No. I.] EVES OF MOLLUSCS AND ARTHROPODS. 85 



continued outwards as four delicate fibres, which produce four 

 minute impressions at each corner of a corneal facet. 



In the spaces between the inner ends of the ommatidia is a 

 third group of cells, the boundaries of which cannot be distin- 

 guished, and it is therefore difficult to determine the exact 

 number belonging to each ommatidium. The nuclei are 

 arranged at various niveaux around the inner ends of the 

 pedicles. These cells contain yellowish fat-like crystals which 

 form, at the inner surface of the ommateum, a narrow and 

 intensely white band. The crystals are insoluble in absolute 

 alcohol, clove-oil, creosote, chloroform, or ether. But a very 

 dilute solution of caustic potash dissolves them at once, with 

 the formation of a purple solution. 



In the spaces between adjacent facets which have been treated 

 with caustic potash, may be seen four groups of fibres, 

 or impressions of the same. They are probably the outer 

 ends of the basal cells just described, although I have not 

 been able to trace any connection between the structures in 

 question. This supposition, if correct, would fix the number of 

 these cells at four, which agrees very well with what appears to 

 be present. 



The basal membrane is composed of Greek-cross-shaped 

 masses of connective-tissue. From the centre of the inner 

 surface of each cross a group of fibres projects inwards and 

 unites with the connective-tissue cells underlying the basal mem- 

 brane. The squares enclosed by the crosses are bridged by a 

 bundle of diagonal fibres. A series of cross-sections of the 

 inner ends of the ommatidia enable us to determine the position 

 that each cell occupies upon the basal membrane. Beyond the 

 base of the pedicle the retinulae suddenly separate, and the 

 stalk of the pedicle dissolves into two groups of fibres, which 

 become attached to the outer surface of the cross. One group is 

 formed of two separate bundles, while the other is also com- 

 posed of two bundles, but so closely placed as to form one 

 figure, the outline of which indicates its dual composition. 

 These four bundles are the inner ends of the retinophorae and the 

 fibres are their root-like terminations. 



To each basal-membrane square are also attached, in regular 

 and constant order, the inner ends of the bacilli and retinulae. 

 Thus each cross of the basal membrane furnishes the support 



