THE EYES OF TRILOBITES 145 



are formed partly of ectoderm and partly of club-shaped nerve-end cells. 

 No trace of a refractive body could be found in the eye. On the whole 

 the resemblance to the eye of Branchipus, as described by Claus and more 

 recently by Patten, also in the scorpion (Chap. 24, Fig. 350, p. 362, 

 Fig. 246, p. 349), is very close." 



Although Miss Robinson alludes to the eye as a single structure, 

 it is obvious that in doing so she was following the customary designation 

 at the time, and that she was well aware of the bilateral nature of the 

 eyes in the nauplius phase of development. The interesting points, 

 apart from the question of bilaterality, in the comparison of the median 

 eyes of Limulus and the Decapod Arthropoda with the median eyes 

 of trilobites, is the fact that in the adult stages of both the living and 

 in many of the extinct forms the eyes lay deeply buried beneath the 

 surface and presumably can have had little or no function. Judging from 

 the appearances seen in many adult specimens of trilobite, the median eyes 

 were already buried and presumably functionless even at the remote epoch, 

 lower Cambrian, when some of the earlier and more simple types of 

 trilobite were alive. On the other hand, indications of the existence of 

 median eyes are present on the surface of the glabella in certain adult 

 forms, e.g. /Eglina prisca, as well as in the nauplius stages and afford 

 valuable evidence of a transitional phase in the phylogeny of the median 

 eyes, namely, an earlier stage when these eyes were superficial and pre- 

 sumably functional and a later stage when in the adult animal they became 

 buried, their function having been taken over by the lateral eyes. Thus in 

 many species, and more particularly those which are more recent and more 

 highly evolved, no indications are present on the glabella of the frontal or 

 median eyes. In view of the recent work by Giinther on the ontogeny of 

 the simple and compound eyes of the water-beetle Dytiscus marginalis 

 (p. 117), in which he has shown that the eye-spot and the six ocelli which on 

 each side precede the development of the compound eye sink beneath the 

 surface, their remnants, however, remaining during life as closed pig- 

 mented vesicles attached to the optic nerve, the absence of indications 

 of their presence on the superficial surface of the glabella in some species 

 is not surprising. The replacement of the median eyes by the com- 

 pound lateral eyes involves the whole problem of the evolution of the 

 compound eye. For since, as pointed out by MacBride, Peripatus, 

 Myriapoda, the lower insects and the larvae of the higher insects agree in 

 possessing only simple, pit-like ocelli, it is fairly clear that the massing 

 together of these ocelli to form a compound eye must have occurred 

 during the evolution of the arthropodan stock and presumably had not yet 

 occurred in the primitive land Arthropoda. But primitive Crustacea, 

 primitive Arachnida, and most Trilobita possess compound eyes. The 



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