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Segmented Worms
Chapter 17
Annelids and Allied Taxa
Phylum Annelida Class
Polychaeta Class
Oligochaeta Class Hirudinida
Phylum Echiura Phylum
Sipuncula
Annelids and Allied Taxa
Members of phyla Echiura and Sipuncula are benthic marine animals with unsegmented bodies.
Molecular sequence data place echiurans within phylum Annelida.
Echiurans - Sister taxon to Annelida.Sipunculans - Sister taxon to a clade
composed of Annelida and Echiura.
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
Annelids can be found worldwide in marine, freshwater, and terrestrial habitats.
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.
Phylogeny
Traditionally, annelids are divided among 3 classes: Class Polychaeta Class Oligochaeta Class Hirudinida
Polychaeta is a paraphyletic class because ancestors of the clitellates arose from within it.
Oligochaeta and Hirudinida form a monophyletic group called Clitellata. Characterized by reproductive structure called a clitellum.
Class Oligochaeta is a paraphyletic group because ancestors of leeches arose from within it.
Class Polychaeta
Polychaeta is the largest, most diverse class. May be
brightly colored, variable shape.
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 (free-moving) 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.
Representative Polychaetes
Clam Worms: Nereis Errant polychaetes Live in mucus-lined burrows
near low tide level. Come out of hiding places at
night to search for food. Prostomium bears a pair of
palps sensitive to touch and taste, a pair of short sensory tentacles, and two small dorsal eyes sensitive to light.
Peristomium has a ventral mouth, a pair of jaws, and four pairs of sensory tentacles.
Representative Polychaetes
Scale worms Flattened bodies
are covered with broad scales.
Some are large, all are carnivores and some are commensals in burrows of other organisms.
Representative Polychaetes
Fireworms Have hollow, brittle
setae that contain poisonous secretions.
Feed on cnidarians.
Representative Polychaetes
Tubeworms Tube-dwellers May line their
burrows with mucus Use cilia or mucus
to obtain food
Representative Polychaetes
Fanworms or Featherduster worms Unfurl tentacular
crowns to feed. Food moved from
radioles to mouth by ciliary action.
Representative Polychaetes
Parchment Worms
Lives in a U-shaped tube.
Modified segments pump water through tube.
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.
http://www.youtube.com/watch?v=2FFnrW_SUdM
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.
Osedax – bone eating worms
http://www.youtube.com/watch?v=URi8KccVkks
Clade Clitellata
Class Oligochaeta and Class Hirudinida Form reproductive structure called a clitellum.
Ring of secretory cells found in a band around the body. Permanent in oligochaetes but visible only during
reproductive season in leeches. Members are derived annelids that lack parapodia. Hermaphroditic (monoecious) animals that exhibit
direct development. Young develop inside a cocoon secreted by the
clitellum, and emerge as small worms.
Class Oligochaeta
Class Oligochaeta includes earthworms and many freshwater worms. They possess setae, but not as much as
polychaetes.
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.
Class Oligochaeta
Darwin wrote about earthworms in The Formation of Vegetable Mould Through the Action of Worms.He noted the beneficial activities of worms,
aeration, moving nutrients up from subsoil, adding nitrogenous products, breakdown of organic matter in dead leaves etc.
An earthworm can ingest its own weight in soil every 24 hours.
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 - 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 segment, 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 - General Behavior
Avoid bright light (negative phototaxis).Chemical stimuli are important in
locating food.Limited learning ability - primarily trial-
and-error learning.
Class Hirudinida
Class Hirudinida includes the leeches. Primarily
freshwater, a few marine & terrestrial.
More common in tropical climates.
Class Hirudinida
Many leeches live as carnivores on small invertebrates.
Some are temporary parasites. Some are permanent parasites – they never
leave their host.
Class Hirudinida
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 Hirudinida - 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 Hirudinida - 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 Hirudinida - 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.
Class Hirudinida
Leeches are highly sensitive to stimuli associated with the presence of prey.Those that feed on mammals are attracted
by warmth.
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.
Phylum Sipuncula
Approximately 250 species of benthic marine worms.
Sedentary, living in burrows of mud or sand, snail shells, coral crevices, or among vegetation.
More than ½ restricted to tropical zones.
Some are tiny, slender worms, but most range from 3 to 10 cm in length.
Some are known as “peanut worms” because when disturbed, they contract to a peanut shape.
Phylum Sipuncula - Form and Function
No segmentation or setae.Slender, retractable introvert or
proboscis at anterior end.Walls of the trunk are muscular.
Phylum Sipuncula - Nutrition
Some appear to be detritivores and others suspension feeders.
Some nutrition may come from dissolved organic matter in the surrounding water.
From burrow or hiding place, they extend tentacles to explore and feed.
Collected organic matter moved from mucus on tentacles to mouth by ciliary action.
Large fluid-filled coelom. Digestive tract is U-shaped.
Phylum Sipuncula - Respiration
Lack a circulatory and respiratory system.
Gas exchange appears to occur across the introvert and tentacles.
Phylum Sipuncula - Nervous and Sensory Systems
Bilobed cerebral ganglion behind tentacles.
Ventral cord extends the length of body.
Phylum Sipuncula - Reproduction
Sexes are separate.Sex organs develop seasonally within
the connective tissue covering the origins of the retractor muscles.
Sex cells are released through the nephridia.
Asexual reproduction occurs by transverse fission.
Phylogeny
Similarities in the early development of molluscs, annelids, and some primitive arthropods indicate that these three groups are probably closely related.Trochophore larvaSpiral cleavageSchizocoelous coelom formation
Evolutionary Significance of Metamerism
No satisfactory explanation for origins of metamerism and coelom has gained acceptance.
Coelom may have been advantageous as a hydrostatic skeleton.
Coelomic fluid would have acted as a circulatory fluid and reduce need for flame cells everywhere.
Coelom could store gametes for timed release. Would require nervous and endocrine control.
Unlikely that segmentation is homologous among annelids, arthropods, and chordates.
Current evidence supports the hypothesis that segmentation arose independently multiple times.
Evolutionary Significance of Metamerism
Selective advantage of a segmented body for annelids appears to lie in the efficiency of burrowing.
However, does not explain segmentation in arthropods given the rigidity of the exoskeleton.
Phylogeny
Molluscs and annelids share many developmental features so are presumed to be closely related.
However, shared features are likely to be a retained ancestral feature for lophotrochozoan protostomes.
Pogonophorans have been included within the annelids as derived polychaetes.
Molecular analyses place sipunculids and echiurans closely related to the annelids.
Phylogeny
With more studies, Echiura, like Pogonophora, may no longer be a valid phylum.
Placement of Sipuncula is contentious. Members are not metameric and lack setae. Larval development similar to annelids, molluscs,
and echiurans. Molecular data may clarify position within
Lophotrochozoa. Presently, depicted as the sister taxon to a clade of
annelids and echiurans. Polychaeta is a paraphyletic group.
