The beautiful pictures of Blanchard, and his description, show how, on the arachnid side, paired diverticula of the stomach are nearly universal in the group. Thus, although they are not present in the scorpions, still, in the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked characteristic of the stomach-region is the presence of four pairs of coecal diverticula, which spread laterally over the prosomatic region. In the spiders the number of such diverticula increases, and the whole prosomatic region becomes filled up with these tubes. Blanchard considers that they form nutrient tubes for the direct nutrition of the organs in the prosoma, especially the important brain-region of the central nervous system. He points out that these animals are blood-suckers, and that, therefore, their food is already in a suitable form for purposes of nutrition when it is taken in by them, so that, as it were, the anterior part of the gut is transformed into a series of vessels or diverticula conveying blood directly to the important organs in the prosoma, by means of which they obtain nourishment in addition to their own blood-supply. 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.]