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| The acorn worm is plentiful
on Chek Jawa but the animal itself is almost never seen above
ground. |
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'Enteropneusta'
means 'gut breathing'. There are about 70 species of
acorn worms.
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All that most people see of the worm is its coiled, grey cast on the
sand.
 Acorn
worms are burrowing worms. They live in shallow waters, many living
in mucus-lined burrows in sandy or muddy bottoms. Some just hide under
stones and shells. Acorn worms are delicate and almost certain to
disintegrate if they are dug up. So please don't try to dig them up.
Worm parts: 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! 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.
Acorn food: Acorn worms swallow
mud and sand and process these for detritus. 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!
Worms with gills like fishes?
Acorn worms are considered more highly specialised and advanced than
other worm-like creatures. They have a circulatory system with a heart
that also functions as a kidney. Acorn worms 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.
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
yet another
photo of the acorn worm larvae (tonaria) |
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Acorn worm cast |
See
also...
Worms of Chek Jawa.
Links
Enteropneusta
on the Canada's Aquatic Environments webpage on the University of Guelph
website: an easy introduction to the more technical aspects of their morphology,
metabolism, reproduction, ecology with an interesting section on their idiosyncracies.
Elusive
acorn worms are ocean's vacuum cleaners on SusanScott.net: an easy-to-read
introduction to the fascinating aspects of acorn worms of Hawaii and the
snails that eat them.
Enteropneusta
from Bio 136 on the University of California, Santa Cruz website: labelled
diagram of the worm
Photo
of adult acorn worm on the American Museum of Natural History website.
Other references
- Barnes, Robert
D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt
College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
- Pechenik, Jan
A., 2000. Biology of the Invertebrates. McGraw-Hill Book Co.,
Singapore. 578 pp.
- Tan, Leo W. H.
& Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore
Science Centre, Singapore. 160 pp. online
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