THE EYES OF TRILOBITES I43 



vision in the form of three independent visual organs. The three 

 pigmented cup-shaped ocelli of certain Copepods and Ostracoda have also 

 developed in connection with a pelagic mode of life as three independent 

 eyes, and in his opinion this also took place in the case of the eyes of the 

 trilobite Tr {nucleus. Moreover, Woodward has interpreted the small 

 depressions on the glabella of trilobites as median eyes ; and in 1897 

 Beecher mentions ocelli as being " rarely present," although Hanstrom 

 thinks it probable that he alluded to the stemmata of Harpes, which were 

 also regarded by Kishinouye (1892-3) as indications of median eyes ; but 

 according to the more recent researches of Richter (191 5-1920 and 1922), 

 these are not median eyes but degenerated remnants of aggregated lateral 

 eyes. Ruedemann (19 16), referring to the median tubercle on the glabella 

 of trilobites, writes that " In studying the structure of the tubercle it 

 was found that the median eye presents all stages of development seen in 

 other crustaceans from mere transparent thinner spots of the test to a 

 lenticular body covered by a thin cornea," as in Harpes. The relation 

 of the median eyes to the surface layers in fossil crustaceans is inter- 

 esting in connection with the work carried out on living representatives 

 of the extinct types, such as that by Lankester and Bourne on Limulus 

 and by Margaret Robinson in 1892 on decapod arthropods, more parti- 

 cularly in adult specimens of Palcemon serratus, Verbius varians, and 

 Pandalus annulicornis . Both in Limulus and in eight separate species 

 of Candidas, paired median eyes which are present in the nauplius stage 

 have been demonstrated as persisting in the adult animals. In these 

 they are covered over by a dense chitinous layer, lined on its under 

 surface by an epithelial stratum. That is to say, they are deeply buried 

 and can have little or no function. An idea of their general appearance 

 and relations may be obtained by reference to Fig. 105, A, B, C, which 

 shows in A the two median eyes exposed by removing the rostrum from a 

 fresh specimen ; C, a transverse section of the same, illustrating the 

 structures lying superficial to the fused median eyes, namely, a chitinous 

 layer, connective tissue and muscle, an epithelial layer and a blood sinus ; 

 it also shows the relation of the pigment cells to the nerve-end cells and 

 the epithelial cords by which the eyes are suspended between the chitinous 

 roof above and the cerebral ganglia below ; B, a transverse section in 

 front of C, shows the relations of the suspensory cords to the epithelium 

 of the roof anterior to the median eye. 



The general disposition of the parts will be realized if we quote in 

 full Miss Robinson's description of her dissection of Pandalus : After 

 removal of the rostrum, " the median eye can be seen as a black speck 

 lying in the centre of the triangle formed by the brain and the stalks 

 of the lateral eyes. The brain is covered by chitin which is lined by a 



