140 



THE PINEAL ORGAN 



is known as the occipital ring and is separated by an occipital or nuchal 

 furrow from the fourth segment. 



According to von Zittel the Trilobites differ from Limulus and all other 

 arthropods in having compound (faceted) eyes supported on free cheek- 

 pieces, which he believed represent the pleura of a head-segment, which is 



Fig. 102. — Longitudinal Section through the Median Eye Tubercle (m. e) 

 of Trinucleus Bucculentus. Adult Specimen. (After L. Stormer.) 



lost except in some forms with stalked eyes and in the cephalic neuromeres 

 of later forms. 



Since the publication of von Zittel's Palceontology in 1900 important 

 recent work, which has been carried out with the employment of modern 

 technique, has confirmed and supplied additional evidence of the presence 

 of median eyes in the larvae and adult specimens of trilobites. Thus 

 Lief Stormer in 1930 demonstrated the presence of a hollow, slightly 

 raised median-eye tubercle in several species of Trinucleidae, Fig. 100 ; 

 he also succeeded in obtaining sections of these, including a transverse 

 section of the median eye of Tretaspis Kiceri (a photograph of which is 

 reproduced in Fig. 101) and a longitudinal section of the median eye of 

 Trinucleus bucculentus (Fig. 102). In Fig. 100, which shows the median 

 eye tubercle of Tretaspis seticornis (seen from above + 80 diameters), there 

 are five small pits arranged like the •' of a playing-card, and a similar 

 arrangement is seen in the meraspid stage II of Tretaspis seticornis. The 

 explanation of the median pit has not been definitely settled, two or three 

 alternative suggestions having been advanced. Thus in Apus, according to 

 the accounts of Patten (191 2) and Holmgren (191 6), the median parietal 

 eye consists of four distinct ocelli, which have migrated inward during 

 development from the sides, and Patten states that they are enclosed in 

 a common sac which opens to the exterior by means of a short duct or 



