PHOTOSPHERIA OF NTCTIPHANES NORVEGICA. 329 



epidermis, from which it projects inwards in the form of a 

 somewhat elongated knob. The principal axis of the organ is 

 oblique to the external surface. The organ is placed imme- 

 diately outside the tissues of the compound eve, a layer of 

 dense pigment separating the reflector from the ocular elements, 

 as is seen in fig. 3, representing a longitudinal section through 

 the photospherion and part of the eye. The reflector is present, 

 and has the same structure as in the other organs, but it extends 

 nearer to the external surface, its free edge being immediately 

 beneath the epidermis. As in the other organs, it is coated ex- 

 ternally by a mosaic of polygonal red pigment-cells. Internal 

 to the reflector is the posterior cellular layer, also having the 

 same structure as in the other organs, but becoming at its 

 edge continuous with the epidermis. Internal to the cellular 

 layer is a layer of straight fibrils. This layer is of a uniform 

 thickness, equal to that of the cellular layer, and it is limited 

 internally by a hollow surface. The hollow is filled up by a 

 mass, which is almost homogeneous at the base near the surface 

 of the fibrillar layer, but passes by gradual transition into a 

 layer of somewhat elongated cells, which themselves are con- 

 tinuous with the epidermis. 



There is sometimes visible in the section a blood space 

 between the exterior of the organ and the surrounding tissues, 

 but we have never seen any muscles or cellular strands like 

 those occurring in the other organs. 



It is to be noted that, with the exception of the laver of 

 straight fibrils and the reflector, every layer in the photosphe- 

 rion of the ocular peduncle is continuous with the epidermis. 

 It seems a necessary inference from this that the organ is pro- 

 duced by differentiation of parts from a simple thickening of the 

 epidermic layer of cells. The reflector is probably a specialisa- 

 tion of subepidermic mesoblastic tissue ; the posterior cellular 

 layer is a specialisation of the deepest portion of the epidermic 

 thickening, and remains continuous with the epidermis at its 

 free edge. Similarly, the central portion of the thickening is 

 modified into the almost homogeneous mass filling the cavity 

 enclosed by the layer of straight fibrils. This layer itself is 



