3. The penis worms (Priapulida)
are a group of marine invertebrates
named for their penis-like shape. All
species of the tubular animals, which
can reach lengths of 39 cm, have an
extensible mouth called a proboscis
that is lined with sharp hooks, teeth,
and spines. Only 20 species of
priapulid are known today, a shadow
of their diverse past, which extends
back for well over 500 million years.
Not commonly seen, the priapulids
have attracted little attention from the
zoology community as a whole.
5. Spines of various sizes and
shapes cover the entire surface of the
introvert. Inside the introvert is a
muscular feeding tube armed with
teeth. The animals use a pair of
muscles to pull the introvert
completely into the trunk.
6. The trunk houses the internal
body organs, in particular the
digestive system and reproductive
organs. The body is filled with fluid
that serves as a skeleton to support
the body when the muscles of the
body wall contract. When priapulans
move, the fluid moves around in the
body cavity and serves the functions
of circulation, waste removal, and
respiration. The tail is continuous with
the body cavity of the trunk and may
be used for respiration.
7. Diet and Feeding Behaviour
Priapulida feed on
slow-moving invertebrates,
such as polychaete worms.
8. With the introvert fully
extended, priapulans grasp prey with
their teeth and rapidly roll it inward.
Priapulans depend on their fluid
skeleton for movement. They use
their extended introvert and the
muscles of the front part of their trunk
to anchor themselves in the sand.
Once anchored, priapulans can pull
themselves through the sand by
contracting their body wall muscles.
9. Reproduction
Priapulans have separate sexes.
They release their eggs and sperm into the
water, where fertilization, or the uniting of
egg and sperm to start development, takes
place and larvae develop. Larvae are
animals in an early stage that change form
before becoming adults.
11. Priapulans live in sand and
mud in all oceans at all depths.
Larger priapulans live in colder
waters. Small priapulans are most
common in shallow tropical waters.
They can live in marine sand and
mud in long, U-shaped burrows, but
some species also live between
rocks.
13. Discovery of a Specie
Discovered in 1973, is perhaps the most
interesting priapulid. Named after the leaders of an ancient
Jewish rebel army, Maccabeus is a midget amongst its
kind—just 1.4 to 2.8 mm and it lives in a tiny, gelatinous
tube in the seabed. What’s really odd about this little
animal is the way it catches its prey—smaller animals,
such as microscopic crustaceans, etc. Much of the body of
Maccabeus is concealed in its tube with the head end lying
flush with the sea bed and its crown of spines poking up
into the water—forming a trap to snare prey. When an
unfortunate animal blunders into this trap, the spines close
over to form a cage from which there is no escape.
Trapped, the prey is sucked into Maccbeus’ mouth and
eaten.
14. Distinct Characteristics
Although rarely seen by casual observers,
priapulids can be very abundant in some places and
densities of 85 adults and 58,000 larvae per square meter
have been recorded. They even seem to thrive in anoxic
sediments (depleted of oxygen) suffused with eggy, highly
toxic hydrogen sulphide—a habitat that few other animals
can tolerate. Not only can they be surprisingly abundant,
but they, and other very poorly known animal lineages,
once collectively known as “lesser lineages,” are
fundamental to our understanding of animal evolution as a
whole, yet we have only scratched the surface of their
biology and ecology.
