2. Introduction to Phylum Annelida:
The phylum Annelida exhibits great diversity of body form. They are
segmented worms and are thus distinguished from other worms like
flat worms, round worms and so on. They include the familiar
earthworms and leeches, in addition to a number of marine and
fresh water species. They vary in size ranging between < 1 mm and
the largest, the giant 3 m. long earthworms of Australia.
Latin: annellus or annelus, a diminutive of anulus, a ring.
3. Scheme of Classification of Phylum
The annelids were previously termed as “worms” and
were grouped under the old phylum Vermis. Cuvier in
1798 pointed out the fundamental differences and
separated them from vermis. Later in 1909, Lamarck
coined the name Annelida.There are about 15,000
living species which have been classified broadly into 3
main groups. The classification followed in this text is
according to the classificatory plan outlined by
Ruppert and Barnes, 1994 (6th edn.)
5. Diagnostic Features of Phylum
.11.Body vermiform and bilaterally symmetrical.
2. The body is metamerically segmented, the division of the body
into similar segments are arranged in a linear series along the
.23. The body cavity is true coelom, which is lined by a peritoneum
or coelomic epithelium and serves as a hydrostatic skeleton
against which muscles can contract.
4. Body cavity often sub-divided by transverse septa.
5. The body wall consists of an outer epithelium covered by a
cuticle and with epidermal bristles or chaetae in bundles or
6. Body wall muscular, often with complete circular muscle layers
and four blocks of longitudinal muscle.
6. 7. A pre-segmental prostomium containing a nervous
ganglion and a post-segmental pygidium is present.
8. Presence of a muscular gut with mouth and anus.
9. Closed blood circulatory system.
10. Excretion takes place by means of metamerically
arranged pairs of coiled tubes called nephridia.
11. The animals are often provided with coelomoducts
which are channels for the outward passage of
12. Nervous system consists of pre-segmental supra-
oesophageal ganglion, circumoesophageal ring and a
ventral nerve cord with segmental ganglia.
13. Development in marine forms are sometimes via a free-
living trochophore larva.
8. Size range and diversity of structure
The length of annelids varies from a
fraction of an inch to more than six
metres (about 20 feet). The width may
exceed 2.5 centimetres (about one inch)
in the contracted state. Free-moving
polychaetes and earthworms include the
largest species. Leeches attain lengths of
about 0.4 metre in the contracted state.
10. 1. Polychaetes:
This group has
only in size but
also in habits
11. polychaetes consists of a head,or prostomium, which may bear two or more eyes; a
preoral segment, with such appendages as antennae, tentacles, and palpi (fleshy
sensory projections); a trunk divisible into distinct segments; and a tail, or pygidium,
which may bear anal cirri (fleshy projections) or plaques and a terminal anus. Each
body segment following the second segment (peristome) usually has paired parapodia;
i.e., fleshy, lateral outgrowths used in feeding, locomotion, or breathing. The parapodia,
generally prominent in free-moving polychaetes, bear bundles of setae, which can be
extended, and aciculae (needlelike structures), which are used for support.The heads
of sedentary polychaetes may be distinct or indistinct. Forms with a distinct head
generally lack head appendages.
12. Branchiae, or gills, which serve for respiration
and as food-gathering organs, are well-
developed in many of the tube-dwelling forms.
Some have tentacles at the anterior end, and
gills arise from the dorsal surface of a few
anterior segments. In these species food is
gathered by the tentacles and respiration is
confined to the gills. The rest of the body is
divided into thoracic and abdominal regions.
Parapodia, if present, are generally simple
lobes; frequently the setae project directly
from the body wall. Many sedentary
polychaetes construct tubes made from a
substance secreted from cells that constitute
the epidermis, or skin. Tubes may consist of
calcium carbonate, parchment, or mucus, to
which sediment adheres. The anus is at the
posterior tip. Tube dwellers generally have an
external fecal groove along which fecal
material passes forward. Eyes are occasionally
present on gills, along the sides of the body, or
on the pygidium in sedentary forms that do
not live in tubes.
13. 2. Oligochaetes:
compared to the
are less varied.
in aquatic form
aperture lies in
front of the
Legless giant amphibian found
in Cambodia: New species can
grow to 1.5m in length and is
NOT a snake: “A new species of
legless amphibian resembling a
giant earthworm or a snake
14. The body of oligochaetes is uniformly
segmented and has conspicuous segmental
lines. The prostomium is usually a simple
lobe overhanging the mouth and lacking
appendages. The microscopically small eyes
are scattered over the body. The clitellum, a
saddle-shaped thickening of the body wall,
is present at sexual maturity. The anus is at
the posterior tip. Setae generally arise from
the ventral (lower) surface of the body.
15. 3. Hirudinea:
Leeches have 34 segments,
and elongation occurs by the
subdivision of these segments.
Leeches have a small sucker at
the anterior end and a large
sucker at the posterior end. A
clitellum is present in the mid-
region during the reproductive
period. The poorly developed
eyes are paired structures at
the anterior end. Setae are
16. Nearly 300 species of
leeches exhibit a
uniformity in their
appearance and struc-
ture. All are
are devoid of typical
like parapodia. Body is
usually adapted for
living as ectoparasite.
members of this group
are discussed below
•*Large earthworms, or night crawlers (Lumbricus terrestris), are cultivated and sold
as bait for freshwater fishes and as humus builders in gardens. The sludge worm
Tubifex, abundant near sewer outlets and thus an indicator of water pollution, is
collected and sold as food for tropical fish. Polychaetes play an important role in
turning over sediment on the ocean bottom.
*The medicinal use of leeches, which dates from antiquity, reached its peak in the
first half of the 19th century. The European species Hirudo medicinalis formerly was
exported throughout the world, and native species also were used. Hirudin, an
extract from leeches, is used as a blood anticoagulant. The estuarine flats of Maine
and Nova Scotia are the principal sources of the bloodworm (Glycera dibranchiata),
which is used as bait for saltwater fishes. Reproductive parts of the palolo (Palola
siciliensis), which break off and are found in great numbers during the reproductive
period, are used as food in Samoa in the south Pacific.
In the polychaetes, sexes are usually separate but cannot be distinguished in the
immature state until gametes (eggs and sperm) appear. Gametes are derived from
the mesodermal linings around the digestive tract. The developing gametes are
shed into the coelom, where they are nourished by nurse cells (eleocytes). The
gametes, especially eggs, are nourished by the breakdown products of muscle
tissue, which are passed on to the gametes via the eleocytes. Ripe eggs and motile
sperm may leave the body through gonoducts, or tubes for the passage of
reproductive cells; through excretory, or nephridial, pores; or through ruptures of
the body wall. polychaetes shed their gametes into the water.
Asexual reproduction is known in a few sedentary polychaete species. In some
genera—Ctenodrilus, Pygospio, and Sabella—fragmentation of the body occurs,
sometimes forming single segments, from which new individuals can develop.
19. *Reproduction in oligochaetes is primarily hermaphroditic; the number, arrangement,
and location of the male and female gonads and their pores vary considerably among
the various species. Lower oligochaetes (Microdrili) have one pair of testes and one
pair of ovaries in successive segments. Higher oligochaetes (Megadrili) retain the two
pairs of testes in segments 10 and 11 and the pair of ovaries in segment 13.
Developing sperm are frequently stored in seminal vesicles before transfer to female
receptacles. Sperm ducts lead from the seminal vesicles to male pores located one or
more segments behind the testes. The ovaries are simple outpouchings (ovisacs), with
oviducts leading to female pores in the next posterior segment.
*The clitellum of the earthworm secretes a case, or cocoon, into which is secreted a
material that serves as nourishment for the young and a mucous substance that aids in
copulation. The cocoon slips forward and receives eggs as it passes the female pores
and sperm as it passes the male pores. Fertilization, therefore, takes place within the
cocoon. The cocoon slides over the peristome, becoming completely sealed as it does so.
*Asexual reproduction is common in aquatic oligochaetes; indeed, sexual reproduction
is virtually unknown in certain naidid species. Some oligochaetes divide to form a
chain of two or more individuals that later break off as young worms. In many genera,
individuals lay self-fertilized eggs capable of development. Others exhibit
parthenogenesis—the production of young without fertilization—a phenomenon
associated with polyploidy (multiple sets of chromosomes) in earthworms and
accompanied by degeneration of male gonads.
20. All leeches are hermaphroditic, and reproduction is always sexual. The testes, from
four to 10 pairs, are arranged by segments, beginning with segment 12 or 13. The
testes on each side of the body are connected with the vas deferens, a duct that leads
indirectly to the male pore. The female reproductive system consists of one pair of
ovisacs containing the ovaries, which, although located in front of the testes, may
extend some length posteriorly, depending on the animal. The ovaries connect to form
an oviduct that forms either a female pore or, in those species that copulate, a vagina.
In one leech family (Gnathobdellae), sperm are transferred by the penis of one animal
into the vagina of another. In two other families (Rhynchobdellae and Erpobdellidae),
sperm are transferred by sperm capsules, or spermatophores, onto the body of the
leech, after which the sperm leave the spermatophore and enter the ovary through
the female pore to unite with the eggs. Leech eggs, numbering from one to more than
100, are usually deposited in cocoons, which may be oval or elongated in shape and
are generally attached to rocks or vegetation. Glossiphoniids produce a membranous
cocoon and attach it to their ventral surface, where development takes place. A
clitellum, which forms only during the reproductive period, secretes the cocoon and
material (albumin) to nourish the developing young.