Download The Phylum Annelida

The Phylum Annelida
Etymology:- From the Latin Annellus a little ring.
Characteristics of Annelida:1)Bilaterally symmetrical and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a true coelom, often divided by internal septa.
4)Body possesses a through gut with mouth and anus.
5)Body possesses 3 separate sections, a prosomium, a trunk and a
pygidium.
6)Has a nervous system with an anterior nerve ring , ganglia and a
ventral nerve chord.
7)Has a true closed circulatory system.
8)Has no true respiratory organs.
9)Reproduction normally sexual and gonochoristic or hermaphoditic.
10)Feed a wide range of material.
11)Live in most environments
1
The Annelida

are a medium sized phylum of more than 9,000 species of
worms.

Most species prefer aquatic environments, but there are also
a number of well know terrestrial species.

Only a few species of annelids are commonly known to
human beings, these include Earthworms that work so hard
to make our soils healthy,


In many countries people are still familiar with Medicinal
leeches, and people who live closer to nature are naturally
more familiar with a much wider range of Annelids than
those who live in cities.
The Annelida

The earthworms, of which there are many species, are
exceedingly important in soil creation, particularly in
temperate areas.

Without them, agriculture and perhaps the whole of human
society as we know it would never have evolved.

Like so much of the unnoticed invertebrate world
earthworms are essential to our very existence.

In marine environments the numerous species of Polychaetes
play a fundamentally important role in the maintenance of
food chains and the whole ecological balance of the seas,
thus supporting the seemingly endless stocks of fish we like
to eat
2
The Annelida

Annelids range in size from the Giant
Earthworms, of which Michrochaetus rappi is the
largest, this magnificent animal has an average
length of 1.36 m (54 ins) and a record breaking
specimen has been recorded that measured 6.7
metres (22 ft) in length.

The smallest Annelid known to science is
Chaetogaster annandalai which is full grown at 0.5
mm (0.02 ins).
3
Characteristics

Annelids have two main modes of existence, they either live rather quietly in holes or
they live more active lives.

The basic Annelid body plan is one of a head followed by a long thin body of numerous
similar segments ending in a small tail.

The head consists of a mouth (prostomium) and sometimes a peristomium , and the tail
is more correctly called a pygidium, as it is not really a tail.

Annelids are coelomate animals meaning they have a true coelom within their body.

They have sets chaetae attached to each body segment, and these can be simple and small
as in the Earthworms or complex and varied as in many Polychaetes.

The head is often reduced and difficult to distinguish in the hole living species, but may be
easily recognised, with eyes and other sensory devices in those species living a more
active life.
coelomate animals

Annelids are coelomate animals (meaning they have a true coelom, even if
this is reduced secondarily).

They normally have long thin bodies composed of a series of identical
segments.

These segments lie between the head, comprised of a prostomium, a
mouth and sometimes a peristomium, and a tail called a pygidium.


Growth occurs both laterally, by enlargement of the segments during the
juvenile stages, and through the addition of new segments.

New segments are produced by the foremost section of the pygidium.

In some species they are produced throughout the animals life but in many
species production stops once a certain set number of segments has been
achieved.
4
The Phylum Annelida
Is divided into 3 classes, one of which the Clitellata could
really be called a Superclass, it contains three subclasses:
 The Oligochaeta


the Branchiobdella

The Hirundinea.

a.
The other two classes are
the Polychaeta which contains the largest number of
species
a.
The Aelosomatida which contains very few species
Scientific or biological
classification
Class:Polychaeta
Class;
Aelosomatida
class:Alitellata
Subclasses
Oligochaeta
Branchiobdella
Hirundinea
5
1-The class Polychaeta

(Poly = many, Chaeta = bristle) are the most diverse and
most speciose group of the Annelida containing over 5,500
species.

They are predominantly marine animals and are divided
ecologically into the Errantia and the Sedentaria

It is useful as it divides the class in two , in terms of the
number of families each group contains.

The Errantia have well developed heads and complex
parapodia (paddles)that they can use for swimming.

They are often dorsoventrally flattened.
6
1-The class Polychaeta

Most polychaetes are gonochoristic (meaning they are either
male or female)

Some are sequential hermaphrodites (meaning they are one
sex first and then change to being the other sex).

Reproduction is often accompanied by the production of
special modified reproductive segments which may, or may
not, become independent of the parent worm before mating.

These segments are destroyed or die during or immediately
after they have released their gametes (sperm and ova).
7
The class Polychaeta
2-The class Aelosomata

contains about 25 species of small to
minute worms with many chaetae.

They live in the interstitial zone of both
fresh and salty water environments.

They are hermaphrodites with each
animal possessing one ovary and two
testis.
8
The class Clitellata

They are normally hermaphrodites, and
possess a clitellum as adults, an organ
which looks like a bandage of skin
wrapped around the animal.

This clitellum, from which the whole
group takes its name has an important
function in sexual reproduction,
otherwise reproduction may asexual by
fission (division).
1-The subclass Oligochaeta

(Oligo = few, Chaeta = bristle) are the second
most numerous group of annelids with around
3,100 species.

Oligochaeta live in marine, freshwater and
terrestrial habitats.

Generally they have a more rounded crosssection

A less distinct head and are less diverse in form
than the Polychaetes.
9
2-The subclass Branchiobdella
 contains 147 species of small (about 1 cm long) aquatic whitish
animals that are either commensals or parasites on Crayfish.
 They are mostly found in the northern hemisphere.
 Different species attach to their hosts at different places on the
body, thus Branchiobdella parastica attaches to the under side of the
abdomen while Branchiobdella astaci attaches to its hosts gills.
 Branchiobdella hexodonta and B. astaci are known to be parasitic
feeding off host tissue

but B. parasitica is thought by some authorities to be a commensal
2-The subclass Branchiobdella

contains 147 species of small (about 1 cm long) aquati
whitish animals .

that are either commensals or parasites on Crayfish.


They are mostly found in the northern hemisphere.

Different species attach to their hosts at different places on
the body, thus Branchiobdella parastica attaches to the under
side of the abdomen while Branchiobdella astaci attaches to
its hosts gills.

Branchiobdella hexodonta and B. astaci are known to be
parasitic feeding off host tissue but B. parasitica is thought by
some authorities to be a commensal
10
3-The subclass Hirundinea

contains the 500 or so species of animals commonly
known as leeches.

Leaches are well known for their blood sucking habits
and their head to tail looping mode of locomotion.

Except for the primitive Acanthobdella peledina
leeches have no chaetae and 33 body segments.

they have two suckers which in most cases are
located one at the anterior (head) end of the body
composed of segments 1-4 and the other at the
posterior (tail) end composed of segments 25-33.
11
Leaches

Like the Oligochaeta from which they are
believed to have evolved the Hirundinea
occur in Fresh water, marine and
terrestrial environments
12
Anatomy
Septum
Muscle
layer
segments
13
Cross section
Anatomy

Annelids are triploblastic protostomes with a
coelom ,closed circulatory system, and true
segmentation.

Protosomes are animals with bilaterial symmetry
where the first opening in development, the
blastophore, becomes its mouth.

Triploblastic means that they have three primary
tissue areas formed during embryogenesis.

A coelom is a fluid-filled body cavity.
14
For drawing
Oligochaetes and polychaetes
 Oligochaetes and polychaetes typically have spacious coeloms; in
leeches, the coelom is largely filled in with tissue and reduced to a
system of narrow canals; archiannelids may lack the coelom entirely
 The coelom is divided into a sequence of compartments by walls
called septa.

In the most general forms, each compartment corresponds to a
single segment of the body, which also includes a portion of the
nervous and (closed) circulatory systems, allowing it to function
relatively independently.
 Each segment is marked externally by one or more rings, called
annuli.
15
Oligochaetes and polychaetes

Each segment also has an outer layer of circular muscle
underneath a thin cuticle and epidermis, and a system of
longitudinal muscles.

In earthworms, the longitudinal muscles are strengthened by
collagenous lamellae; the leeches have a double layer of
muscles between the outer circulars and inner longitudinals.


In most forms, they also carry a varying number of bristles,
called setae, and among the polychaetes a pair of appendages,
called parapodia.
16
Digestive system
17
Oligochaetes and polychaetes

Anterior to the true segments lies the prostomium and
peristomium, which carries the mouth, and posterior to
them lies the pygidium, where the anus is located.
The digestive tract is quite variable but is usually specialized.
For example, in some groups (notably most earthworms) it has
a typhlosole (internal fold of the intestine or intestine inner
wall), to increase surface area, along much of its length.


Different species of annelids have a wide variety of diets,
including active and passive hunters, scavengers, filter feeders,
direct deposit feeders that simply ingest the sediments, and
blood-suckers.
Nervous and Digestive systems
18
Oligochaetes and polychaetes

The vascular system and the nervous system are separate
from the digestive tract.

The vascular system includes a dorsal vessel conveying the
blood toward the front of the worm, and a ventral
longitudinal vessel that conveys the blood in the opposite
direction.

The two systems are connected by a vascular sinus and by
lateral vessels of various kinds, including in the true
earthworms, capillaries on the body wall.

The nervous system has a solid, ventral nerve cord from
which lateral nerves arise in each segment.
Circulatory system
hearts
19
Excretory system
20
Excretory system
Reproductive system
21
Reproduction

Depending upon the species, annelids can reproduce both
sexually and asexually.

Asexual reproduction

Asexual reproduction by fission is a method used by some
annelids and allows them to reproduce quickly.



The posterior part of the body breaks off and forms a new
individual.
The position of the break is usually determined by an
epidermal growth ,for example, to reproduce by the body
breaking into such fragments.
22
Reproduction
 Earthworms and other oligochaetes, as well as the leeches, are
hermaphroditic and mate periodically throughout the year in
favored environmental conditions.
 They mate by copulation.
 Two worms, which are attracted by each other's secretions, lay
their bodies together with their heads pointing in opposite
directions.
 The fluid is transferred from the male pore to the other worm.
 Different methods of sperm transference have been observed in
different genera, and may involve internal spermathecae (sperm
storing chambers) or spermatophores that are attached to the
outside of the other worm's body.
23
Reproduction





Sexual reproduction allows a species to better
adapt to its environment.
Some annelida species are hermaphroditic, while
others have distinct sexes.
Most polychaete worms have separate males and
females and external fertilization.
The earliest larval stage, which is lost in some
groups, is a ciliated trochophore, similar to those
found in other phyla.
The animal then begins to develop its segments,
one after another, until it reaches its adult size.
24
Relationships

The arthropods and their kin have long been
considered the closest relatives of the annelids on
account of their common segmented structure.

However, a number of differences between the
two groups suggest this may be convergent
evolution rather than a feature passed on by
common descent.

The other major phylum that is of definite
relation to the annelids is the mollusk, which
shares with them the presence of trochophore
larvae.
Fossil record

The annelid fossil record is sparse, but a
few definite forms are known as early as
the Cambrian.

There are some signs they may have been
around in the later Precambrian.

Because the creatures have soft bodies,
fossilization is an especially.
25
Fossil record

The annelid fossil record is sparse, but a
few definite forms are known as early as
the Cambrian.

There are some signs they may have been
around in the later Precambrian.

Because the creatures have soft bodies,
fossilization is an especially.
26
27