22 – Worms
259
ignored by predatory fish as their body tissue often contain toxins. The bright colours may also warn potential
predators that they are poisonous to eat. Others mimic
other poisonous animals so that while they are not themselves poisonous the animal that they are mimicking is.
While flatworms are hermaphrodites they appear
not to self fertilise. When two mature individuals meet,
one slowly glides over the other and raises the front
half of the body. A penis armed with a hypodermiclike stylet is pushed into the body wall of the other
worm and the sperm are injected into the animal, they
then move through the body to find an egg in the oviduct where fertilisation occurs. In some other species
the worms copulate tail to tail and the sperm are delivered into the female part of the reproductive system,
being released into the oviduct as required. Fertilised
eggs are placed in a capsule that is attached to the substratum and after a while miniature adults are produced, which just creep away from the capsule that
then disintegrates.
Flatworms are abundant and diverse on the GBR,
and range in size from very small (less than 1 mm) to
several centimetres in length. Colour patterns are useful in species identification, although these are often
lost when the material is preserved. Identification from
good quality coloured photos is often possible, although
many species still await formal description.
N SIPUNCULANS
Sipunculans are soft-bodied, unsegmented coelomate
worm-like animals, commonly called peanut worms
(Fig. 22.4G). They consist of a muscular trunk or body
and an anteriorly placed, more slender introvert that
bears the mouth at the anterior extremity. The introvert
is highly elastic, capable of considerable extension; at
other times it is partially or completely retracted within
the body cavity. The mouth leads to a long, recurved,
spirally wound alimentary canal that lies within the
coelom. Tentacles either surround the mouth or are
associated with it. Chaetae are absent. One or two pairs
of nephridia are present. The body wall is usually thick
and often rubbery and has well developed circular and
longitudinal muscles and occasionally a thin layer of
oblique or diagonal muscle is also present. The skin
often contains pigmented papillae and the surface of
the introvert may be covered in hooks. Around the
anus a caudal shield or calcareous knob or cone is often
present. Sexes are separate, fertilisation is external, and
a free swimming trochophore larva is produced.
While sipunculans are typically regarded as sedentary, some species, especially those that burrow in sand
or silt, show considerable activity when they dig. When
threatened, most species are able to rapidly retract their
sensitive and probing introvert. Species respire through
their body wall and tentacles, and cells in both the vascular and coelomic systems contain a red respiratory
pigment, haemerythrin, which only gives up its oxygen at a very low partial pressure—much lower than
that commonly found in sea water. It has been suggested that it functions only in adverse situations, such
as may occur on intertidal reef flats on the reef especially during low tides in the middle of the day during
summer.
Sipunculans are abundant on the reef, although not
often seen unless sediment samples are collected or
reef rock broken off (Fig. 22.4H). They live in characteristic shaped burrows in reefal substrate or in tubes in
the sediment; others bore into solitary corals or inhabit
empty gastropod shells. The solitary coral, Heteropsammia michelini, commonly contains the sipunculan Aspidosiphon muelleri. Rock boring species secrete an
acid-like substance and after settling as larvae metamorphose and bore into the substrate, forming a flaskshaped burrow where they are effectively entombed
whilst retaining a narrow passage to the exterior
through which they obtain their oxygen and release
their gametes. They are detritus feeders, using their
tentacles and extensible introverts to collect algae, sediment and detritus. Sipunculans themselves are selectively fed upon by some molluscs and humans eat them
in selected localities in the Indo-Pacific (although
whether they were eaten by Australian Aborigines is
not known).
Six families of sipunculans are known and numerous genera and species have been recorded from the
GBR but many remain to be described. They appear to
have phylogeographic affinities with other Indo-Pacific
species. Juveniles are difficult to identify as they lack
many of the adult characteristics.
259
ignored by predatory fish as their body tissue often contain toxins. The bright colours may also warn potential
predators that they are poisonous to eat. Others mimic
other poisonous animals so that while they are not themselves poisonous the animal that they are mimicking is.
While flatworms are hermaphrodites they appear
not to self fertilise. When two mature individuals meet,
one slowly glides over the other and raises the front
half of the body. A penis armed with a hypodermiclike stylet is pushed into the body wall of the other
worm and the sperm are injected into the animal, they
then move through the body to find an egg in the oviduct where fertilisation occurs. In some other species
the worms copulate tail to tail and the sperm are delivered into the female part of the reproductive system,
being released into the oviduct as required. Fertilised
eggs are placed in a capsule that is attached to the substratum and after a while miniature adults are produced, which just creep away from the capsule that
then disintegrates.
Flatworms are abundant and diverse on the GBR,
and range in size from very small (less than 1 mm) to
several centimetres in length. Colour patterns are useful in species identification, although these are often
lost when the material is preserved. Identification from
good quality coloured photos is often possible, although
many species still await formal description.
N SIPUNCULANS
Sipunculans are soft-bodied, unsegmented coelomate
worm-like animals, commonly called peanut worms
(Fig. 22.4G). They consist of a muscular trunk or body
and an anteriorly placed, more slender introvert that
bears the mouth at the anterior extremity. The introvert
is highly elastic, capable of considerable extension; at
other times it is partially or completely retracted within
the body cavity. The mouth leads to a long, recurved,
spirally wound alimentary canal that lies within the
coelom. Tentacles either surround the mouth or are
associated with it. Chaetae are absent. One or two pairs
of nephridia are present. The body wall is usually thick
and often rubbery and has well developed circular and
longitudinal muscles and occasionally a thin layer of
oblique or diagonal muscle is also present. The skin
often contains pigmented papillae and the surface of
the introvert may be covered in hooks. Around the
anus a caudal shield or calcareous knob or cone is often
present. Sexes are separate, fertilisation is external, and
a free swimming trochophore larva is produced.
While sipunculans are typically regarded as sedentary, some species, especially those that burrow in sand
or silt, show considerable activity when they dig. When
threatened, most species are able to rapidly retract their
sensitive and probing introvert. Species respire through
their body wall and tentacles, and cells in both the vascular and coelomic systems contain a red respiratory
pigment, haemerythrin, which only gives up its oxygen at a very low partial pressure—much lower than
that commonly found in sea water. It has been suggested that it functions only in adverse situations, such
as may occur on intertidal reef flats on the reef especially during low tides in the middle of the day during
summer.
Sipunculans are abundant on the reef, although not
often seen unless sediment samples are collected or
reef rock broken off (Fig. 22.4H). They live in characteristic shaped burrows in reefal substrate or in tubes in
the sediment; others bore into solitary corals or inhabit
empty gastropod shells. The solitary coral, Heteropsammia michelini, commonly contains the sipunculan Aspidosiphon muelleri. Rock boring species secrete an
acid-like substance and after settling as larvae metamorphose and bore into the substrate, forming a flaskshaped burrow where they are effectively entombed
whilst retaining a narrow passage to the exterior
through which they obtain their oxygen and release
their gametes. They are detritus feeders, using their
tentacles and extensible introverts to collect algae, sediment and detritus. Sipunculans themselves are selectively fed upon by some molluscs and humans eat them
in selected localities in the Indo-Pacific (although
whether they were eaten by Australian Aborigines is
not known).
Six families of sipunculans are known and numerous genera and species have been recorded from the
GBR but many remain to be described. They appear to
have phylogeographic affinities with other Indo-Pacific
species. Juveniles are difficult to identify as they lack
many of the adult characteristics.