384 THE AMERICAN NATURALIST. | [Vor. XXXVI. 
trilobites, Merostomata, and other arthropods, but not with 
the three frontal ocelli of adult insects. In Limulus, where 
the structure and development of this organ is best known, the 
retinas of the median ocelli arise from two pairs of segmental 
sense organs, that during the closing in of the brain migrate 
from the margins of the cephalic lobes to the roof of the fore- 
brain vesicle. Here the ocelli come to lie at the blind end of a 
long tubular outgrowth of the brain roof. The distal end of the 
tube then divides into two vesicles, lying in the median line, one 
in front of the other. The united retinas of one pair of ocelli 
form one of the vesicles and, at a considerably later period, lie 
in a degenerate condition, deeply buried beneath a median 
tubercle on the dorsal surface of the head. The other two reti- 
nas lie close together in the second vesicle, beneath two median 
lenses. These two terminal vesicles are found in a more or less 
modified form in many Crustacea and without doubt in the trilo- 
bites and Merostomata also, since the arrangement of their sur- 
. face lenses is, in some case, precisely the same as in Limulus. 
The median eye of Tremataspis, like that of Limulus, probably 
consists of a complex group of three ocelli derived from the 
incomplete fusion of two pairs. They were, no doubt, true 
cerebral eyes, lying at the end of a tubular outgrowth of the 
brain. The distal end of this tube was probably bifurcate, the 
anterior vesicle containing one pair of ocelli lying beneath, or in, 
the median pit and the posterior vesicle lying in thepaired median 
orbits. The anterior and posterior vesicles of Limulus and 
Tremataspis are represented in true vertebrates by either the 
vesicular ends of two separate outgrowths from the brain roof, 
one behind the other, or by two terminal vesicles, one in front 
of the other, arising from a common tubular outgrowth. 
According to this view, the visual organs of vertebrates are 
derived from three pairs of segmental sense organs, originally 
situated near the margins of the cephalic lobes. The median 
eyes, which were the most anterior, were the first to be con- 
verted into cerebral eyes of the vertebrate type. This change 
took place in the arthropods, the various steps in the process 
being clearly seen in insects, crustaceans, and arachnids. 
The transfer of the lateral eyes to the cerebral vesicles and 

