ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 1 83 



oblongata, their branches should, as I think they do, represent a 

 pneumogastric nerve.^ 



Finally the visceral nerves answer to the sympathetic. 



The next question to be considered is, in what manner these 

 typical ganglia are arranged and combined to form the almost 

 infinite varieties in the actual nervous systems of the Cephalous 

 MoUusca. 



We find— 



1. The ganglia concentrated into a mass above the oesophagus, 

 e.g., Doris, Phyllirrhoe, the majority of the Nudibranchiata. 



2. The ganglia concentrated into a mass below the oesophagus, e.g., 

 Pteropoda. 



3. The ganglia concentrated around the cesophagus, some above 

 and some below, e.g., Buccinum, Helix, Onchidiiim, Cephalopoda. 



4. The ganglia scattered and separated in pairs, e.g., Heteropoda, 

 Tectibranchiata, and many Pectinibranchiata. 



Among these the parieto-splanchnic ganglia may either be united 

 by apposition with the pedal ganglia, and with the cerebral by com- 

 missure, as occurs most commonly, e.g.. Octopus, Nautilus, Haliotis ; 

 or they may be united with the cerebral ganglia by apposition, and 

 with the pedal by commissure, as in Strovibus and Pteroceras. 



Patella, Aplysia, and Bulleea form a gradual transition from one of 

 these conditions to the other. 



It will be seen at once from this enumeration that the concentration 

 of the nervous system is by no means a test of high organization in the 

 MolJusca, but rather the reverse. 



A peculiarity of the mollusks belonging to the second and third 

 categories, viz., that their infracesophageal nervous mass is often per- 

 forated by the aorta, may be accounted for by the narrowness of the 

 angeial ring, consequent upon the concentration of the elements of 

 the parieto-splanchnic system, so that they unite directly above the 

 aorta. 



The singular variation in the arrangement of the different portions 

 of the nervous system, whereby the Mollusca as a class differ so widely 

 from the other great classes of Vertebrata and Articulata, may, I t hink, 

 find an explanation in the well-known law, that the constancy of a 

 given arrangement of organs greatly depends upon the period at which 

 they appear in embryonic life. If certain organs are formed early, 

 those which come later must obviously accommodate themselves to 



^ Von Siebold compares the nerves which arise between the nerves to the ganglion stel- 

 latum in Cephalopoda to a par vagum. Vergl. Anat. p. 379. 



