ON THE EYES OF SOME INVEETEBRATA. 675 



The side-wall (cornea) does not project very much over the 

 surface of the head, and is divided into a great number of plano- 

 convex lenses (facets) which are separated from one another 



by narrow shallow furrows. 

 'l will here recall to mind quite shortly what Grenacher 



says about the construction of the eye-units : 



Below each cornea-lens is found a coniform space (PL XLV, 

 fig. 1, a) filled with a liquid or jelly ; its shape is apparently fixed 

 by the enveloping pigment-cells. This is the pseudoconus. Its 

 truncated point ends at the four crystal cells (4), which are 

 also set together, so as to form a cone with its broad base 

 outwards, the point inwards. They are the matrix-cells of the 

 pseudoconus, and surrounded like it by the two principal 

 pigment-cells (2). 



The crystal cells are touched by the retinula (6), which 

 diminishes in breadth as it passes inwards, and so represents a 

 very much elongated cone, which surpasses the pseudoconus in 

 length more than sixfold. The retinula consists of seven cells 

 fused together longwise, and forming the wall of a tube, at the 

 inside of which the staves (rhabdomeres) (5) project as circular 

 led«-es ; they touch each other only at the foremost somewhat 

 thickened end. One of the rhabdomeres always lies in the 

 midst of the six other ones carried by a narrow ledge of the 

 cell-body. 



Between the exterior ends of the ommatidia lie clear- coloured 

 and spindle-shaped pigment-cells (3). 



Of the seven nuclei of the retinula-cells five lie in the upper- 

 most end of the retinula, while the sixth one lies somewhat 

 more distant from them, and the seventh one quite isolated in 

 the lower third of the retinula, so that one can distinguish 

 clearly in a stained vertical section of the eye, three nucleus 

 layers, the second and third of which, formed each by one row 

 of nuclei, are placed concentrically to one another. 



At the end of the retinula, immediately above the basal 

 membrane, lie pigment-cells with very small nuclei (7), the 

 numerous processes of which wrap round in a subtle pigment- 

 knot the undermost part of the retinula. 



