THE COMPOUND EYE 



157 



however, that among the eai'Hest fossils known to man — the Trilobites, Arthro- 

 pods which crept over the ooze of the sea-bed, and the Eurypterids, enormous 

 marine spider-Hke creatvires sometimes over 6 feet in length, which flourished 

 in the Palaeozoic era more than 300 million years ago and are long since extinct — 

 both median ocelli and lateral compound eyes are present which have reached a 

 high stage of complexity (Figs. 136 and 137) (Brink, 1951). It would seem, 

 therefore, that both types of eye were derived from a simple ancestral stock 



Fig. 136. — Reconstruction of the 

 Fossil Tbilobite, ^^aiixA prisca. 



On the glabella {gl) there are impres- 

 sions of a median, m, and paired lateral 

 ocelli, /. The compound eyes, CE, 

 are very large (after Barrande). 



Fig. 137. — Reconstbuction of the 

 Fossil Eubypterid, PiERraoTUS 



AyCLlCU.'i. 



An ancient extinct Arachnid found 

 in the Old Red Sandstone rocks in 

 Scotland. It is possessed of elaborate 

 compound eyes, E, as well as two 

 dorsal ocelli, Oc. 



before the beginning of known geological time, that each has evolved in its diffe- 

 rent way along diverging lines, and that their general form as seen today has been 

 essentially the same since the early Pakeozoic period.^ 



The Structvre of the Co7npou7id Eye 



The essential structure of each ommatidiuin is relatively simple. 

 Most superficially the cuticle forms a corneal facet (Fig. 134) 

 underneath lies the crystalline cone, usually with two convex 

 surfaces, the two together acting as a light-collecting system. The 

 remainder of the organ is occupied by the sentient elements arranged 

 in tubular form ; this associated grouj) of cells is called the retinule 

 the cells of which rest upon a fenestrated basement membrane and are 

 arranged so that their differentiated inner borders together form a 



1 Compare p. 754. 



