24 
William Patten. 
E. The Median Eye. The three anterior median openings are compared with the tri- 
ocnlar median eye of Limulus, Apus, Trilobites, Merostomata and other Arthropods, but 
not with the three frontal ocelli of adult insects. InLimulus, Patten ’89, and’93, where the 
structure and development of tliis organ is best known, the retinas of the median ocelli 
arise trom two pairs of segmentai sense organs, that duriug the closing in of the brain 
migrate from the ipargins of the cephalic lobes to the roof of the fore-brain vesicle. Неге 
the ocelli corne to lie at the blind end of a long tube-like outgrowth of the brain roof. The 
distal end of the tube divides into two vesicles, lying in the median line, one in front of 
the other. The retinas of one pair of ocelli unité to form one of the vesicles, and, in a dege- 
nerate condition, lie deeply buried beneath a median tubercle on the dorsal surface of the 
head. The other two retinas 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 Trilobites and Merostomata also, since the arrange¬ 
ment of their surface lenses is precisely the same as in Limulus. The Median Eye of Tre- 
mataspis , like that of Limulus, probably consists of a complex group of three ocelli derived 
from the incomplète fusion of two pairs. They were true cerebral eyes lying at the end of 
a tubular outgrowth of the brain. The distal end of this tube was probably bifurcate, as in 
Limulus, the anterior vesicle containing one pair of ocelli lying beneath, or in, the median 
pit, and the posterior vesicle lying in the paired 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 beliind the other, or by 
two tei minai vesicles, one in front oi the other, arising from a common tubular out¬ 
growth. 
According to this view, the Visual organs of Vertebrates are derived from three pairs 
of segmentai sense organs, originally situated near the margins of the cephalic lobes. The 
median eyes, which were originally the most anterior in position, were the first to be con- 
verted into cerebral eyes of the Vertebrate type. This change took place in the Arthropods, 
and the various steps in the process are clearly seen in Insects, Crustacea, and Arachnids. 
The tiansler of the lateral eyes to the cerebral vesicles and their conséquent inversion took 
place rnucli later, probably in the intermediate type of animais to which the Tremataspidae 
belong. It was during this period that the median eyes reached their highest development 
and the lateral eyes degenerated, or in some cases disappeared completely from the surface. 
We hâve no means of knowing whether their temporary décliné in functional importance 
was the cause, or the resuit, of their transformation into eyes of the cerebral type. 
F. The Post-orbital Openîng contained the forerunner of the Vertebrate olfactory organ. I 
bave identified it with the frontal organ of Limulus , Branchipus, Apus and other Arthropods. 
This organ in the Arthropods présents extraordinary variation in its position, but it can 
always be identified by the peculiar histological structure of the terminal organ and 
