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Phylum Annelida: summary of
Phylum Annelida: summary of
characteristics
characteristics
Name from Latin a
Name from Latin a nnulus
nnulus meaning a ring.
meaning a ring.
Vermiform. Possess tissues and organs.
Vermiform. Possess tissues and organs.
Muscular gut with mouth and anus.
Muscular gut with mouth and anus.
Body divided into segments.
Body divided into segments.
Outer epithelium with clumps of bristles (except in forms
Outer epithelium with clumps of bristles (except in forms
with suckers). May be covered with a cuticle.
with suckers). May be covered with a cuticle.
Body wall muscular with both circular and longitudinal
Body wall muscular with both circular and longitudinal
muscles.
muscles.
Closed circulatory system.
Closed circulatory system.
Nervous system with supraoesophageal ganglion,
Nervous system with supraoesophageal ganglion,
circum-oesophageal ring and ventral nerve cord.
circum-oesophageal ring and ventral nerve cord.
Nephridia responsible for most excretion
Nephridia responsible for most excretion
Phylum Annelida
Phylum Annelida
The annelids (L.
The annelids (L. annelus
annelus : a little ring) are the
: a little ring) are the
segmented worms.
segmented worms.
Annelids are coelomate, protostomes and the
Annelids are coelomate, protostomes and the
body is metameric being composed of serially
body is metameric being composed of serially
repeated segments or metameres.
repeated segments or metameres.
Each segment is separate from the next
Each segment is separate from the next
segments being divided by partitions or septa.
segments being divided by partitions or septa.
Segmentation
Segmentation
The evolution of segmentation is the great
The evolution of segmentation is the great
evolutionary innovation of the annelids.
evolutionary innovation of the annelids.
Segmentation allows annelids to make
Segmentation allows annelids to make
more precise body movements than
more precise body movements than
organisms that have a hydrostatic
organisms that have a hydrostatic
skeleton, but lack segmentation e.g. the
skeleton, but lack segmentation e.g. the
pseudocoelomate nematodes.
pseudocoelomate nematodes.
Segmentation
Segmentation
Because the coelom is divided by septa the
Because the coelom is divided by septa the
force of muscle contraction in a segment is not
force of muscle contraction in a segment is not
transmitted throughout the body, but instead is
transmitted throughout the body, but instead is
confined to the single segment.
confined to the single segment.
Thus, one segment may elongate while the
Thus, one segment may elongate while the
adjacent one contracts and this allows the
adjacent one contracts and this allows the
animal to make fine, controlled movements.
animal to make fine, controlled movements.
Movement
Movement
Because they have fine control of movement
Because they have fine control of movement
annelids have evolved a relatively sophisticated
annelids have evolved a relatively sophisticated
nervous system.
nervous system.
Most annelids are burrowing forms and as an
Most annelids are burrowing forms and as an
adaptation to this lifestyle bear short chitinous
adaptation to this lifestyle bear short chitinous
bristles called setae on each segment. The
bristles called setae on each segment. The
setae enable the annelid to gain traction against
setae enable the annelid to gain traction against
the side of the burrow.
the side of the burrow.
Movement
Movement
In other annelids longer hair-like setae
In other annelids longer hair-like setae
assist the animal in swimming.
assist the animal in swimming.
For the annelids that live in burrows or in
For the annelids that live in burrows or in
tubes the setae help to prevent the animal
tubes the setae help to prevent the animal
from being pulled out.
from being pulled out.
Annelids
Annelids
The typical annelid body has a two part head
The typical annelid body has a two part head
made up of a
made up of a prostomium
prostomium and a
and a peristomium
peristomium , a
, a
series of segments, and a terminal
series of segments, and a terminal pygidium
pygidium
which contains the anus.
which contains the anus.
Neither the head nor the pygidium are
Neither the head nor the pygidium are
considered true segments. In growth, new
considered true segments. In growth, new
segments form anterior to the pygidium. If an
segments form anterior to the pygidium. If an
annelid is cut in two the posterior segments can
annelid is cut in two the posterior segments can
be regrown.
be regrown.
Figure 17.03a
Annelid Classification
Annelid Classification
There are approximately 12,000-15,
There are approximately 12,000-15,
species of annelids divided into 4 classes:
species of annelids divided into 4 classes:
Polychaeta: polychaete worms
Polychaeta: polychaete worms
Oligochaeta: earthworms
Oligochaeta: earthworms
Hirundinea: leeches
Hirundinea: leeches
Siboglinidae: pogonophorans
Siboglinidae: pogonophorans
Class Polychaeta
Class Polychaeta
The polychaetes are the largest of the
The polychaetes are the largest of the
annelid classes and include more than
annelid classes and include more than
10,000 described species, most of which
10,000 described species, most of which
are marine. Morphologically very diverse.
are marine. Morphologically very diverse.
The name “poly” “chaete” refers to the
The name “poly” “chaete” refers to the
numerous chaetae or bristles they
numerous chaetae or bristles they
possess.
possess.
Figure 17.03e
Figure 17.03c
11.3B
Figure 17.02b
Fanworms
Fanworms
Most of the sedentary polychaetes, which
Most of the sedentary polychaetes, which
inhabit burrows or build tubes, are filter
inhabit burrows or build tubes, are filter
feeders and consume plankton or detritus.
feeders and consume plankton or detritus.
Forms such as fanworms extend long,
Forms such as fanworms extend long,
modified feathery crowns of stiff prostomial
modified feathery crowns of stiff prostomial
tentacles to feed. Ciliary action draws in
tentacles to feed. Ciliary action draws in
food, which is trapped in mucus and
food, which is trapped in mucus and
delivered down grooves to the mouth.
delivered down grooves to the mouth.
