THE EVIDENCE OF THE ORGANS OF VISION 113 
pointed out by the authors, it is a remarkable exception in the 
animal kingdom that absorption should occur in that portion of the 
gut which is anterior to the part in which digestion occurs. In all 
these animals the two anterior diverticula extend forwards over the 
brain, and, as we have seen in Artemia, the anterior extremity 
of each one is so intimately related to a part of the brain—viz. 
the retinal ganglion—as to form a lining membrane to that mass 
of nerve-cells. It follows, therefore, that the nutrient fluid absorbed 
by the cells of this part of the gut-diverticulum must be primarily 
for the service of the retinal ganglion. In fact, the relations of 
this anterior portion of the gut to the brain as a whole suggest 
strongly that the marked absorptive function of this anterior 
portion of the gut exists in order to supply nutrient material 
in the first place to the most vital, most important organ in the 
animal—the brain and its sense-organs. This conclusion is borne 
out by the fact that in these lower crustaceans the circulation of 
blood is of a very inefficient character, so that the tissues are mainly 
dependent for their nutrition on the fluid immediately surrounding 
them. It stands to reason that the establishment of the anterior 
portion of the gut as a nutrient tube to the brain would necessitate 
a closer and closer application of the brain to that tube, so that the 
process of amalgamation of the brain with the single layer of colurmnar 
epithelial cells which constitutes the wall of the gut (which we see 
in its initial stage in the retinal ganglion of Artemia), would tend 
rapidly to increase as more and more demands were made upon the 
brain, until at last both the supra- and infra-cesophageal ganglia, as 
well as the retinal ganglia and optic nerves, were in such close 
intimate connection with the ventral wall of the anterior portion of 
the gut and its diverticula as to form a brain and retina closely 
resembling that of Ammocoetes. 
Such an origin for the lateral eyes of the vertebrate explains in a 
simple and satisfactory manner why the vertebrate retina is a com- 
pound retina, and why both retina and optic nerve have an apparent 
tubular development. 
At the same time one discrepancy still exists which requires 
consideration—viz. in no arthropod eye possessiug a compound 
retina is the retina inverted. All the known cases of inversion 
among arthropods occur in eyes, the retina of which is simple, and 
are all natural consequences of the process of invagination by which 
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