86 PATTEN. [Vol. I. 



for a single ommatidium, and both these structures correspond 

 in number. 



In longitudinal sections one sees that a bundle of pigmented 

 nerves-fibres passes to each of the openings leading into the 

 square spaces enclosed by the crosses ; just before reaching the 

 basal membrane it breaks up into smaller branches, one of 

 which goes to each cell attached to a basal-membrane cross. 

 Besides the pigmented fibres there are four colorless ones 

 which, arising from as many main nerve-branches, ascend the 

 four angles of the cross, and extend along the outer surface of 

 the four cells composing the retinophora. Lastly, a single color- 

 less branch enters the base of the cross and issues from the 

 centre of the opposite surface, to be continued straight upward 

 through the centre of the style to the crystalline cone. 

 Although the basal-membrane crosses, and Ihe enclosed 

 squares, as well as the principal nerve-bundles, coincide in 

 number, each ommatidium is supplied with nerve-fibres from 

 four different bundles. It is probable that the superficial 

 fibres distributed over the wall of the calyx communicate with 

 the axial nerve by means of cross fibrillae, just as in Pecten 

 and Area. In the outer ends of the crystalline cone, in that 

 part which is densest and most hyaline, I have not been able to 

 demonstrate anything like cross lines or fibres. 



In Galathea, Palczmon and Pagurus, one may easily observe 

 the corneal hypodermis the general characters of which differ 

 but little from that of Penaeus. In both Palcemoji and Pagu- 

 rus there are two peripherally placed nuclei for each quadri- 

 lateral facet. In Galathea there is a remarkable modification 

 of these cells to form, for each ommatidium, an iris with a slit- 

 like contractile opening, the walls of which may be expanded 

 by means of radiating contractile fibres. In Galathea I have 

 followed the external longitudinal nerve-fibres of the style, as 

 well as the central, axial fibres, up to the calyx, where the latter 

 nerves extend into the centre of the crystalline cone, and the for- 

 mer give rise to branching fibres spread over the wall of the 

 calyx. In Galathea there are four lateral thickenings of the 

 calycal wall. In Galathea, Palcsmon, and Pagurus, the pedicles 

 are composed of two sets of plates similar to those oi PincBiis, the 

 markings in one set being at right angles to those of the 

 other. 



