The Annelida are segmented worms in which
the body wall, coelom (body cavity), epidermis,
circular muscle, longitudinal muscle, and
peritoneum are arranged into a longitudinal series
of rings or segments. True segmented animals
exhibit metamerism, a repetition of a structure or
organ from segment to segment. Each segment
has the same fundamental structures as all the
others.With the exception of the digestive system,
the major organ systems of the annelids are metameric
in structure. Young annelids generally have
few segments, but as they grow, new segments are
formed by the division of the terminal segment.
Annelids represent the most highly organized animals
capable of complete regeneration.
General Characteristics of the Annelids
The mouth lies between the first and second segments
and forms one segment called the prostomium.
In leeches, the mouth contains suckers
for attaching to the body of a host. The brain originates
in the prostomium and develops a pair of
circumpharyngeal nerve rings that reach around
the pharynx to form the ventral nerve cord, which
appears as a chain of ganglia, one pair in each segment.
In the Polychaetes, a pair of swimming
or crawling parapodia are located on most of the
segments. Both the Polychaetes and Oligochaetes
contain external setae to assist in locomotion.
The annelid body is covered with a thin cuticle.
Each segment has a ring of circular and longitudinal
muscles that contract to either elongate or
shorten the segment. Aspacious coelom, divided
by septa, lies between the body wall and an internal
digestive tract. The coelom is filled with fluid
and serves as a hydrostatic skeleton in all annelids
except the leeches. The coelom also contains the
circulatory and excretory systems. A system of
large vessels (hearts) pump blood through a ventral
vessel into capillary beds that invade all of the
tissues. The blood is returned to the hearts via the
dorsal vessel. Each segment, except the first and
last, contains a pair of nephridia, which collect
wastes and deliver them to the outside.
Reproduction in the Annelids
In many species of polychaete worms, fertilization
is external and takes place in the open sea
water. The palolo worm provides a good example
of polychaete reproduction. Through most of the
year, the worms exist as sexually immature animals called atokes, but during the breeding season,
the posterior segments develop gonads, and
the coelom becomes filled with gametes. On the
night of breeding, individuals back out of their
holes, and the posterior portion, called an epitoke,
breaks free. The epitokes swim to the surface for a
few minutes and burst, shedding eggs or sperm
and leaving a rapidly disintegrating body. Reproduction
in the palolo worms is tied to an annual
cycle designating the month, a lunar rhythm designating
the day, and a diurnal cycle designating
the hour of reproduction. Over 90 percent of the
population breeds within a single two-hour period
of the entire year.
Oligochaete worms are hermaphroditic, with a
pair of testes in the tenth and eleventh segments
and a pair of ovaries in the thirteenth segment.
During copulation, two worms exchange sperm.
Once the eggs are fertilized, the clitellum, a swollen
glandular region of the epidermis, secretes a
membrane that slips forward along the body so
that the eggs are laid directly into it as it passes.
Finally, the cocoon slips off the head, and the eggs
develop into tiny worms, which later emerge from
the cocoon. The reproductive system of leeches is
very similar to that of the Oligochaetes.
Classification:
Kingdom: Animalia
Subkingdom: Bilateria
Phylum: Annelida
Classes: Polychaeta (with parapodia and numerous
setae), Archiannelida (small marine annelides with
simple body), Oligochaeta (parapodia absent and
few setae), Hirudinea (leeches, parapodia and setae
absent)
Orders: Polychaeta—Errantia (palolo worms), Sedentaria
(lugworms); Oligochaeta—Lumbricus (earthworms);
Hirudinea—Rhynchobdellida (no jaws,
pharynx eversible, colorless blood), Gnathobdellida
(three jaws, red blood)
Geographical location: Found all over the world
Habitat: Polychaeta—mostly marine, found near the
shore or on the bottom of shallow areas, with a few
species in living in brackish water or freshwater;
Archiannelida—marine; Oligochaeta—freshwater
and terrestrial forms found burrowing in soil or leaf
mold; Hirudinea—freshwater and terrestrial environments
often found attached to the body of a host
Gestational period: Varies among species, but most
species lay eggs within a few days after fertilization;
eggs usually hatch within a few days to a few weeks
after being deposited
Life span: Varies among species; can be as short as a
year in some polychaetes and up to several years in
some earthworms
Special anatomy: Elongated, metameric (segmented)
bilateral invertebrates with appendages (parapodia)
and chitinous setae in many species but lacking in
Hirudinea; possess a true coelom (body cavity lined
with epithelial tissue) divided by septa with a closed
circulatory system, complete digestive system and
an excretory system (nephridia) in each segment;
leeches have specially adapted mouth parts for attaching
to the body of the host
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