ON THE ORIGIN OF VERTEBKATES. 263 



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 constituted the wall of the gut, which we 

 see in its initial stage in the retinal ganglion of Branchipus, 

 would tend rapidly to increase as more and more demands were 

 made upon the brain, until at last both 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. 



I conclude from this evidence that the gut in the more 

 generalised early Crustacean forms, such as the Trilobites, which 

 were allied on the one hand to the Arachnids, and on the other 

 to the Vertebrates, terminated headwards in a pair of anterior 

 diverticula, which were in close relation to the lateral eyes and 

 optic nerves. 



Such an origin for the lateral eyes of the vertebrate explains 

 in a simple and satisfactory manner why the vertebrate retina 

 is a compound 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 the retina is formed. On the other hand, eyes with 

 an inverted compound retina are not entirely unknown among 

 invertebrates, for the eyes of Pec ten and of Spondylus possess 

 a retina which is inverted after the vertebrate fashion, and still 

 may be spoken of as compound rather than simple. It is clear 

 that an invagination, the effect of which is an inversion of the 

 retinal layer, would lead to the same result, whether the retinal 

 optic nerve fibres were short or long, whether, in fact, a retinal 

 ganglion existed or not. Undoubtedly the presence of the 

 retinal ganglion tends greatly to obscure any process of in- 

 vagination, so that, as already mentioned, many observers, like 

 Parker, consider the retina of the crustacean lateral eye to be 



