NERVOUS SYSTEM — BUCCAL AND VISCERAL GANGLIA. 
219 
Electrical or mechanical stimulation of the suboesophageal ganglia 
of Helix causes vigorous muscular agitation, but the animal only 
suiTives their removal about twenty-four hours. 
Some Opisthobranchs possess in addition to the usual commissural 
connection between the pedal ganglia, a second, or as it is termed, 
a parapedal commissure. 
In the Pelecypoda the pedal ganglia are usually placed at the root 
of the foot near the junction of the visceral and muscular parts, and 
usually partially or closely fused together, with the otocysts in close 
proximity, although the otocystic nerves arise from the cerebral centre. 
The Buccal or Stomato-gastric ganglia are usually placed near the 
outlet of the salivary ducts, and vary greatly in their position in 
relation to the cerebral ganglia, according to the amount of protrusion 
of the buccal bulb ; they are connected together beneath the origin of 
the oesophagus, and, unlike other centres, are connected solely with 
the cerebral ganglia, and have only indirect communication with the 
pedal or visceral ganglia ; they innervate the stomach, oesophagus, 
salivary glands, anterior aorta, the pharyngeal muscles, etc., and are 
to some extent sympathetic in function. 
In the Streptoneura they were primitively permanently posterior to 
the cerebral ganglia, which were fixed in advance of the buccal bulb, 
but in many of the Euthyneures and more highly developed Strep- 
toneures, the buccal bulb is capable of protrusion beyond or retraction 
behind the cerebro-visceral nerve ring ; and as the buccal ganglia are 
fixed beneath the origin of the oesophagus, they participate in its 
movements and may therefore be in front of or behind the cerebral 
ganglia according to the state of protrusion or retraction of the 
buccal mass. 
In the Pelecypoda the buccal or stomato-gastric ganglia, which in 
the primitive mollusk innervated the anterior portion of the alimen- 
tary canal, are not apparent, but in correlation with the diminished 
importance of the head region, have degenerated and probably fused 
with the cerebro-pleuro-visceral commissures, which now supply the 
nerves to the early course of the alimentary tract. 
The Visceral ganglia, variously named the parieto- splanchnic, 
viscero-pleural, postero-superior, median or inferior, may be regarded 
as the characteristic nerve centre of the mollusca, and chiefly 
innervate the mesodermic organs; they are usually unsymmetrical 
