NERVOUS SYSTEM OF MOLLUSCS. 391 



lemma) and crowded on the surface of the ganglion ; * 

 and Owen assimilates this pigment to " an arachnoid 

 membrane between the dense outer membrane and 

 the ganglions." t These explanations are easily dis- 

 proved. On opening one of the ganglia (in a fresh 

 specimen) and pressing out the contents, I found that 

 the colour was not due to pigment in the membrane, 

 but to the contents of the ganglion, both cellular and 

 liquid ; and by careful pressure, the whole contents 

 were ejected, leaving the colourless membrane behind, j 

 Unimportant as this observation was, it was the start- 

 ing-point of a long series of investigations. Finding 

 the contents of the ganglion were coloured, I inferred 

 that the coloured spots, irregularly distributed over the 

 upper portion of the nerve-trunks, and throughout the 

 bands connecting the ganglia into a collar, were also 

 due to ganglionic cells ; this being proved, it followed 

 that the cells and granules of the ganglion were not 

 anatomically separable from the cells and granules of 



* Von Siebold, Comjyarative Anatomy, p. 233. So also Delle 

 Chiaje, Istituzioni di Anatomia et Fisiologia Comparata, p. 147: 

 *'Nel centro principalmente h rosso-rancio, che ravvisasi pure nei 

 gangli. E circondato da valida raembrana contenente molte glandu- 

 lette giallastre." 



i" Owen : Led. on Comp. Anat. of Inverielrata, p. 550. 



X The error is probably owing to a generalisation from the fact that 

 in many animals the pigment is distributed over the membi-ane. 

 Leydig (Histologie d. Mensch. u. d. Tkiere, 1856, p. 50) confirms what 

 I have said in the text: "Diese Pigmentirung ist diffuser Art, sie 

 riihrt her von einer rothen Fliissigkeit, welche das ganze Ganglion 

 durchtrankt, und nachdem des Neui-ilem eingerissen ist, in Tropfen 

 herausquillt." 



