614 L. H. nouGH. 



processin Admetus very closely and conspicuously. Accord- 

 ing to tlie Sarasins tlie accessory brain oE Helix Waltoni 

 takes its origin out of the so-called cerebral tubes; by 

 analogy with Scorpions, it is perhaps possible to prove the 

 same for Admetus, though. I can only find it by comparing 

 the papers of Brauer (7) and Laurie (29). Laurie says that 

 the median eyes of Scorpions arise out of an invagination, 

 which takes place just in front of the Anlage of the brain ; 

 he describes the cells forming it as resembling brain-cells, 

 but having smaller, darker stained and more densely packed 

 nuclei ; in another paper (31) he states that the brain 

 of Phrynus resembles that of Scorpions inasmuch as 

 in both cases we find a division of the brain in a 

 part containing smaller, darker, and more densely packed 

 nuclei, and a part with lighter, larger, and less dense nuclei. 

 According to Brauer the median eyes arise on a small pro- 

 jection, which lies beyond the invagination mentioned by 

 Laurie, this invagination becoming deep, and giving rise to 

 an accessory part of the brain. Brauer, however, omits to 

 mention whether he found the nuclei couaposing this part to 

 differ from those forming other parts of the brain. 



According to Kleinenberg (25) part of the nervous system 

 of Lopadorhynchus is also formed out of primitive sense- 

 organs, which afterwards become converted entirely into 

 ganglion-masses. 



We are thus led to suppose that the central nervous system 

 oi: Arthropods to some extent corresponds in origin and 

 structure to that of Vertebrates ; namely, as regards the 

 origin of the thi'ee layers (" Sauleuschicht," " Mittelzone," 

 and ''Randschleier" '' surface pit,'^ germ-group, and com- 

 missural substance), and as regards the conversion of sense- 

 organs into ganglia. To some extent it corresponds to that 

 of Molluscs, namely, as regards the origin of ganglia out of 

 sense-organs, and perhaps in possession of an accessory 

 brain. Lastly to that of Worms, namely, as regards the 

 origin of ganglia out of sense-organs. 



The Median Eyes. — As yet only the median eyes have 



