ß42 William Patten 



tacea, biit neither time, uor the limits of this paper (already long-er 

 than I originally intended), have allowed me to carry out the plan. 

 Still, I have made a number of observations upon the eyes of the above 

 named genera, eoucerning the most importaut points I desired to il- 

 lustrate, and which, although by no means so complete as those upon 

 Penaeus, still, as far as they go, fully confirm what I have already 

 described for that genus. A more complete description of the ommatidia 

 will be reserved for some future time. 



In all three genera, one may easily demonstrate the presence of 

 the corneal hypodermis, the general characters of which dififer but little 

 from those of Penaeus. In both Paiaemon and Pagurus there are two 

 peripherally placed nuclei for each quadrilateral facet (PI. 31, fig. 115 

 n.c.h.). In Galathea, however, there is a remarkable moditication of 

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

 tractile opeuing, the walls of which may be expanded by means of 

 radiating, contractile tibres, attached to the periphery of the cells (PL 31, 

 fig. 114). The iris is formed of the two corneal hypodermic cells, the very 

 indistinct nuclei of which are situated so close together, that they often 

 appear like a single oval body, placed exactly over the centre of the 

 hexagonal facets, instead of on the periphery. The cell protoplasm is 

 thickened into two opaque, triangulär curtains enclosing a clear, slit- 

 like space , in the centre of which are situated the two nuclei. When 

 the closed iris is observed from below, in surface preparations , one 

 sees simply the thickened edge of the adjacent cell walls enclosing a 

 very narrow, central space. In an open iris, prepared in the same 

 manuer, the free edge of the triangulär curtain, which otherwise pro- 

 jects inwards between the slightly expanded outer ends of the retino- 

 phorae, is seen to have been drawn towards the periphery of the cell, 

 thus enlarging the central openiug, or pupil, and also affording a better 

 view of the shape of the curtain (fig. 1 14 cu.) ; the latter is composed of a 

 granular protoplasm, filled with what appear to be vacuoles of varying 

 size. The edge of the curtain is continuous with many fine , radiating, 

 protoplasmic fibres , which , at their opposite extremities , unite with a 

 thickened, p*eripheral ring of protoplasm, x. 



In surface preparations, made as described for Penaeus. the iris 

 of each facet may be seen in various stages of expansion and contrac- 

 tion. The two nuclei, placed in the centre of the openiug, are extremely 

 transparent and refractive, and these facts together whit their lenticular 

 shape might lead one to consider them as minute lenses. The curtain 

 of the iris , in the living condition, is probably filled with fat globales, 



