302 SIMILAR EYES IN COGNATE CRUSTACEANS. 



vision, each eye is nearly hemispherical, and placed on a 

 peduncle by which it is projected to the distance requisite 

 to effect this purpose. (See PI. 45, Fig, 3, b, and b'.) 



In the Scrolls (PI. 45, Fig. 6. b'.,) the disposition of the 

 eye, and its range of vision, are similar to those in the Tri- 

 lobite; but the summit of the eye is less elevated; as the flat 

 back of this animal presents little obstruction to the rays of 

 light from surrounding objects.* 



In the Limulus (PI. 45, Fig. 1.,) vv^here the side eyes (b, 

 b') are sessile, and do not command the space immediately 

 before the head, two other simple eyes (b") are fixed in 

 front, compensating for the want of range in the compound 

 eyes over objects in that direction.f 



In the above comparison of the eyes of Trilobites, with 

 those of the Limulus, SeroUs, and Branchipus, we have 

 placed side by side, examples of the construction of that 

 most delicate and complex organ the eye, selected from 

 each extreme, and from a midway place in the progressive 

 series of animal creations. We find in Trilobites of the 

 Transition rocks, which were among the most ancient forms 

 of animal life, the same modifications of this organ which 

 are at the present time adapted to similar functions in the 

 living SeroHs. The same kind of instrument was also em- 

 ployed in those middle periods of geological chronology 

 when the Secondary strata were deposited at the bottom of 

 a warm sea, inhabited by Limuli, in the regions of Europe 

 which now form the elevated plains of central Germany. 



• Fig. 1. b', Fig. 3. b'. and Fig. 6. b'. are magnified representations of 

 the eyes to which these figures arc respectively adjacent. Figs. 10. and 11. 

 are differently magnified forms of the eye of Asaphus caiidatus, which in 

 Fig. 9. is represented of its natural size. A few of these lenses are semi- 

 transparent; they are still set in their original rims, or frame-work of t!ie 

 cornea, the whole being converted into calcareous spar. 



■|- These eyes are placed so close together, that, having been mistaken for 

 a single eye, they caused the name of Monoculus Polyphemus to be applied 

 to this animal by Linnxus. 



