SEGMENTAL SENSE-ORGANS OF ARTHROPODS. 
WILLIAM PATTEN. 
THE cephalic lobes of Acilius are composed of three seg- 
ments, each of which contains a segment of the brain, optic 
ganglion, and optic plate, the latter bearing two pairs of 
segmental sense-organs, or eyes. That these characters are 
common to all insects is very probable since the larve of many 
Coleoptera, Neuroptera, and Lepidoptera agree with those of 
Acilius in having six pairs of ocelli and three optic ganglia, 
while in the adult the ganglion of the convex eye is generally 
composed of three lobes probably derived from three inde- 
pendent ganglia. 
The segmental nature of the eyes is more clearly seen in the 
embryos of Scorpions, Spiders, and Limulus, where it can be 
shown that they are serially homologous, with one or more pairs 
of sense-organs on each segment of the thorax. 
In Scorpions, whose cephalic lobes are composed of three 
segments similar to those of Acilius, the ganglionic invagina- 
tions of the first segment, which has no eyes, unite to form a 
transverse furrow that is soon converted into a closed sac, the 
walls .of which are formed by the He segment of the optic 
plate, optic ganglion, and brain. 
The second ganglionic invagination is at first like that of 
Acilius ; but the optic plate itself is soon infolded, forming the 
outer wall of the ganglionic sac, the inner wall being the optic 
ganglion. The two cavities finally unite in the median line, and 
the median eyes appear as a pair of thickenings on the inverted 
optic plates. A small group of upright cells soon appears on the 
posterior edge of each eye, and pushing its way over the inverted 
cells, gives rise to the permanent retina. The primitive retin- 
ophore are finally converted into the irregular pigment cells, 
filling the inner portion of the eye, some of which Lankester 
supposed to be of mesodermic origin. A thin part of the inner 
wall of the ganglionic sac, belonging neither to the optic plate 
nor the optic ganglion, forms the capsule of the eye. 
