NATURAL HISTORY OF THE SHIP-WORMS. 207 
or exhalent siphon is without papillae or tentacles and is shorter and less muscular 
than the respiratory or inhalent siphon, which bears a number of tentacles (fig. 5, 6, 
9). These are sensory structures, but they also serve mechanically to close very 
quickly the entrance of the respiratory siphon against the entrance of enemies and 
injurious objects from the outside. 
Between the duplicature at the posterior edge of the shell and the collar the 
mantle is very uniform. Its structure is as follows: Externally, the surface epithe- 
lium is composed of flattened, nonciliated cells, which secrete the calcareous lining 
of the burrow. Internal to the outer epithelium are the weak muscles of the 
mantle, consisting of the longitudinal layer; a layer in which the fibers cross 
obliquely to the longitudinal fibers; and internal to these the circular layer. The 
internal surface of the mantle is lined by cells which in general are columnar and 
ciliated. Opposite the ends of the gills, the mantle is strongly ciliated and contains 
numerous mucous gland cells which empty into the internal surface of the mantle 
cavity. This region is indicated diagrammatically in figures 27-32, where on 
either side ventral to and outside the ciliated, glandular area the mantle wall is 
thickened so as to form a groove opposite the groove of the gill. In life these two 
grooves are in apposition and together form a canal along which the food is swept 
forward to the mouth by the ciliated cells lining the canal. 
Between the two epithelial layers of the mantle there is a reticular network 
formed of connective tissue, with a small amount of muscle and nerve fibers, ete. 
The spaces so formed are filled by a peculiar substance whose nature I have not been 
able to determine. In living specimens the mantle is of a light grayish, translucent 
appearance. But specimens in alcohol become of almost a chalky whiteness, due 
to the masses of this peculiar material. Each lacunar space is filled by a more or less 
spherical nodule, which is just visible to the naked eye. Examined by transmitted 
light, these nodules are very opaque and seem composed of granular particles; by 
reflected light they are white. They are insoluble in acids, but soluble in water and 
quickly disappear in aqueous solutions. Deshayes described them as nonnucleated 
mucous cells. They are apparently the ‘‘siliceous particles’ which Hancock 
observed, and with which he supposed the burrow to be formed. They are not 
cells, but deposits of some sort. They are probably to be regarded as constituting 
a reserve of calcium, containing material of some sort for rapid use, as occasion 
may require, in the formation and thickening of the calcareous tube which lines the 
burrow. 
Special gland of the mantle.—Lying between the two epidermal layers of the 
mantle, in the middorsal region near the extreme posterior end of the body, there 
is a small special gland which seems to be peculiar to the ship-worms. The extent 
and the details of structure of this organ are shown in figure 21, which repre- 
sents a transverse section of the whole gland in a specimen about a half a centimeter 
long. The gland consists of numerous more or less spherical, vesicular acini whose 
average diameter is about a fortieth millimeter. They are lined by flattened, 
nonciliated, slightly granular cells. From the gland a median duct passes pos- 
teriorly to open on the dorsal outer surface. 
This gland appears in the young ship-worm soon after attachment as a single 
median small vesicle of apparently ectodermal derivation. As the animal grows, 
