character of later acquisition and that S. myops exhibits the more primitive condition.
Another striking feature of this nepionic form not seen in the larva of Limulus is the great size of the lateral eyes which suggests very strongly the megalops stage in the zoea of many crustaceans. The large eyes appear in most specimens as disks bearing a central node [pl. 51, figs. 1, 5]; in some as globose projections with an apical depression [pl. 51, fig. 2]. By tracing these parts through later stages to the adult form, it becomes apparent that the disks or semiglobes are the large visual nodes with an outer orbital ridge and that the apical nodes are composed of the crescentic visual surface[1] and the included apical area as in the mature S. excelsior. With this conception of the parts of the eye, it follows that the visual node was protruded enormously as in the megalops stages of the crustacean larvae. This protrusion disappears entirely, as figures 7–13 show, and only the small visual node surrounded by the visual surface on the outer side remains. In figure 5 the outer orbital ridge is still very well seen; in figure 6 it seems to have merged into the semilunar ridge that runs parallel to the anterior and lateral margins of the carapace.
The original position of the visual surfaces, or eyes proper, in the earliest larvae seems to have been about halfway between the anterior and posterior, margins and nearer to the lateral margin or border than to the median ridge. This position is retained throughout the neanic stages. It is identical with that observed in the larval Limulus. With the beginning of the ephebic stage the compound eyes wander inward and become
- ↑ The visual surface itself has not been observed in the individuals of nepionic age, we presume on account of the extremely small size of the specimens and the usual compression of the visual area into a narrow crescentlike band or slit which is very frequently obscure even in the mature specimens. On account of this failure to see the visual surface itself, we must concede the possibility that in this nepionic stage the whole apical or central node may have constituted the visual surface and the latter been gradually reduced to the crescent-shaped band.
