Eyea of MoUuscs and Arthropoda. 731 



less cells, or retinophorae , become double, and acquire axial nerve 

 fibres; (3) the retinidial cuticula beconics thickened, and over each cell 

 divides iuto Segments, the rods, containing specialized parts of the retia 

 terminalia; and (4) the retinophorae develop argeutinulae. Groups of 

 such onimatidia may reniain upon an even surface, or becomé in- 

 vaginated; in either case they form a retineum (e.g. pseudo-lenticulate 

 eyes of Area or HaUotis). If both the retinulae and retinophorae retain 

 their rods. which form a continuous layer. a primitive retineum is 

 produced (e. g. HaUotis). If the retinulae still retain their pigmeut, but 

 lose their rods. a secondary retineum is formed (Heteropods and 

 Alciopidae); this is a condition prior to the formation of a retina. If 

 the ommatidial tract becomes convex, the retinulae lose their rods and 

 form a protective ring around the highly developed rods of the retino- 

 phorae. The rods, therefore, no longer form a continuous layer, and 

 an ommateum is produced (compound eyes oi Area). 



In all Molluscs the rods retain their primitive, terminal position. 

 Arthropods are characterized by the frequent presence of axial rods, , 

 the terminal ones being found only in the simplest ocelli. In the latter : 

 case the ommatidia form invaginate retinea which may w^ithout other 

 change be gradually converted into an invaginate ommateum , by the 

 development of axial rods in the retinophorae (Spiders, Scorpions and 

 Limulus). In the latter genera, the retinophorae increase to fi ve or ten 

 in number. By the evagination of the cup-like ommateum, this form 

 reaches its highest perfection, giving rise to the compound eyes of In- 

 sects and Crustacea; during this proeess, the retinophorae of each 

 ommatidium increase to four in number. 



By the great development of the retinophorae and their rods, and 

 the conversion of the retinulae into colorless, ganglionic cells, a retina 

 is formed [Pccten and Vertebrates). The latter always contains a great 

 many ganglionic cells in intimate association veith the retinophorae. 

 In a retineum, there are very few ganglionic cells; in the ommateum 

 there are none. 



In a retina, the inverted position of the rods is due to the devel- 

 opment of the outer wall of the optic vesicle, instead of the inner. 



Some of the more important groups of ommatidia may be arranged 

 in the foUowing manner ; 



