Download Slide 1 - NGHS

The Segmented Worms
Annelids and Allied Taxa
• Phylum Annelida
– Class
Polychaeta
– Class
Oligochaeta
– Class Hirudinea
• Phylum Echiura
• Phylum
Sipuncula
Phylum Annelida
• Annelids are
protostome coelomates
in superphylum
Lophotrochozoa.
– Spiral, determinate
cleavage.
• Nervous system more
centralized & circulatory
system more complex
than in previous phyla.
Phylum Annelida
• Annelids are segmented worms.
• They have bodies composed of a series of
fused rings.
• Earthworms, leeches, clam worms.
Phylum Annelida
• The evolutionary innovation shown by
annelids is segmentation
(metamerism).
– Segmentation evolved separately in
annelids, arthropods, and chordates.
• The body is divided into a series of
segments, each having similar
components of all major organ systems.
– Built in fail-safe.
– Allows for specialization.
Phylum Annelida
• Many annelids have
chitinous bristles
called setae.
– Help in locomotion
– Anchor worm in place
– Deter predators
Phylum Annelida – Body Plan
• Prostomium –
anterior part followed
by segmented body.
• Pygidium – terminal
portion.
Phylum Annelida – Body Plan
• Peritonia (layers of
mesodermal
epithelium) of
adjacent segments
meet to form septa.
• Fluid-filled coelom
acts as a
hydrostatic
skeleton.
Phylum Annelida
• Tissues and organs differentiated
• Segmentation highly developed
– Mesodermal “blocks,” each with own coelom
– Septa between segments
Polychaetes
• Members of class Polychaeta
– Possess paddlelike parapodia that function as
gills and aid in locomotion
Parapodia
Polychaetes
• Resembling giant
lipsticks, tubeworms
(Riftia pachyptila) live
over a mile deep on the
Pacific Ocean floor near
hydrothermal vents.
• They may grow to about 3
meters (8 ft) long.
• The worms’ white tube
home is made of a tough,
natural material called
chitin (pronounced “kitein”).
Class Polychaeta
• Polychaetes have
some features other
annelids do not:
– A well developed head.
– Paired appendages,
parapodia, that
function as gills and aid
in locomotion.
– No clitellum.
• Many setae
Class Polychaeta
• Polychaetes are
mostly marine and
mostly benthic.
– May live under rocks,
burrow into sediment,
or build their own
tubes.
– Some are planktonic.
Class Polychaeta
• Sedentary and
errant (freemoving) forms.
• Sedentary forms
often have
elaborate devices
for feeding and
respiration.
– Filter or deposit
feeders.
Class Polychaeta
• Errant forms
include pelagic
and benthic
types and are
often predators
or scavengers.
Class Polychaeta Reproduction
•
•
•
•
Gonads are temporary structures in polychaetes.
Sexes usually separate.
Fertilization is external.
Early larva is a trochophore.
Circulation and Respiration
• Most have parapodia and gills for gaseous exchange.
– Others use the body surface.
• Circulation varies.
– In Nereis a dorsal vessel carries blood forward and a ventral
vessel carries blood posteriorly.
- Blood flows across
between these major
vessels in networks
around the parapodia
and intestine.
• In some, septa are
incomplete and coelomic fluid
serves circulatory function.
• Many polychaetes have
respiratory pigments Hemoglobin, chlorocruorin or
hemerythrin.
Excretion
• Excretory organs vary, from protonephridia to
metanephridia, and mixed forms.
• One pair per metamere.
• Inner end (nephrostome) opens into the
coelomic cavity.
• Coelomic fluid enters the nephrostome.
• Selective resorption occurs along the
nephridial duct.
Nervous System and Sense
Organs
• Double ventral nerve cord runs length of the
worm with ganglia in each metamere.
• Sense organs include:
– Eyes, nuchal organs and statocysts.
– Eyes vary from simple eyespots to well-developed
image-resolving eyes similar to mollusc eyes.
– Nuchal organs are ciliated sensory pits that are
probably chemoreceptive.
– Some burrowing and tube-building polychaetes use
statocysts to orient their body.
Clade Siboglinidae
(Pogonophorans)
• Formerly members of
phylum Pogonophora
(beardworms).
• Discovered in 1900.
• 150 species described.
• Most are small, less
than 1 mm in diameter.
• Giant beardworms that
live in deepwater
hydrothermal vents are
3 m long and 5 cm in
diameter.
Clade Siboglinidae
(Pogonophorans)
• Most live in mud on ocean floor at depths of 100 to 10,000
m.
• Sessile animals that secrete and live in long chitinous tubes.
• Tubes have general upright orientation in bottom sediments.
• Tubes are generally three or four times the length of the
animal.
Clade Siboglinidae
(Pogonophorans)
• Long cylindrical body covered with cuticle.
• Divided into a short anterior forepart, a long slender
trunk, and a small, segmented opisthosoma.
• Tentacles are hollow extensions of the coelom and
bear minute pinnules.
Clade Siboglinidae
(Pogonophorans)
• No mouth or digestive tract.
• Nutrients such as glucose and amino
acids absorbed from seawater through
pinnules and microvilli of tentacles.
Clade Siboglinidae
(Pogonophorans)
• Most energy derived from a mutualistic
relationship with chemoautrophic
bacteria that oxidizes hydrogen sulfide.
– Trophosome, derived embryonically from
midgut, houses the bacteria.
Clade Siboglinidae
(Pogonophorans)
• Sexes are separate.
• Research suggests that cleavage is unequal
and atypical.
– Appears to be spiral.
• Coelom formed by schizocoely.
• Embryo
– Worm-shaped and ciliated.
– Poor swimmer.
– Probably carried by water currents until it settles.
Class Oligochaeta
• Earthworms are the
most familiar
oligochaetes, found in
moist, rich soil.
– They can burrow deep
underground and remain
dormant in a slime
chamber during dry
weather.
– Setae help prevent
slipping while burrowing.
Oligochaetes
• The largest
earthworm ever
found was in South
Africa and was 22
feet long!
The Earthworm
Giant Earthworm – Amazon Basin
Class Oligochaeta - Feeding
– Food is stored in a thin-walled crop.
– Muscular gizzard grinds food into small
pieces.
– Digestion and absorption occur in intestine.
Class Oligochaeta - Excretion
• Each somite, except the 1st
three and terminal one, have a
pair of metanephridia.
• A ciliated funnel, the
nephrostome, draws in wastes
and leads through the septum.
• These coil until the nephridial
duct ends at a bladder that
empties outside at
nephridiopore.
• Wastes from both the coelom
and the blood capillary beds
are discharged.
• Aquatic oligochaetes excrete
toxic ammonia.
Class Oligochaeta - Circulation
and Respiration
• Coelomic fluid and blood transport food, wastes, and
respiratory gases.
• Blood circulates in a closed system with five main
trunks running lengthwise in the body.
• Dorsal vessel contains valves and functions as a true
heart.
– Pumps blood anteriorly into 5 pairs of aortic arches.
– Aortic arches ensure steady pressure in ventral vessel.
Class Oligochaeta - Nervous
System and Sense Organs
• Central nervous system and
peripheral nerves.
• Pair of cerebral ganglia
connect around the pharynx to
the ganglia of the ventral nerve
cord.
• Neurosecretory cells in brain
and ganglia secrete
neurohormones.
– Regulate reproduction,
secondary sex characteristics,
and regeneration.
• Lack eyes but have many
photoreceptors in the
epidermis.
• Free nerve endings in
tegument are probably tactile
structures.
Class Oligochaeta - Reproduction
• Earthworms are hermaphroditic – male and
female organs in the same animal.
• When mating, two worms are held together by
mucus secreted by the clitellum.
Class Oligochaeta - Reproduction
• After mating, a
cocoon forms
around the
clitellum, as it
passes forward it
gathers both
gametes, and
fertilization
occurs inside.
Class Oligochaeta - Reproduction
• Development occurs
inside the cocoon
and young worms
hatch out.
– Development is
direct, no larval
stage.
Class Oligochaeta - General
Behavior
• Avoid bright light (negative phototaxis).
• Chemical stimuli are important in locating
food.
• Limited learning ability - primarily trialand-error learning.
3 ecological groups of earthworms
Endogeic example:
Aporrectodea
caliginosa (angle
worms)
Epigeic example:
Lumbricus rubellus
(leaf worms)
Anecic example:
Lumbricus terrestris
(nightcrawlers)
Earthworm Cross Section
Dorsal Blood
Vessel
Intestine
Coelom
Nephridia
Epidermis
Muscle
Layer
Ventral
Nerve Cord
Ventral Blood
Vessel
Class Hirudinea (leeches)
• Most inhabit fresh
water, or moist
terrestrial
environments
• Feed on small
invertebrates or
are parasitic
• Have significant
medicinal value in
treating bruising,
and as a source of
Pharmaceuticals
Hirudinea
Class Hirudinea
• Many leeches live as carnivores on small
invertebrates.
• Some are temporary parasites.
• Some are permanent parasites – they never
leave their host.
• Can be a benefit in medicine.
Class Hirudinea
• Leeches are hermaphroditic
and have a clitellum (only
appears during breeding
season), like oligochaetes.
• Leeches do not have setae.
• They’ve developed suckers
for attachment and a
specialized gut for storing
large amounts of blood.
Class Hirudinea - Respiration and
Excretion
• Some fish leeches have
gills.
• All other leeches
exchange gases across
epidermis.
• 10 to 17 pairs of
nephridia.
• Coelomocytes and other
special cells may assist
in excretion.
Class Hirudinea - Nervous and
Sensory Systems
• Two “brains”
– Anterior fused ganglia form a ring around
the pharynx.
– Seven pairs of posterior fused ganglia.
• 21 pairs of segmental ganglia in between
along a double nerve cord.
• Epidermis contains free sensory nerve
endings and photoreceptor cells.
• Pigment-cup ocelli are present.
Class Hirudinea - Circulation
• Coelom reduced by invasion of
connective tissue.
– Forms system of coelomic sinuses and
channels.
• Some have a typical oligochaete
circulatory system.
– Coelomic system is auxiliary.
• Some lack blood vessels and coelomic
sinuses serve as only vascular system.
Phylum Echiura
• Approximately 140
species of marine
worms that burrow into
mud or sand.
• Live in empty snail
shells or sand-dollar
tests, or rocky crevices.
• Found in all oceans.
• Length varies from a few
millimeters to 40 or 50
cm.
Phylum Echiura – Form and
Function
• Sausage-shaped.
• Inextensible proboscis anterior
to the mouth.
• Often called spoon worms”.
• Simple nervous system with a
ventral nerve running length of
the body.
• Ciliated groove on the
proboscis allows them to
gather detritus over the mud
while lying buried.
• Muscular body wall is covered
by a cuticle and epidermis
which may be smooth or
covered by papillae.
Phylum Echiura
• Large coelom.
• Digestive tract long and coiled.
• Pair of anal sacs may serve an
excretory and osmoregulatory
function.
• Most have a closed circulatory
system with colorless blood.
– Hemoglobin found in certain cells
and in coelomic corpuscles.
• Respiration probably occurs in
hindgut which is continually filled
and emptied by cloacal irrigation.
Phylum Echiura - Reproduction
• Sexes are separate.
• Gonads produced by special regions in
peritoneum in each sex.
• Fertilization usually external.
• Early cleavage and trochophore stages
similar to annelids.