> Phylum Hemichordata > Class
learn only 3 things about them ...
are rarely seen above ground.
are advanced worms and play a role in recycling nutrients.
They are very delicate. Don't dig them up!
Evidence of this curious animal is commonly seen on many of our shores.
On sand bars, in sandy areas especially near seagrasses. The animal
itself is almost never seen above ground. All that most people see
is its coiled, grey cast (although many may use a less polite word).
Sometimes if you are lucky, you might spot a bit of its backside poking
out of the ground as it creates its cast. Acorn worms are delicate
and almost certain to disintegrate if they are dug up. So please don't
try to dig them up.
What are acorn worms? Acorn worms
are unsegmented worms belonging to Phylum
Hemichordata. There are about 70 species of acorn worms. They are
quite different from the segmented worms that we are more familiar
with such as earthworms and bristleworms.
These belong to Phylum Annelida.
Features: 9-45cm long, some reaching
over 2m long! They are usually slimy brown or pink worms. Acorn worms
are burrowing animals. They live in shallow waters, in mucus-lined
burrows in sandy or muddy bottoms. Some just hide under stones and
An acorn worm's body is made up of three sections: the proboscis,
collar and trunk. The proboscis is the front end of the animal. It
is short and conical (looking much like an acorn) and is used to collect
food and to burrow. The proboscis is connected by a short stalk to
the collar. The collar is narrow and contains the mouth.
Most of the acorn worm is made up of a long trunk that has gill slits.
Oxygenated water is drawn into through the mouth and expelled through
these gill slits. In this way, the acorn worm breathes with part of
its gut! 'Enteropneusta' means 'gut breathing'.
The acorn worm's skin is densely covered with cilia (tiny hairs) and
glands which secrete a mucus that covers the body. Some produce a
bromide compound that gives them a medicinal smell and might protect
them from bacteria and predators.
Worms with gills like fishes?
Acorn worms are considered more highly specialised and advanced than
other worm-like creatures. They have a complex respiratory and circulatory
system with a heart that also functions as a kidney. They have a gill-like
structure similar to primitive fish and are thus sometimes considered
a link between vertebrates and invertebrates. Their larvae appear
similar to those of echinoderms and thus suggest that vertebrates
and echinoderms have a common ancestor.
What do they eat? Acorn worms
swallow mud and sand and process these for edible bits. At low tide,
they stick out their rear ends at the surface and excrete coils of
processed sediments. Called the cast, this is all that most people
will see of an acorn worm!
Acorn babies: Acorn worms have
separate genders that release eggs and sperm into the water for external
fertilisation. In some, eggs develop into free-swimming larvae that
look very similar to echinoderm larvae. These eventually settle down
and change into tiny acorn worms. Here is a photo
of an acorn worm larvae on Image Quest 3-D Marine Library; and
photo of the acorn worm larvae (tonaria)
one was seen in a pool of water.
Chek Jawa, Aug 02
back end of this worm may stick out
of its burrow as it was creating its cast.
Chek Jawa, Apr 04
worm must obviously manipulate its butt
in order to 'build' such a neat coil!
Chek Jawa, Jul 03
Changi, Apr 09
worms on Singapore shores
- Edward E.
Ruppert, Richard S. Fox, Robert D. Barnes. 2004.Invertebrate
Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963
Jan A., 2005. Biology
of the Invertebrates.
5th edition. McGraw-Hill Book Co., Singapore. 578 pp.