ON THE ORIGIN OF VERTEBRATES. 259 



Artemia, for it is universally acknowledged that these forms 

 are the nearest living representatives of the Trilobites. If it 

 therefore 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. 



As already referred to in Part I.^ of this series of papers, I 

 requested Mr W. B. Hardy to examine the central nervous 

 system of the crustacean group for the purpose of comparing 

 it in detail with the vertebrate central nervous system. The 

 result was that most excellent paper published in the Phil. 

 Trans. ^" in which the nervous system of Branchipus and Artemia 

 were taken into consideration, as well as that of Astacus. 

 Further, in the course of this work, series of sections through 

 the whole animal were made by him, and incidentally he 

 pointed out to me how the gut in Artemia was prolonged head- 

 wards into two anterior diverticula, and how closely the two 

 lateral eyes were attached to these diverticula. This observation 

 of Hardy gave me the clue to the meaning of the lateral eyes of 

 Vertebrates, and with his permission I reproduce here one of his 

 sections showing one of the two anterior diverticula cut across, 

 and the retinal ganglion so closely attached that the lining 

 cells of the ventral part of the diverticulum form a lining to 

 the retinal ganglion (fig. 10). I have plotted out by means of 

 a camera lucida the series of sections, with the result that the 

 retina appears as a bulging attached ventro-laterally to the 

 extremity of each gut diverticulum, as is shown in fig. 11: it 

 is instructive to compare with this figure Scott's picture ^ of 

 the developing eye in Ammocoetes, where he figures the retina 

 as a bulging attached ventrally to the extremity of the narrow 

 tube of the optic diverticulum. In fig. 12, A, I reproduce this 

 figure of Scott, and by the side of it, fig. 12, B, I have repre- 

 sented the origin of the vertebrate eye as I believe it to have 

 occurred. 



We see, then, this most striking fact, that in the most primi- 

 tive of the Crustaceans, not only are there two anterior diver- 



1 This Journal, vol. xxxii. p. 528. 



2 p/;;7. Trans. Roy. Soc, 1894, B, p. 83. 

 ■* Journ, of MurphoJ., vol. i. 



