The universality of such diverticula among the arachnids makes it highly probable that their progenitors did possess an alimentary canal with one or more pairs of anterior diverticula. In the { 110}vertebrate, however, the paired diverticula are associated with a compound retina, a combination which does not occur among living arachnids; we must, therefore, examine the crustacean group for the desired combination, and naturally the most likely group to examine is the Phyllopoda, especially such primitive forms as Branchipus and Artemia, for it is universally acknowledged that these forms are the nearest living representatives of the trilobites. If, therefore, it be found that the retina and optic nerve in Artemia is in specially close connection with an anterior diverticulum of the gut on each side, then it is almost certain that such a combination existed also in the trilobites.
[Illustration: FIG. 45.--SECTION THROUGH ONE OF THE TWO ANTERIOR DIVERTICULA OF THE GUT IN ARTEMIA AND THE RETINAL GANGLION.
The section is through the extreme anterior end of the diverticulum, thus cutting through many of the columnar cells at right angles to their axis. _Al._, gut diverticulum; _rt. gl._, retinal ganglion.]
{ 111}[Illustration: FIG. 46.--THE BRAIN, EYES, AND ANTERIOR TERMINATION OF THE ALIMENTARY CANAL OF ARTEMIA, VIEWED FROM THE DORSAL ASPECT.
_Br._, brain; _l.e._, lateral eyes; _c.e._, median eyes; _Al._, alimentary canal.]
[Illustration: FIG. 47.--A, THE FORMATION OF THE RETINA OF THE EYE OF AMMOCOETES (after SCOTT); B, THE FORMATION OF THE RETINA OF THE EYE OF AMMOCOETES, ON MY THEORY.
_R._, retina; _l._, lens; _O.n._, optic nerve fibres; _Al._, cephalic end of invertebrate alimentary canal; _V._, cavity of ventricles of brain; _Al.d._, anterior diverticulum of alimentary canal; _op.d._, optic diverticulum.]
My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the course of his work he cut serial sections through the whole animal, and, as mentioned in my paper in the _Journal of Anatomy and Physiology_, he discovered that the retinal ganglion of each lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion (Fig. 45). In this animal there are only two gut-diverticula, which are situated most anteriorly. I have plotted out this series of sections by means of a camera lucida, with the result that the retina appears as a bulging attached ventro-laterally to the extremity of each gut-diverticulum, as is shown in Fig. 46. It is instructive to compare with this figure Scott's picture of the developing eye in Ammocoetes, where he figures the retina as { 112}a bulging attached ventrally to the extremity of the narrow tube of the optic diverticulum. In Fig. 47, A, I reproduce this figure of Scott, and by the side of it, Fig. 47, B, I have represented the origin of the vertebrate eye as I believe it to have occurred.