MEDIAN EYES IN INVERTEBRATES 351 



median-eye tubercle arranged in a manner similar to the five of a playing- 

 card. An almost identical disposition of ocelli in a group of three with 

 the apex forward is found in certain recent fossil and amber-preserved 

 insects, e.g. Xylocopa senilis, Apis, a fossil bee (Fig. 239, A), and Priono- 

 myrmex longiceps, a fossil ant (Fig. 239, B), and this characteristic arrange- 

 ment is maintained in the living representatives of the same species and 

 other insects, e.g. Porthetis spinosus (Fig. 241) and Copiophora cornuta 

 (Fig. 242). There is considerable difference of opinion among the older 

 writers as to whether these markings really correspond to ocelli or not, 

 but recent workers, e.g. Stormer, employing new methods of micro- 

 scopical technique, have demonstrated not only the presence, but details 

 of the structure of the median eyes in the glabella of several Norwegian 

 species of TrinucleidEe both in the larval and adult specimens. 1 It is 

 with his kind permission that we have been enabled to reproduce the fol- 

 lowing photographs, which serve to illustrate some of the more important 

 points which he has described in this most interesting group of trilo- 

 bites and of the frontal ocelli of insects in general, and of some arthropods. 



Fig. 100 (Chap. 11, p. 139) shows the general position on the cephalon 

 of the median-eye tubercle of Tretaspis seticornis, in the meraspid stage II, 

 when viewed from the side as in B and as seen from above in A ( <8o). 

 C and D show the tubercle in adult specimens. Both in the meraspid 

 stage and in the adult five impressions are visible on the dorsal aspect 

 of the tubercle. The appearance of the tubercle when seen in section 

 are well shown in Fig. 1 01, which is a transverse section through the median- 

 eye tubercle of an adult Tretaspis Kiaeri, n. sp., and Fig. 102, which is 

 from a photograph of a longitudinal section through the median-eye 

 tubercle of Trinucleus bucculentus . 



Stormer comes to the following conclusions : 



(1) The median tubercle found on the top of the glabella in several 



Trilobites must be regarded as a true median-eye. 



(2) The median-eye which has been found in four species of Trinu- 



cleidae shows a different structure to the lateral eyes. 



(3) The surface of the median-eye tubercle shows five distinct im- 



pressions, indicating four or five ocelli below. 



(4) The structure resembles that of the median eyes in recent Phyllo- 



poda (Apus, Fig. 67, Chap. 11, p. 104, and Fig. 247, and Figs. 

 248, 250, Chap. 24, Lepidurus, Fig. 68, A, Chap. 11, p. 106). 



(5) No lens such as that in the lateral eyes has been found. 



(6) The ontogeny of the Trinucleidas shows that the median eye is 



highly developed in the early larval stages, when the lateral 

 eyes are small and little developed. 



1 Lief Stormer, Scandinavian Trinucleidce, with Special References to Norwegian Species 

 and Varieties, I to III, 1939, Skrifter ut gin av det Norske Videnskaps, Akademi i Oslo. 



