The process of building the mud tubes of the Sabellæ is a
very interesting one. It is performed, according to my own
observations,[148] mainly by means of the gill-filaments and their
pinnal grooves. The filaments are bent-over, till the inner or grooved
face comes in contact with the soft mud on which the animal is lying,
when the sensitive pinnæ close on a minute portion of the mud, taking
it up in a pellet, which is then fashioned by the form of the groove;
the filament is now erected, and the pellet, passing down the groove
to the bottom by means of the cilia, is delivered to the care of two
delicate moveable organs, like leaves or flaps, which place it on the
edge of the tube, and then shape and mould it, smoothing both surfaces.
Doubtless, either from these organs, or from some other part of the
circumjacent region, the glutinous secretion is at the same time poured
out, which consolidates the mud, and forms the true basis of the tube.