 



