Eyes of Molluscs and Atthropods. 637 



of similar crystals in the outer ring of the pigmented collar of the calyx. 

 On observing these crystals imder the mieroscope, lunconsciously recalled 

 tomind similar crystals, so commonly found in embryo insects, and which 

 in Blatta I eonsidered to be crystals of urie acid. Whether there is 

 more than a superticial resemblance between the two substances or not, 

 I was unable to determine. When treated with caustic potash, one often 

 sees, in the Spaces between adjacent corneal facets, four groups of 

 fibres, or impressions of the same (fig.Töy). They are probably the outer 

 ends of the basal cells just described , although I have been unable to 

 trace any connection between the structures in question. This supposi- 

 tion, if correct. would tix the number of these cells at four, a number 

 which agrees very well with that which appears to be present. 



The basal membrane in Penaeus has reached an extremely 

 high stage of complication , and supplies us with facts which lead to 

 very valuable results; at the same time, it may serve as a type, from 

 which similar structures in other genera will probably not vary greatly. 

 Our knowledge of the so-ealled basal membrane may be summarized 

 In the oft repeated Statement that it is a membrane provided with 

 holes for the passage of nerve fibres 'the »durchlöcherte« membrane 

 of the Germane . No attempt to determine the relation between the 

 holes of the membranes and the superimposed ommatidia has yet been 

 made. The fact, that only in this manjier one can obtain a definite 

 knowledge of the Innervation of the ommatidium, and, at the same 

 time, valuable facts concerning their general morphology, has led me 

 to consider this subject in some detail. The facts have been obtained 

 in two ways; by tangential sections, and by isolating the membrane by 

 means of maceration. In both cases it is necessary to dissolve the in- 

 numerable, fat-like crystals, described above, with dilute caustic potash. 



We will consider first, the structure of the membrane, second. the 

 arrangement of the ommateal cells upon it, and, third, the bundles of 

 nerve fibres by which it is penetrated. 



The basai membrane consists of a mass of connective tissue fibres, 

 in certain places so compactly fused as to form hyaline, and apparently 

 structureless masse s, connected with each other by a network of fine 

 fibres. Their shape is nearly that of regulär Greek crosses, the inner sur- 

 face being smooth and compact in structure (fig. 107), — while toward 

 the outer surface their fibrous nature is more clearly seen. The general 

 shape and combination of the crosses which form the membrane cannot 

 be better described than by referring to the drawings (figs.106,108 and 

 110). Throughout the entire membrane, the arrangement and direction 



