COMrOUND EVES OF ARTHROPODS. 297 



c. Camharus. 

 (Pig. 4, PI. XXIX; Pig. 35, PI. XXXI ; Pigs. 71, 79, PI. XXXV.) 



In Fig. 4, PI. XXIX, three ommatidia of Camharus are 

 represented, a, h and c. The ommatidium may be described as 

 in the two preceding cases, as consisting of three strata of cells, 

 each stratum characterized by the special chitinons product 

 which it secretes. The outermost stratum of cells, the corneagen 

 {c.gY secretes a slightly biconvex corneal facet (c). 



Next below the corneagen comes the stratum of vitrellae, 

 consisting of four cells, each with an extremely elongated internal 

 process. The median surface of the vitrella which corresponds to 

 the chitingsecreting surface of the vitrella in Serolis and Tnlor- 

 chestia secretes a perfectly transparent, homogeneous suT)stance, 

 rather refractive in its nature, forming the crystalline cone 

 [c. c). I cannot say anything of the chemical nature of this 

 substance in its relation to the chitin. The cells which secrete 

 this substance in the crayfish are morphologically identical with 

 the vitrellae in Se7'oli'< and Talorchestla which secrete chitin, 

 and the use this body subserves in the ommatidium of the cray- 

 fish is identical with that of the chitinous body, the crystalline 

 cone, in the two Arthropods we have already examined. There 

 can be no question of the homology of this body to the crys- 

 talline cone of other Arthropods. I have therefore represented 

 this body in the yellow color as in Serolis and Talorchei<tia. 

 The outline of the vitrella cell in the figure has been made 

 diagramatic to a certain extent. 



Next below the vitrellae come the retinulae, the whole being 

 aggregated into a spindle-shaped bundle.- On the distal end of 



' Rcichenbach {Sludien zur EntwicklungsgeschichU des Flusskrebses, p. 93) 

 speaks of these cells as "Semper'schen Zellen." In examining his figure 

 (Pig. 225, Taf. XIV) we find what he regards as " Semper's cells," are those with 

 "Semper'schen Kerne" (ri. K.) The term "Semper's nucleus" as originally 

 used by ClaparOde and adopted by others does not mean the nucleus of the 

 corneagen, as Reichenbach uses it, but means that of the vitrella. In fact the 

 existence of corneagen has been ignored by several earlier writers. It was 

 Patten who emphasized its importance. 



= This spindle stratum does not come from any part of the "augenfallc" of 

 Reichenbach, but arises in the region where he locates the ectodermal and 

 mesodermal fiigment cells, luc. cit. Pig. 225, Taf. XIV. 



