Glands in the vestimentum secrete the tube, increasing its length at the upper end.
Its wall is leathery, a combination of ∼25% chitin and the remainder protein. The
bottom end is closed. There can be septa closing off older, unoccupied sections, so
probably tube material can be secreted by epidermis well posterior to the
vestimentum. Tube growth and worm growth are extremely fast; tube extension was
estimated from photographic series by Lutz (1994) as 85 cm yr−1, and by others from
changes in size-modes over a few weeks as twice that. In either case, it is the most
rapidly growing large invertebrate. There are two posterior sections of the worm, the
trunk and a short, spinose anchor bulb called the opisthosome. If the plume is nipped
by a crab or poked by a submarine arm, the plume can be rapidly retracted into the
protective tube.
(^) In Riftia (Fig. 15.3) and its relatives, symbiotic bacteria live in a large organ in the