I38 THE PINEAL ORGAN 



Phacopidae. The visual area consists of small round or polygonal openings 

 occupied by separate corneal facets between which is an interstitial test 

 or sclera. 



But beside the faceted or lateral paired eyes of Trilobites there are 

 indications in some of the more primitive types of eye-spots or ocelli. 

 These are simple in type and may be paired or unpaired. The existence 

 of such ocelli is indicated in Harpes (Fig. 97, B), and some specimens of 

 Trinucleus (Figs. 100, 101, 102). They appear in the form of one to three 

 simple elevations or small tubercles, on the fixed cheeks at the ends of the 

 eye sutures, while the ordinary faceted eyes on the movable cheeks are 

 absent. They are regarded as being correlated with the ocelli or median 

 paired eyes which are present in many of the Crustacea and in Limulus ; 



.eye 



Fig. 99. — Deiphon. Silurian. Bohemia. (After Barrande.) 



Glabella globular, without lateral furrows. Free cheeks minute. Fixed cheeKs 

 produced on either side into a long curved spine. Eyes small at base of 

 spine. 



and probably also those of Arachnida, Insecta, and other Classes of 

 invertebrates. 



The meaning of the impressions on the glabella of certain Trilobites 

 is not certain, but the existence of two pairs of ocelli on the carapace of 

 certain living crustaceans, Arachnida, and Insecta, either with or without 

 lateral eyes, in the same relative position as in the Trilobites, suggests that 

 there may have been two pairs of ocelli on some Trilobites and that either 

 one or both of these pairs might in some cases have fused so as to form a 

 single or median eye ; or that indications on the superficial surface of the 

 glabella of either one or both pairs might have disappeared altogether. 

 Further, that these median eyes were already degenerating in some of the 

 earliest known examples of Trilobite and that they were being replaced 

 by lateral paired eyes of the faceted type, the variations in size and com- 

 plexity of which points to the great length of the period during which this 

 differentiation had been taking place. 



