Phylum Phoronida (Phoronids)
- Bilaterally symmetrical; lophophore at anterior end.
- Pelagic larvae.
- Small sedentary worms which exist in chitinous tubes.
- Exist on benthic substrate; suspension feeders.
- Protostomate; even still, the cleavage of the eggs is
radial and indeterminate.
- Tripartite body plan, each region has its own separate
body cavity "ologomeric".
- Vascular system with haemoglobin and two excretory organs.
- Silmutaneous hermaphrodites, occasional gonochoristic species.
Phoronids, commonly called horseshoe worms, are sessile
organisms that live exclusively in marine environments. They are
attached to a substrate and live in a chitin-like tube that is made
from secretions in their earlier strages of life. These tubes
eventually become decorated with debris and gives the horseshoe worm camouflage. The body is composed of three regions.
The first region of the tripartite body
is the prosome which is small and overhangs the mouth. The second
region, the mesosome, is small but contains the large lophophore, the
mouth, and the mesocoel which is used as the lophophoral hydraulic
system. The last and largest is the metasome which contains the other
body organs and the anus. Most phoronids are hermaphroditic and their
sex cells exit through the excretory ducts. The egg hatches into a
pelagic larva called an actinotrocha. The larva already has a gut and
tentacles present. It then settles to the bottom where it starts to
secrete its chitin-like tube.
Some Interesting Facts:
- Phoronids attach to pilings, build tubes in soft benthic
substrate, or burrow into calcareous rocks and shells in shallow
- They feed by means of a lophophore. This structure is a
specialized mass of tentacles. The lophophore drives a current of
water down to the tentacles and food particles are then rolled in
mucus and passed on to the mouth.
- Phoronids never leave their tube. The tentacles are thrusted
outwards for feeding on the falling nutrients. When the
animal is alarmed, the tentacles are quickly pulled into the tube for safety.
Some Pertinent Books Found in Lamson Library:
- Bertin, Leon. 1976. Larousse Encyclopedia of Animal
Life. New York: The Hamlyn Publishing Group Limited.
- Banister, Dr. Keith and Dr. Andrew Campbell. 1985. The
Encyclopedia of Aquatic Life. New York: Facts on File.
(Jon D. Podgorni)
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