438 W. H. GASKELL. 



cells are grouped round the diverticula of the central canal in 

 the region of the cerebral and olfactory lobes ; how the com- 

 mencing optic lobes are formed of a great lateral bank of 

 these cells, spreading dorsally on each side of the aqueduct of 

 Sylvius; and how, too, the commencing cerebellum is largely 

 formed of an increase of these same kind of cells ; further, we 

 see, as already mentioned, how the cortex of the large 

 ganglion habenulse is formed of similar cells, arranged in a 

 strikingly similar manner to those of the optic ganglion of an 

 Arthropod. 



Similarly, in the Crustacean the most striking difference 

 between the higher parts of the nervous system — the supra- 

 oesophageal and infra-oesophageal ganglia — and the lower 

 ganglia of the ventral chain is the great increase of these small- 

 celled masses, which are in connection with the reticulated 

 substance (Punctsubstanz), and do not possess well-defined axis 

 cylinder processes as in the case of the large pear-shaped nerve- 

 cells. 



In factj the central nervous system of the Aramocoetes, and 

 therefore of all other Vertebrates, is the direct descendant of 

 the Arthropod nervous system in all respects ; and it is for this 

 reason, and not because similarity cf function requires similarity 

 of structure, as suggested by Bellonci (27), that the remarkable 

 resemblance exists between the structure and connections of 

 the olfactory lobes in the Vertebrates and in the higher 

 Arthropods which he has pointed out in his paper (27) , in the 

 ^Archives Italiennes de Biologie.'' In his conclusions he says : 

 *' La structure et les rapports des lobes olfactifs presentent 

 chez les arthropods superieurs et les Vertebres le meme plan 

 fondamental. Dans les uns comme dans les autres les fibres olfac- 

 tives et les fibres de connexion des lobes olfactifs se resolvent 

 en un fin reticule, qui, se groupant par places, forme ce que 

 j'ai nomme le glomerule olfactif." In the Ammocoetes the 

 olfactory glomeruli resemble exactly in appearance, and in their 

 reaction to staining with osmic,the reticulated substance (Punct- 

 substanz) of the Arthropod nervous system ; here more clearly 

 than anywhere else in the Vertebrates, we see the characteristic 



