LATERAL AND CENTRAL EYES OF SCORPIO AND LIMULUS. 199 



the ommateum is deeply cupped leaving a tubular cavity imme- 

 diately below the cuticular lens. This condition is a step to- 

 wards the complete pinching in of the perineural cells of the 

 ommateum, and their separation as an anterior " vitreous 

 layer " from the deeper lying nerve-end cells. 



No Arthropod eye has as yet been described which is so 

 strictly " monostichous " as the lateral eye of the Scorpion, 

 that is to say, which presents so little evidence of any tendency 

 of the perineural cells to take up a position in front of the nerve- 

 end cells. The position of the ommateum in relation to the lens 

 in the lateral eye of the Scorpion is more nearly like that of 

 the ordinary hypodermis cells in relation to their cuticle, and 

 may be called " epistatic," whilst the monostichous eyes de- 

 scribed by Grenadier (in Myriapods as well as in Insect 

 arvse), are all characterised by a tubular cupping of the om- 

 mateum, which may be called " apostatic." 



The relation of the long axes of the cells of the ommateum 

 to the geometrical (and optical) axis of the lens is widely different 

 in the two cases. Similar facts as to the direction of the axes 

 of the cells of the ommateum, which are not unfrequently to 

 be observed in the eyes of Arthropods, must be taken into 

 account in any attempt at an explanation of the Arthropod 

 eye as an optical apparatus; but we are not at present in a 

 position to make such attempt. 



The features in which the lateral eye of Scorpions is more 

 elaborate than that of the larval Dytiscus are (1) the exist- 

 ence of interneural cells in the former, and (2) the tendency in 

 the former of the rhabdomeres of neighbouring nerve-eud cells 

 to unite as rhabdoms. 



The two agree in the possession of a well-marked ommateal 

 capsule continuous with the basement membrane of the hypo- 

 dermis, and in the " purity^' of the ommateum, that is to 

 say, its freedom from intrusive connective tissue. Since such 

 intrusive connective tissue, when it does enter into the omma- 

 teum, appears to enter there with the function of a pigmentary 

 investment to the optical elements, we may call an ommateum 

 which is devoid of such adventitious pigmentiferous tissue 



