204 BULLETIN OF THE BUREAU OF FISHERIES. 
sheath, we get an arrangement very like that in Nucula and Yoldia, in which the 
posterior half of the stomach has the same structure and function as the sheath 
in other forms, though no style is formed. Having left, in the development of 
more specialized forms, its poit of origin in the median position at the extreme 
posterior end of the stomach, the intestime has remained attached to various 
parts of the sheath of the style, as in Cardiwm, and has reached its greatest dis- 
placement in forms like Teredo and Pholas, in which it leaves the stomach from one 
side and in which the sheath of the style forms a large blind pouch. If this view 
of the relations of the stomach, intestine, and sheath be the correct one, then the 
sheath is not a structure which has been acquired in the more highly specialized 
forms. It is homologous with the posterior part of the stomach of primitive 
forms like Nucula and Yoldia. The intestine has left its original median attach- 
ment to the posterior end of the stomach and has become attached to one side of it. 
THE ADULT. 
After the preceding description of the larva and its metamorphosis, the general 
plan of organization of the adult ship-worm will be easily understood. This is illus- 
trated for Yylotrya in figures 5, 6, 10. Figure 5 represents a left view of a young 
specimen 4 inches long, as it is taken from its tube. At the anterior end (on the 
left) the ‘‘head”’ is covered by the small shell, over whose dorsal and posterior por- 
tions duplicatures (the ‘‘cephalic hood’’) of the mantle project. Behind the shell 
extends the long, naked body, tapering so that the whole ‘‘worm”’ forms a very 
long truncated cone. At the posterior end are shown the points of attachment to 
the caleareous tube, and from it project the palettes and siphons. From the anterior 
end, between the gaping shell valves, projects the pestle-shaped foot. 
The outer wall of the naked, projecting part of the body is the mantle. If it is 
removed, as represented in figure 6, the long gills are exposed posteriorly and the 
large visceral mass anteriorly. The latter is continuous with the foot and extends 
about two-fifths of the length of the body. It contains the viscera (alimentary | 
canal, genital organs, etc.). Dorsal to it (fig. 10) lies the large pericardial cavity 
with its contained and associated structures (heart, kidneys, visceral ganglion). 
Dorsal to the pericardial cavity is the long, narrow anal canal, into which the rectum 
opens above the posterior adductor muscle, and which is continued into the epibran- 
chial cavity posterior to the visceral ganglion. The shell cavity 1s occupied mostly 
by the foot, the cecum and anterior part of the stomach, and the two adductor 
muscles which are common to most lamellibranchs. 
SHELL. 
The newly attached ship-worm larva possesses a typical bivalve shell. The valves 
are equal in size and united dorsally by a well-developed hinge apparatus. The 
shell in side view is wider than long; the transverse diameter is about equal to the 
longitudinal. The right valve (fig. 11) bears three equal hinge teeth; the left, two. 
Dorsal to the teeth is an external hinge ligament. In each valve the apophysis of 
later stages is present as a rudiment. Up to this time growth has taken place along 
concentric lines. From this time on rapid, very unequal growth in different parts 
