680 William Patten 



ance of all evidenee of such a change, — with the slight modificatious 

 necessary to convert an ocellus like the lateral one of Spiders into a 

 similar organ, the evidenee in favor of the latter supposition is so 

 strong that there is no course left bnt to adopt it. 



We bave, therefore, arrived at the following conclusions: (1) that 

 the ancestral forms of ali Arthropods were probably pro- 

 videdwith a small number of eyes placedoueach side of the 

 head; (2) these eyes consisted of closed optic vesicles, 

 formed by invaginations lying dose beneath the hypo- 

 dermis, which formed a continuous layer over them ; (3) the 

 deep wall of the vesicle formed a retineum, similar to that 

 of Worms and certain Mollusca, composed of colorless, 

 double retinophorae, hearing terminal rods and contain- 

 ing an axial nerve fibre: each retinophora was surrounded 

 by circles of rodless pigment cells: (4; the outer wall of the 

 optic vesicle secretes a cuticular, vitreous body, similar 

 to that found in the optic vesicle of Worms (Alciopidae and 

 Molluscs [Fissurella etc) ; (5) the hypodermis overlying the 

 optic vesicle (corneal hypodermis) gave rise to alenticular 

 thickening of the cuticula, the lens. Such a primitive form, 

 which closely resembles typical Molluscan. and Worm eyes, has under- 

 gone no great modifìcation in the simpler ocelli. In general, the modifi- 

 cation has been in two directions : (1 ; an increase in the number of ocelli, 

 accompanied by a decrease in the number of their ommatidia: this is in 

 turn accompanied by an increased complexity of the ommatidia; and 

 (2 , a decrease in the number of ocelli accompanied by an increase in 

 the number and complexity of the ommatidia. The condition of the 

 lateral eyes of Limulus indicates that the development of axial rods, 

 in place of the terminal ones, was the first step in the modifìcation 

 of the ommatidia. Afterwards, eame a decrease in the number of omma- 

 tidia with an increase in the number of retinophorae. 



According to this description , then , the median eyes represent a 

 less modifìed condition of the archaic eyes, than the lateral ones. The 

 eyes oi Limulus represent the extreme modifìcation, in this direction, 

 of the ancestral one , while those of Scorpio bave progressed in the 

 same direction, but bave stopped at an intermediate point. Lankester 

 regards the lateral eye of Limulus as a »monostichous, polymeniscus« 

 eye, and therefore a modifìcation of a single ocellus into a compound 

 eye. I am, on the coutrary, as has already been said, inclined to regard 

 it as a group of ocelli, whose ommatea are reduced to single, complex 



