developed into an operculum-like organ, smooth and of horny texture,
which closes the narrow end of the tube. The other extremity is more
elaborately guarded, the anterior segment being fringed with a frontal
membrane, while the second segment forms a disc, the minute mouth orifice
with the true tentacles and gills being debased to the third segment.

Confronted by danger, the animal closes its front door by retracting
until the disc presses immovably against the circumference of the tube,
the retraction being so sudden that a frail spurt betrays the whereabouts
of an otherwise secret dwelling-place. In the centre of the disc is the
first segment, from which the frontal fringe is extended in the form of
an array of keen bristles as a defensive weapon. With the lid at one end
and the armed disc at the other the animal enjoys security and comfort,
and when unsuspicious the "shoulders" protrude, the head meekly
following. The tentacles are serrate and glitter like tinsel, possibly
for the fascination of the minute forms of life which the tube-dweller
consumes. To enable it to retract and emerge quickly the animal is
provided with a series of tufts of bristles on the back and on the
ventral surface of the body with a row of toothed "pads," which fulfil
the dual office of grapplers and feet.

With what skill and patience does this pectinarian construct its ornate
habitation! How artfully does it pick and choose among the tiny shells
and grit! With what rare discretion rejects the unfit, and with what
satisfaction retains a neat and dainty item of building material! How
deftly does it arrange its courses and bonds, cementing each fragment in
its place until a perfect cylinder, proportionate in dimensions,
uniformly expanding in circumference, smooth within, rugged without,
scientifically correct in design, is the result! How apt, too, the
frictionless lining! And all this laborious neatness and precision