NATURAL HISTORY OF THE SHIP-WORMS, 221 
given off which innervate the kidneys and other viscera, the genital papillae and the 
osphradium, at least in part (fig. 60). From the anterior end a pair passes forward 
to supply the genital organs and perhaps other viscera. From the middle of the 
ganglion a pair passes laterally to innervate the genital papille and the kidneys 
(fig. 63). Leaving the posterior lateral angles of the ganglion, the largest pair of 
nerves pass backward under the visceral, and divide each into two parts. The 
one, somewhat larger than the other, passes dorsally to enter and be lost in the 
mass of the visceral ganglion. The other passes laterally to innervate the osphra- 
dium. 
The visceral ganglion proper of the adult (fig. 10, 60), because of the great devel- 
opment of the posterior part of the body innervated by it, has attained greater com- 
parative size than the cerebral and pedal. It forms a somewhat three-lobed mass, 
in which the larger central part consists of the completely fused pair of visceral 
ganglia of the larva, while the lobe on either side consists of the respiratory ganglia 
of young stages which have come to lie adjacent to the visceral proper. 
From the visceral ganglion several pairs of nerves are given off, whose connec- 
tion with the visceral is through the lateral masses (fig. 60). Passing forward on 
either side are two small nerves (fig. 60, 1 and 2) which accompany the kidneys and 
anal canal and innervate the posterior adductor muscle and the mantle anteriorly. 
Given off slightly posterior to them, a large nerve (3) goes directly to the middle part 
of the mantle. Posteriorly, a pair of large pallial nerves (pn, fig. 31-33, 60) passes 
backward to innervate the posterior part of the mantle, including the siphons and 
the muscles of the palettes. The branchial nerves (fig. 60, bn) pass laterally, 
closely associated with the osphradium, and then innervate the gills. 
This description of the nervous system differs essentially from that of Quatre- 
fages (1849) which has heretofore been accepted. He thought the two cerebral 
ganglia closely fused, and the pedal rudimentary and separate. I have no doubt 
that he mistook the pedal ganglia for the cerebral; his figures seem to show this. 
But what he observed and figured as the two very small pedal ganglia I do not know. 
It has been seen that while the pedal ganglia are not so large as in forms with a large 
foot, they are not at all rudimentary. 
Otolithic vesicles.—The larva leads an active, free-swimming life, and some means 
of orientation is very essential. This is the function of a pair of otolithie vesicles in 
the usual position in the foot. But the adult ship-worm may assume any position 
and the otolithic vesicles become useless and degenerate. After attachment they 
soon cease to grow and, in specimens 2 or 3 mm. long, their function seems lost. 
They persist as small masses of cells in the adult, though without a lumen and with- 
out the otolithic concretions. 
Sense organ of the genital papilla—One of the pairs of nerves of the anterior 
ganglion has been described as going to the kidneys and genital papille. Situated 
just at the junction of the ectodermal genital duct with the sexual organ there is an 
organ which, by its structure, seems to be for special sensation. The nerve to it (fig. 
63, 64), after a very short course, is distributed to sensory cells which lie adjacent to 
the epithelial lining of the genital duct. The sensory cells are long spindle-shaped, 
and send their peripheral ends to terminate among the epithelial cells lining the 
