A few additional comments on glass sponges.  First, as Tom Watters
indicated - but it might be well to state it outright - these are not
mollusks.  They are members of an entirely different phylum - the
Porifera - sponges.  The "watering pots" mentioned by Jr in his
inquiry, and the glass sponges are both elongate and tubular in
shape, and both have a perforated plate covering their upper end.
But they are not related at all.  Watering pots are bivalve mollusks.
 
Most sponges have tiny needlelike spicules of either calcium or
silica embedded in their body wall.  The shape and chemistry of these
spicules is an important characteristic used in classifying sponges.
In the glass sponges, the silica spicules are fused together into a
rigid latticework that resembles lace.  Some of them are quite
beautiful, and they are surprisingly strong considering that they
weigh almost nothing.  Also, as you might expect, being made of fused
silica (essentially natural "fiberglass") they are very chemically
resistant, and in spite of their delicate appearance can be safely
soaked in bleach or other chemicals to clean them (other kinds of
sponges would dissolve or disintegrate).  Most glass sponges are deep
water species, usually dredged at depths of 800 to 1500 meters, and
some have been taken at depths of 3 miles.
The story of the commensal shrimp mentioned by Tom is also
interesting.  An immature male and female shrimp are just the right
size to enter the tubular body of the sponge through one of the
natural holes in the lacelike skeleton.  But, once they mate, spawn
and molt, they are too large to get back out through the same holes.
So they spend the rest of their lives together in a "glass cage",
subsisting on plankton brought in by the feeding currents of the
sponge.  In old Japan, a glass sponge with the pair of enclosed
shrimp was traditionally given as a wedding gift, symbolizing the
permanence of marriage.
Paul M.
Rhode Island