202 PROFESSOE E. RAY LANKESTER AND A. G. BOURNE. 



of Liimilus with as many as fifteen nerve-end cells. Possibly 

 there is a superior and inferior series of retinula cells interlock- 

 ing with one another, and amounting to fifteen in number, 

 which might in certain sections show ten in others, fifteen areas 

 in section ; but we have not obtained definite evidence that such 

 is the case, and are disposed to consider the retinula as having 

 the arrangement of cells shown in fig. 19 and in fig. 20. The 

 nuclei of some of the retinula cells lie nearer to the lens 

 extremity, in others nearer to the filamentary extremity. Each 

 retinula cell gives off a coarse nerve filament from its filamentary 

 extremity. 



The rhabdom of the retinulee of the lateral eyes of Limulus 

 is formed by the union of ten rhabdomeres, as shown diagram- 

 matically in fig. 20. That end of the retinula nearest the lens 

 touches the conical extremity of the secondary lens, but leaves 

 an axial space, which is filled neither by the lens nor by the 

 rhabdom (xy). The rhabdom itself is hollow in its more ante- 

 rior portion, the constituent rhabdomeres only thoroughly 

 uniting along the common axis in the deeper region of the 

 retinula. This is seen in the transverse sections of three reti- 

 nulsp. at different horizons drawn in fig. 26, where the section 

 with hollow rhabdom is more anterior (that is, nearer the lens) 

 than is that to the left with solid rhabdom. 



The perineural cells (fig. 19 y) are delicate columnar cells, 

 much elongated where they adjoin the retinula, and deeply 

 charged with pigment. They pass over in the lateral regions of 

 the lens-cones into ordinary pigmentiferous hypodermis cells (c). 

 Vertical sections through a lens-cone and subjacent omma- 

 teum (de-pigmented as a matter of course) show, besides the 

 retinula cells and the perineural cells, small cells, which are dis- 

 posed upon and between the adjacent large nerve-end cells of the 

 retinula. At first sight these might be interpreted as perineural 

 cells, but their position and distribution do not seem to admit 

 of this view of their nature. They are apparently intrinsive 

 connective tissue, which enters the ommateal capsule with 

 the large group of nerve-fibres attached to the retinula cells. 

 These cells are seen in fig. 19 and fig. 22, where they are 



