LATERAL AND CENTRAL EYES OF SCORPIO AND LIMULUS. 211 



ommateum, the more archaic, and those eyes with diplos- 

 tichous oramateum derived from the monostichous condition. 

 A few examples clearly transitional between the monostichous 

 and the diplostichous condition have been described by Gren- 

 acher (among Myriapods). 



It is especially to be noted in reference to the comparison of 

 monomeniscous and polymeniscous eyes, that the comparison 

 yields totally different results accordingly as we may choose to 

 compare with the non-facetted eye of a Spider or of an Insect 

 larva, on the one hand, a single eyelet of a "grouped " eye or 

 of a *' compound " eye, or on the other hand, the whole group 

 or the whole " compound " eye. 



Explanatory List of Terms introduced in this Memoir. 



Onimateura. — All the soft tissues of an Arthropod eye, as distinguished from 



the cuticular lens. 

 Nerve-end cells. — The cells of the ommateum, in which the filaments 



of the optic nerve terminate. 

 Perineural cells. — Cells having the same ectodermal (hypodermis) origin as 



the nerve-end cells, and surrounding a group of the latter ; the rudimentary 



vitreous body aud marginal cells of monostichous eyes. 

 Interneural cells. — Cells having the same ectodermal (hypodermis) origin 



as the nerve-end cells, but remaining small and unrelated to nerve fila- 

 ments whilst wedged in between the bases of adjacent nerve-end cells. 



Only known as yet in the lateral eyes of Scorpions. 

 Monostichous. — Of an ommateum which consists of a single layer of cells. 

 Diplostichous. — Of an ommateum which consists of two layers of cells, 



one superimposed on the other. 

 Vitreous body. — The anterior cell-layer of a diplostichous ommateum. 

 Retinal body. — The posterior, or deep-layer, of a diplostichous ommateum, 



often, but not always, separated from the anterior by a septal membrane. 

 Retinulate. — Of an ommateum in which the nerve-end cells are segregated 



to form definite groups, or " retinulse." 

 Rhabdomere. — The axial or lateral hard-piece which is frequently formed 



by a nerve-end cell in front of its nucleus. 

 Rhabdom. — The compound hard-piece formed in the axis of a retinula by 



the union of the laterally formed rhabdomeres of its constituent nerve-end 



cells. 



