1 82 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



To sum up, the typical number of ganglia in the Cephalous 

 Mollusca is three pair, with which accessory ganglia and visceral 

 ganglia may be connected in \-ariable number. The primary ganglia 

 are united by commissures which form — i, two greater nervous rings, 

 the cephalo-pedal, connecting the cephalic and the pedal ganglia, and 

 the cephalo-splanclinic, connecting the cephalic and parieto-splanchnic 

 ganglia : these rings surround both the oesophagus and the aorta. 2, 

 Two lesser nervous rings, the cephalo-buccal, uniting the cephalic and 

 buccal ganglia, and encircling the oesophagus, and the parieto-angeial, 

 uniting the parieto-splanchnic and angeial ganglia, and sometimes 

 surrounding the aorta alone : this ring does not seem to be invariably 

 present. 



The homology of these ganglia with those of other animals does 

 not, I think, present any very great difficulty. 



It is needless to point out their identity with those of the Acephala 

 Lamellibranchiata. 



In the Articulata we have corresponding cerebral ganglia, while 

 the suboesophageal ganglionic chain answers to the pedal and parieto- 

 splanchnic ganglia united. The nerves of the latter s}'stem appear in 

 a distinct form as the transverse nerves of Insects. 



It seems possible that the series of lateral ganglia in certain 

 Annelida (^Amphiiioine) may correspond with the parieto-splanchnic 

 ganglia of mollusks. 



On the other hand, the stomato-gastric nerves with their ganglia 

 in Articulata appear to correspond with the visceral nerves of 

 Mollusca. 



To what portions of the nervous system of the Vertebrata do these 

 various ganglia answer? This is a problem which has been variously 

 solved. Unless, with Von Baer, we deny the homology of the centres 

 of the nervous system in the Invertebrata with those of the Vertebrata, 

 an argument whose worth can onlj' be decided by a careful and 

 laborious study of development, it would seem clear that the cerebral 

 ganglia are homologous with the corpora striata and thalami of Verte- 

 brata. Their accessories, the buccal ganglia, answer to the trigeminal 

 ganglia, and supply similar parts. 



The cephalo-pedal and cephalo-splanchnic commissures correspond 

 with the crura cerebri ; the pedal and parieto-splanchnic ganglia an- 

 swering to the spinal cord and medulla oblongata. The origin of the 

 auditory nerves would then correspond with that of the seventh pair 

 in Vertebrata, and the pedal nerves with the spinal nerves in function 

 and position. 



Again, if the parieto-splanchnic ganglia represent the medulla 



