_O.n._, axial core of cells in optic nerve; _p._, pigment epithelium; _p.c.r., pars ciliaris retinæ_; _m.f._, Müllerian fibres; _l._, lens.]
It is then absolutely clear that the essential parts of the eye may be considered as composed of two parts--
1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the accompanying Fig. 44, and the left side of Fig. 41.
2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described. In this part supporting elements are found, just as in any other compound retina, of the nature of neuroglia, which are independent of the Müllerian fibres.
{ 109}Of these two parts we have already seen that the second is to all intents and purposes a compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is continuous with the single-layered epithelial tube of the central nervous system--_i.e._ with the cephalic part of the gut of the arthropod ancestor--it follows with certainty that the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the wall of which, near the anterior extremity, the compound retina has amalgamated on either side, just as the infra-oesophageal ganglia have amalgamated with the ventral wall of the main gut-tube. In this way, and in this way alone, does the interpretation of the structure of the vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions already arrived at.
The question therefore arises:--Have we any grounds for believing that the ancient forms of primitive crustaceans and primitive arachnids, which were so abundant in the time when the Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the consideration of the vertebrate eye strongly indicates must have been the case?
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.
