V 



THE PRESENCE OF A MEDIAN EYE IN TRILOBITES 1 



Plates 34-30 



The purpose of this paper is to assert the occurrence of median 

 or parietal eyes in the trilobites. We do not know whether the, 

 presence of such eyes has been proved before; at any rate, we 

 have been unable to find statements to that end in the literature 

 accessible to us, while on the other hand, none of the recent text- 

 books and treatises mention the occurrence of such eyes in trilo- 

 bites, and some, as the excellent Manual of Paleontology by 

 Nicholson and Etheridge, state directly that " trilobites differ from 

 the Merostomata in having no "ocelli" (p. 527). It is true 

 " ocelli " have been mentioned of trilobites, but in every case some- 

 thing else than the median or parietal eyes of the crustaceans, 

 which alone, as a rule, occur as " ocelli," is meant. Thus Wood- 

 ward has considered as ocelli two pores or funnel-shaped depres- 

 sions which are found, one on each side of the glabella, in the dorsal 

 furrows mostly alongside the frontal lobe. McCoy thought that 

 these " cephalic pores " might have been the points of origin of a 

 pair of antennae, and Nicholson and Etheridge considered them as 

 the points of origin of deep internal processes of the exoskeleton to 

 which muscles were attached. Whatever their function may have 

 been, they were not ocelli, because these would not be located at 

 the bottom of furrows. 2 



In his paper on Trinucleus and in his chapter of the Zittel- 

 Eastman textbook (p. 612), Beecher has stated that the visual 

 organs of certain genera, as Harpes and Trinucleus which present 

 from one to three simple elevations or granules on the fixed cheeks 

 at the end of " eye lines,' shoold be correlated with the ocelli of 



1 Paper read before the Paleontological Society of America at the Washing- 

 ton meeting, December 29, 191 5. 



2 We do not claim, however, that they may not have been originally larval 

 ocelli,- for in the larvae of insects and other arthropods there occur a pair 

 of frontal ocelli, which, however, in arachnids and crustaceans (phyllopods 

 and entomostraca) are highly modified, as two sets of " frontal organs," 

 two paired and one unpaired. In Limulus they become the " olfactory 

 organs" (see Patten, no. 13, p. 126) and we consider it quite possible that 

 these pores also had that function in the 'trilobites. 



127 



