ANATOMY AND PHYSIOLOGY OF ATRINA RIGIDA. 433 
The cerebral ganglia are situated wide apart, there being one on each side of the 
esophagus. They are connected by a nerve ring which passes over the esophagus. (Fig. 
10 and 12, cc .) They give off a number of nerves, usually seven pairs, to the palps. A 
very large nerve leaves the anterior end of the ganglion and passes parallel to the 
cerebral connective for a short distance 
and then bends outward and enters the 
mantle. Just before it enters the mantle 
it gives off a branch, which continues 
forward for a short distance above the 
anterior retractor muscle of the foot. 
This branch then bends outward and 
ventrally, penetrates the tissue of the 
retractor muscle, from which it finally 
emerges and enters the anterior adductor. 
(Fig. i2.) The mantle nerve having 
entered the mantle divides into a number 
of branches, all of which unite with the 
circumpallial nerve, to be described later. 
Each cerebral ganglion communicates 
with the corresponding pedal ganglion by 
a short, thick connective. A complete 
nerve ring is thus formed around the 
esophagus. (Fig. 10.) 
The visceral ganglia are situated on 
the ventral face of the adductor, just 
posterior to the kidney. They lie near 
together and are connected by a very thick commissure which contains many 
nerve cells. (Figs, n and 12, vg.) A large cerebro- visceral connective passes 
through the kidney and visceral mass between the cerebral and visceral ganglia. 
(Fig. 12, c.) The visceral ganglia give off four pairs of nerves to the posterior, 
which pass over the ventral surface of the adduc- 
tor. (Fig. 19, 1, 3, 3, and 4.) Three of these 
finally bend outward and enter the mantle. Their 
course in the mantle may be seen in figure 12,1,2, and 
3. Most of the branches of these nerves unite with 
the circumpallial nerve, but a few from the most 
anterior of the three lose themselves in the tissue 
of the mantle. The fourth pair of nerves, described 
above as lying on the ventral surface of the adduc- 
tor, does not reach the mantle, but passes near 
the rectum and enters the muscles of the mantle 
gland. Judging from their close connection with the 
muscles of this organ, one is led to believe that they are distributed to the muscles only. 
Another pair of nerves which arises from the visceral connectives (fig. 11,5) passes on 
the surface of the adductor in the opposite direction from those just described and enters 
the tissue of the mantle gland. These nerves are most likely distributed to the glandular 
48299° — Bull. 29 — 11 28 
Fig. 11. — Drawing of the visceral ganglia, r, 2, 
and 3, nerves to the mantle; 4 and 5, to 
mantle gland; 6, to adductor; 7, to gills; 
S, to kidneys; 9, the cerebro visceral con- 
nective. 
connectives, eg, Cerebral ganglion; pg, pedal ganglia; cv, 
cerebro visceral connective; cc, cerebral connective; in, nerve 
to mantle and anterior adductor; 1, 2, 3, 4, 5, and 6, nerves to 
the palps. 
