246
22. Worms
P. A. Hutchings
earthworms and leeches do form a single clade (the
clitellates) their relationship to the polychaetes is still
being debated. So, today the term ‘polychaetes’ is
widely used rather than the term ‘Annelida’ and this
group includes all the traditional polychaete families
as well as the leeches, earthworms, and the siboglinids,
sometimes known as ‘beard worms’. This latter group
until recently was considered either as one or two phyla
and included the Vestiminifera, which are restricted to
cold and hot vents, and the Pogonophora, or thin
worms found in sediments. But recent studies have
shown that these worms are closely related to each
other and represent a family within the polychaetes. To
date, no examples of siboglinids have been found in
the GBR, but some may occur in deeper waters off the
outer barrier reef. While it is widely accepted that the
leeches, earthworms, siboglinids and probably the
echiuroids and polychaetes are all closely related, their
relationships are still being debated and numerous
molecular and morphological studies have failed to
completely resolve these issues.
Polychaetes consist of two presegmental regions,
the prostomium and peristomium, a segmented trunk
and postsegmental pygidium (Fig. 22.1A). The body
wall consists of circular and longitudinal muscle layers
enclosing a body coelom. Usually they have a well
defined head with sensory and/or feeding appendages, followed by numerous body segments that may
The common name ‘worms’ can refer to any of several
groups of animals, including polychaetes (segmented
worms), nemerteans (ribbon worms), sipunculans
(peanut worms), echiuroids (spoon worms), myzostomes, nematodes (that may be free living or parasitic),
and the parasitic worms, the platyhelminthes, which
includes the cestodes (tapeworms), trematodes (liver
flukes) and the free living turbellarians (flatworms). All
these groups are abundant on the Great Barrier Reef
(GBR), but the amount of information that is known
about them varies according to the group. Each of these
groups will now be discussed in terms of their diversity, where they occur on the reef and their feeding and
reproductive ecologies. At the end of this chapter a
series of references are given that will facilitate the
identification of each group and provide additional
information about them.
N POLYCHAETES
Polychaetes are the most diverse and abundant of the
worm groups and originally were referred to as a class
within the Phylum Annelida, together with the earthworms (Class Oligochaeta) and the leeches (Class
Hirudinea), and this classification will be found in
many textbooks. Recent studies (both morphological
and molecular) on the annelids have failed to show
that they are a monophyletic group, and while the
22. Worms
P. A. Hutchings
earthworms and leeches do form a single clade (the
clitellates) their relationship to the polychaetes is still
being debated. So, today the term ‘polychaetes’ is
widely used rather than the term ‘Annelida’ and this
group includes all the traditional polychaete families
as well as the leeches, earthworms, and the siboglinids,
sometimes known as ‘beard worms’. This latter group
until recently was considered either as one or two phyla
and included the Vestiminifera, which are restricted to
cold and hot vents, and the Pogonophora, or thin
worms found in sediments. But recent studies have
shown that these worms are closely related to each
other and represent a family within the polychaetes. To
date, no examples of siboglinids have been found in
the GBR, but some may occur in deeper waters off the
outer barrier reef. While it is widely accepted that the
leeches, earthworms, siboglinids and probably the
echiuroids and polychaetes are all closely related, their
relationships are still being debated and numerous
molecular and morphological studies have failed to
completely resolve these issues.
Polychaetes consist of two presegmental regions,
the prostomium and peristomium, a segmented trunk
and postsegmental pygidium (Fig. 22.1A). The body
wall consists of circular and longitudinal muscle layers
enclosing a body coelom. Usually they have a well
defined head with sensory and/or feeding appendages, followed by numerous body segments that may
The common name ‘worms’ can refer to any of several
groups of animals, including polychaetes (segmented
worms), nemerteans (ribbon worms), sipunculans
(peanut worms), echiuroids (spoon worms), myzostomes, nematodes (that may be free living or parasitic),
and the parasitic worms, the platyhelminthes, which
includes the cestodes (tapeworms), trematodes (liver
flukes) and the free living turbellarians (flatworms). All
these groups are abundant on the Great Barrier Reef
(GBR), but the amount of information that is known
about them varies according to the group. Each of these
groups will now be discussed in terms of their diversity, where they occur on the reef and their feeding and
reproductive ecologies. At the end of this chapter a
series of references are given that will facilitate the
identification of each group and provide additional
information about them.
N POLYCHAETES
Polychaetes are the most diverse and abundant of the
worm groups and originally were referred to as a class
within the Phylum Annelida, together with the earthworms (Class Oligochaeta) and the leeches (Class
Hirudinea), and this classification will be found in
many textbooks. Recent studies (both morphological
and molecular) on the annelids have failed to show
that they are a monophyletic group, and while the