Vol. 50.] SYSTEMATIC POSITION OF THE TKILOBITES. 



421 



[The eye is sunk beneath the surface in a 

 water-sac, and is therefore not in contact 

 with the outer cuticle ] 



furrow between the glabella and cbeek in trilobites which appear 

 to be eyeless, may be the same structures, only closer to the eyes. 

 Perhaps too the curious marks on each side of the glabella in 

 Phacops Volborthii and Ph. fecundus may also come under the same 

 head : that is, they may all be openings, or the remains of openings, 

 into water-sacs over the eyes. 



(d) According to this interpretation of the facts, water-sacs must 

 have originally been present over the eyes of all these primitive 

 Crustacea, completely 



degenerating, however, ^ig. 8.— Diagram of the eye of Apus. 

 in later forms. The tri- 

 lobites afford some inter- 

 esting, though indirect 

 and not conclusive, evi- 

 dence on this point. 

 As is well known, in 

 the earliest trilobites 

 the ' eye-membrane ' is 

 generally wanting. 

 Gerstaecker l would ac- 

 count for this as due to 

 the enormous pressure to 

 which the Lower Silurian 

 fossils were exposed. I 

 would suggest, as a more 



probable interpretation, that the eye proper was not in actual 

 contact with the outer cuticle, but lying in a pocket which would 

 fall away from the outer cuticle as the animal tissues decayed. In 

 Apus, the eye, not being attached to the outer cuticle, but belonging 

 to the thin cuticle of the water-sac (see fig. 8), easily falls away from 

 the former in the process of section-cutting ; only as the water-sacs 

 degenerated (as they have done in the higher Crustacea), and as the 

 eyes became secondarily attached to the external cuticle, would they 

 be preserved. 



We may, then, suppose that in the earlier trilobites the external 

 cuticle was differentiated, above where the eyes were situated in 

 the water-sacs, so as to form a kind of thin and membranous cornea, 

 which would be easily destroyed. This would explain the frequent 

 collapse of the ' eye-membrane.' Again, in other trilobites, the 

 external cuticle above the eyes may have shown no such differentia- 

 tion into a smooth membranous cornea, the eyes lying in the water- 

 sacs under a generally transparent cuticle. These trilobites would 

 now appear to have been blind, whereas their eyes were more 

 probably in pockets under the external cuticle. Microdiscus has no 

 eyes visible. It is interesting to note M'Coy's observation (quoted 

 by Dr. Woodward, op. supra cit.) that the pores above mentioned are 

 most obvious in ' blind ' trilobites. 



(e) The eyes which do appear in trilobites show very marked 

 differences, which Burmeister, with great ingenuity, endeavoured to 



1 Bronn's ' Klassen und Ordnungen,' vol. v. p. 1168. 



