202 BULLETIN OF THE BUREAU OF FISHERIES. 
by only a snort connective. The pleural ganglia (pl) are still separate from the cere- 
bral (ec) and lie lateral to them. The pedal ganglia (p) are high up in the foot and 
are closely fused together, as in the adult. The two visceral ganglia (v) are still 
wide apart and lie against the anterior face of the posterior adductor muscle. In 
front of each is a separate respiratory ganglion (7g). From each pleural ganglion 
a pleuro-visceral connective (pv) extends to the corresponding visceral ganglion, 
and from the cerebral ganglion a very short cerebro-pedal connective (cp) to the 
pedal. I think there is still a distinct pleuro-pedal connective, but this I can not 
state positively. The ganglia contain the ganglion cells and the connectives con- 
tain only nerve fibers. 
The kidneys (k, fig. 7) consist of large vacuolated cells, and open externally in 
front of the posterior adductor. The internal pericardial openings I have not been 
able to find. iying around the cerebral ganglion is a gland which, so far as known, 
is peculiar to ship-worms and which, in later stages, becomes so greatly developed 
in connection with the gills. The glandular portion (g D, fig. 24) contains spherical 
cells, and from it a duct opens to the exterior (d D) under the cerebral ganglion. 
METAMORPHOSIS. 
The duration of the free-swimming life of the larva is not known, but it is perhaps 
a month, more or less. Ina very much shorter period after attachment, the peculiar 
ship-worm has been developed, with the adult organization essentially completed. 
The first change is the sudden entire loss of the velum. Within a few hours after 
the larva has settled, the velum begins to disintegrate and its constituent cells are 
cast off and eaten by the larva. The lower lip is projected forward under the cavity 
of the velum, and as the cells are cast off they can not pass to the exterior, and 
so are eaten. The basement membrane of the velar epithelium contracts rapidly 
and the cavity of the velum is very quickly obliterated (within a few hours). 
Accompanying the loss of the velum, the long byssus thread has been secreted. 
As soon as the larva is secured by means of the byssus it seeks a place to burrow, 
and, in preparation for its future life, its whole organization begins to change. The 
fusion of the two sides of. the mantle ventrally progresses rapidly, and within two 
or three days (fig. 2) only the opening for the foot is left. The siphons elongate 
rapidly, and very early (within three days) the mantle grows out posteriorly so as 
to project beyond the shell. The shell grows and changes rapidly after attachment. 
Within two days the first row of teeth has been formed on the anterior edge of each 
valve; the greater growth on the ventral edges causes the two valves to gape, to 
permit the protrusion of the foot in front and the growth of body behind; the knobs 
have been formed on dorsal and ventral portions; the apophyses have been formed 
and the retractor muscles of the foot have become attached to them instead of to 
the umbonal region of the shell. The foot, meanwhile, has become pestle-shaped. 
The alimentary canal takes an important part inthe general change. Thecscum 
of the stomach, present in the larva as a rudiment, enlarges very rapidly and, even 
before woody material has been ingested in quantity, projects as a large vesicle into 
the foot. In the early stage shown in figure 8 (four days attached, a half milli- 
meter long) it already forms a large part of the visceral mass. As the ingestion 
