No. 3.] SEGMENTAL SENSE-ORGANS OF ARTHROPODS. 601 
The ganglionic invagination of the third segment is not as 
extensive as the preceding ones, and the optic plate which bears 
the lateral eyes is not involved in the infolding. If the cephalic 
lobes of Scorpions could be stretched out, the eyes would lie, as 
in Acilius, on the thickened outer edge of each segment. This 
thickened edge is represented in the post-oral region by the 
pleurze of the thoracic segments, each of which bears two large 
sense-organs, close together near the outer edge of the base of 
the legs. It is clear that the eyes are serially homologous with 
these thoracic sense-organs. The latter contain a cavity, shaped 
like the bowl and stalk of a goblet, lined with striated cuticula 
similar to that found at an early stage over the eyes of Acilius. 
In Spiders the structure of the cephalic lobes is the same as 
those of Scorpions. The two anterior median eyes belong to 
the second segment, and are homologous with the median eyes 
of Scorpions, the development being the same in both cases. 
The three remaining pairs belong to the third segment, and 
are homologous with the lateral eyes of Scorpions. They are 
invaginated to form optic cups in the same way as those of 
Acilius. The retina is apparently cut into two separate layers 
by the tapetum; but the tapetum is really pierced by regularly 
arranged holes, through each of which passes a pair of retino- 
phore. Above the narrow openings, each cell expands into an 
oblong body containing two rods like those of Acilius. Beyond 
the rods the cell terminates in a short, rounded swelling, con- 
taining the primary nucleus of the retinophora. The secondary 
nucleus lies at the opposite extremity, where the cell is continu- 
ous with a bundle of nerve fibres. The segmental sense-organs 
at the base of the legs are smaller than in Scorpions and easily 
overlooked. 
In Limulus the cephalic lobes are composed of three highly 
modified segments, the first and second being invaginated to 
form two sacs. An evagination of that part of each sac formed 
by the optic plate then gives rise to a tube, at the end of which 
is formed an eye. A pair of these tubes from the first segment 
unites with a similar pair from the second to form the stalk of 
the median eye, the eye itself being formed by the union of the 
four eyes at the ends of these tubes. The two posterior eyes 
are therefore homologous with the median eyes of Scorpions 
and Spiders. 
