220 BULLETIN OF THE BUREAU OF FISHERIES. 
in figure 8, the concentration is almost as great as in the adult. Each visceral 
ganglion of the larva gives off a respiratory nerve which bears a respiratory gan- 
glion still far apart from the visceral. While the visceral ganglia are becoming more 
closely fused, the commissures between the cerebral and pleural ganglia are becom- 
ing obliterated. The pleural ganglia persist as separate masses in the young ship- 
worm of three or four days (fig. 8), but soon afterwards fuse completely with 
the cerebral, though sections of later stages still indicate by their structure the 
double origin of the so-called cerebral ganglia. 
While these concentrations of the visceral ganglia with each other and of the 
pleural with the cerebral have been taking place, the cerebral commissure is con- 
stantly elongating, along with the growth of the cesophagus, so that in the adult 
the cerebral ganglia are separated from each other by a comparatively long com- 
missure. 
Nervous system of the adult——Along with the great change in the general rela- 
tions of the various systems that has taken place during the transformation from 
the larva to the adult ship-worm, the nervous system has changed principally in 
the altered position of the visceral ganglia, which lose their place in front of the 
posterior adductor muscle, and come to lie much posterior to it. The same 
three pairs of ganglia are present in the ship-worms, however, as in other types of 
lamellibranchs. The general arrangement is shown in figure 60. Lying almost 
at the sides of the mouth are the two cerebral ganglia (c, fig. 10, 60), well developed 
and separated from each other by a long commissure (c¢ ¢), which is composed only 
of nerve fibers. From near the outer end of each a single large pallial nerve passes, 
to be distributed to the parts of the mantle which underlie the shell and form the 
cephalic hood. From near the inner ends of the ganglia large connectives pass 
around the sides of the mouth to the pedal ganglia (p); and from the posterior 
outer ends the cerebro-visceral connectives pass posteriorly to the visceral ganglia. 
The pedal ganglia give off several pairs of large nerves to the foot. 
Visceral ganglia——The two visceral ganglia of the larva fuse into the single 
mass which lies very far posteriorly in the adult (v, fig. 10, 60). After leaving the 
cerebro-pleural ganglia, the cerebro-visceral connectives pass along the sides of 
the ‘‘head”’ under the anterior gill filaments; but posterior to the large adductor 
muscle they take up a more median position, among the tissues of the liver and 
reproductive organs. In front of the visceral ganglia they come to lie close to- 
gether, internal to the large, ductlike portions of the reproductive organs. But 
before entering the visceral ganglia, they pass dorsal to a small ‘‘anterior ganglion”’ 
which hes just in front of the latter. In passing through they give to it a small 
number of nerve fibers (fig. 63), which are lost in it. Then the connectives enter 
the visceral ganglion but little diminished in size. This anterior ganglion was first 
described by Pelseneer (1888) for the ship-worms, and seems peculiar to them and 
their allies. It is a small ganglionic mass lying distinctly in front of the visceral 
ganglion in well-preserved specimens. From the. fact that fibers cross between the 
sides, it seems composed of two halves, quite completely fused together. As has 
been stated, the cerebro-visceral connectives in passing send fibers ventrally into 
it, to be completely lost there. From this ganglion several pairs of nerves are 
