l8o ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



These three pairs of primary gangHa are the Cephahc, the Pedal, 

 and the Parieto-splanchnic. 



I. Tlie Cephalic Ganglia. — These are ahvays either in apposition, or 

 are united by a commissure above the cesophagus. They give off 

 either immediately or from the connecting commissure, the following 

 nerves : — 



1. Labial, to the lips and anterior parts of the head. 



2. Olfactory, to the tentacles. 



3. Optic. 



4. Buccal, to the buccal mass, tongue, and jaws. 



Accessory ganglia may be developed upon all these nerves. They 

 are found upon the labial nerves in Gastcropteron, three upon each 

 side (Souleyet and Blanchard) ; upon the olfactory nerves in many 

 Nudibranchiata (Alder and Hancock, Souleyet, &c.) ; upon the optic 

 nerves in Cephalopoda and Heteropoda. 



The presence of ganglia upon the buccal nerves is almost constant. 

 There seems to be only one inferior buccal ganglion in some Cephalo- 

 poda, while in others there are two, one above and one below. In the 

 Heteropoda, Pteropoda, and most Gasteropoda, there is a pair of 

 ganglia placed laterally at the re-entering angle of the oesophagus and 

 buccal mass. In Patella, Haliotis, and Fissurella, I have found four, 

 two in the latter position, and two anteriorly, just where the buccal 

 nerves come off. 



The buccal ganglia are ahvays united by a commissure, so that 

 when the cerebral ganglia are above the oesophagus, an anterior 

 nervous ring is formed ; when they are at the side or below, as in 

 Pteropoda, there is no anterior nervous ring. 



II. The Pedal Ganglia. — These are either in contact or are united 

 by a commissure ; below the oesophagus they give off — 



1. Auditory nerves. 



2. Pedal nerves. 



The auditory nerves are not commonly present, as their organs are 

 generally sessile ; however, they exist in Cephalopoda, and in Stronibus 

 and Ptcroceras. As has been stated above, the Heteropoda make an 

 extraordinary exception to the usual position of the auditory organs, 

 since in them these nerves appear to be given off from the cephalic 



See also the memoir of Hancock and Embleton, before cited, in which the first complete 

 demonstration of the true sympathetic system in the Gasteropoda is given ; and Alder and 

 Hancock's British Nudibranchiata, in which are contained the most beautiful descriptions 

 and figures of the anatomy of the MolUisca e.xtant. 



