400 SIMILAR EYES IN COGNATE CRUSTACEANS. 
water, and requires universal 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 Serolis (PI. 45, Fig. 6. b'.), the dispo- 
sition of the eye, and its range of vision, are 
similar to those in the Trilobite ; 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.), where 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 Trilo- 
bites, with those of the Limuhis, Serohs, and 
Branchipus, we have placed side by side, exam- 
ples of the construction of that most delicate and 
* Fig. 1. b'. Fig. 3. b'. and Fig. 6. b'. are magnified repre- 
sentations of the eyes to which these figures are respectively 
adjacent. Figs. 10. and 11. are diflTerently magnified forms of 
the eye of Asaphus caudatus, 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 the 
cornea, the whole being converted into calcareous spar. 
t These eyes are placed so close together, that, having been 
mistaken for a single eye, they caused the name of Monoculus 
Polypliemus to be applied to this animal by Linnseus. 
