NERVOUS SYSTEM — VISCERAL GANGLIA AND STREPTONEURY. 221 
originally left posterior organs become placed on the right side of the 
now anterior mantle cavity, and as the pallial ganglia have been in- 
volved in this movement, the originally right pallial ganglion has 
passed to the left side of the body above the intestine, and has therefore 
been distinguished as the Supra-intestinal ganglion, and innervates the 
primitively right ctenidium and osphradium, which the torsion of the 
body has also transferred to the left side of the anterior pallial cavity, 
while the primitively left pallial ganglion has passed beneath the 
Fig. 134. 
Fig. 435. 
Fig. 436. 
Fig. 437. 
Diagrams illustrating the process by which the crossing of the visceral nerves or Streptoneury is 
accomplished (after Lang). Fig. 434 shows the primitive untwisted condition of the nerves. 
Fig. 435 and Fig. 436 show the process partially accomplished, and Fig. 437 shows the crossing of 
the commissures completed, the semi-rotation of the visceral sac being accomplished. The arrows 
indicate the direction of the movement. 
a. anus ; /. foot; //. heart ; Lc. left ctenidium, with osphradium at base; l.v, left visceral or 
pallial ganglion ; i?. oesophagus ; p.Lc. primitively left ctenidium and osphradium, now however 
placed on the actual right of the body owing to the rotation of the visceral sac; p.l.v. primitively 
left, but now the actually right visceral or pallial ganglion ; p.rx. primitively right, but now the 
actually left ctenidium and osphradium ; p.r.v. primitively right, but now actually left visceral or 
pallial ganglion ; r.c. right ctenidium and osphradium ; r.v. right visceral or pallial ganglion. 
intestine to the right side of the body and is now known as the 
Sub-intestinal ganglion; the commissures connecting these ganglia 
to the pleural and abdominal ganglia, which were straight and un- 
twisted in the hypothetical primitive mollusk, are by this movement 
crossed and form a figure of 8, this feature constituting Streptoneury. 
In the Euthyneura the aggregation of the various constituents of 
the visceral ganglia is earned out to the greatest extent, the whole 
of the ganglia having usually become more or less concentrated and 
fused together around the pharynx, owing to which and to the partial 
detorsion the visceral sac has undergone, the pleuro-abdominal 
commissures are not crossed and the osphradium, when retained, is 
placed at the side of the body to which it morphologically belongs, 
while the increased importance of the cephalic region, and the reduc- 
tion and displacement of the mantle posteriorly has resulted in the 
innervation of the mantle devolving upon the pallial ganglia. 
In the Pelecypoda the visceral or parieto-splanchnic ganglia are 
widely distant from the cerebro-pleural centre, and as the pleural 
