LATERAL AND CENTRAL EYES OF SCORPIO AND LIMULtJS. 197 



polymeniscous eye, but a number of contiguous secondary 

 simple (monomeniscous) eyes. 



The polymeniscous eye presents various elaborations of 

 structure. In the lateral eye of Limulus we have a monostichous 

 polymeniscous eye of relatively simple character. On the other 

 hand, polymeniscous eyes which have developed by the differen- 

 tiation of diplostichous monomeniscous eyes exhibit the highest 

 elaboration of structure known in the Arthropod series. In the 

 first place the retinulse become exceedingly well defined, and 

 separated from one another by intrusive connective tissue; that 

 is to say, by connective tissue which, not belonging originally to 

 the hypodermis layer from which the nerve-end cells and vitreous 

 body-cells are developed, yet pushes its way in amongst these 

 elements. This intrusive connective tissue is pigmentiferous. 



We have seen it already making its appearance (figs, y, 10, 

 11, 14) in the monomeniscous retinulate central eye of Scor- 

 pions. It is more strongly developed in the typical " compound 

 eye," and serves to isolate very completely each retinula from 

 its neighbours. 



A further speciality of that higher form of polymeniscous eye 

 known as the compound eye (of Insects and Crustacea) is the 

 segregation of a third element of the eye in addition to the 

 segregation of retinulse and lens facets; this third element is 

 the vitreous body. Whilst in the monomeniscous diplostichous 

 eye, even when retinulate (central eye of Scorpion) the vitreous 

 body remains homogeneous, consisting of uniformly distributed 

 columnar cells, in the higher form of polymeniscous diplo- 

 stichous eye (compound eye of Insects and Crustacea), the 

 vitreous body, as might be expected, joins in the segregation 

 which characterises the cuticular lens formed upon it. Not 

 only do we find the cells of the vitreous body arranging them- 

 selves in isolated groups similar to, and super-imposed each 

 upon a retinula, but the intrusive connective tissue advances 

 into the \itreous layer and cuts off with its pigment cells each 

 group of vitreous cells from its neighbour, in the same way as it 

 separates neighbouring retinulse. 



Each group of vitreous cells corresponding to a retinula 



