NERVOUS SYSTEM OF MOLLUSCS. 371 



of comparative histology. The disposition of nerves and gan- 

 glia, and the structure of particuhir parts, such as the brain 

 and spinal chord, have been studied with splendid results ; but, 

 as far as my reading extends, no one has thought of making 

 minute and extensive comparisons of the various specialties 

 of nervous tissue ; and the reason has been that men have 

 assumed a nerve to be always of one structure, a ganglion 

 always resembling every other gangHon, forgetting Moliere's 

 humorous wisdom : it y a fagots et fagots. 



When I first examined the nervous system of the Doris 

 and Pleurobranchus, I was surprised to find the brain (oeso- 

 phageal ganglia) of an orange-red, and yellow colour, instead 

 of white or grey. The fact is familiar to anatomists, but the 

 explanation given is more than questionable. Von Siebold 

 attributes it to pigment scattered through the investing 

 sheath (neurilemma) and crowded on the surfiice of the 

 ganglion ; * and Owen assimilates this pigment to " an 

 arachnoid membrane between the dense outer membrane and 

 the ganglions." -f- These explanations are easily disproved. 

 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 pressm^e, 

 the whole contents were ejected, leaving the colourless mem- 



* Von Siebold, Comparative Anatomy, p. 233. So also Delle Chiaje, 

 Istituzioni di Anatomia et Fisudogia Comparata, p. 147 : " Nel ccntro princi- 

 palmente h rosso-rancio, cbe ravvisasi pure nei gangli. E circondato da valida 

 membrana contenente molto glandulette giallastre." 



t Owen : Led. on Camp. Anat. of Inverlehrata, p. 550. 



