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 columnar 

 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 Ammoccetes. 



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 possessing 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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