Phylum Annelida

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Phylum Annelida

• The Segmented Worms Ch 11

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Annelida Characteristics
Triploblastic
Organ level of organization
Bilateral Symmetry
Cephalization
Eucoelomate
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Eucoelomate
Have a true body cavity that is completely
surrounded by mesoderm
ectoderm
coelom
mesoderm
endoderm
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Annelid Characteristics
the coelom

• is a closed, fluid filled cavity that surrounds
  the gut
• mesodermal membranes (mesenteries) suspend organs
  in the coelom

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Annelida Characteristics
Protostome

• Development is characterized by
• blastopore becomes the mouth
• Spiral / determinate cleavage
• Schizocoely

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Annelida Characteristics
Metamerism

• The body is made up of serially repeating,
  coordinated segments called metameres that are
  separated from one another by septa.
• Each metamere contains sets of repeating organs
• e.g. blood vessels, nerve cord, excretory organs

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Metamerism
septa
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Annelida Characteristics
Nervous system

• 2 cerebral ganglia
• a ventral nerve cord with 2 ganglia per
  metamere.
• In some species, sensory organs such as eyes,
  palps, and tentacles have arisen

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Annelida Characteristics
Locomotion

• both longitudinal and circular muscles
• most have chaetae setae (chitonous bristles
  secreted by the epidermis) that aid in locomotion
  and burrowing

Skeletal System

• fluid in coelom acts as a hydrostatic skeleton

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Annelid Characteristics
Gas exchange

• mainly by diffusion
• Class Polychaeta often has specialized
  structures for gas exchange
• (e.g. parapodia, gills)

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Annelid Characteristics
Digestive System

• complete
• regional specialization

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Annelid Characteristics
Circulatory System

• closed circulatory system composed of blood
  vessels (some of which are contractile and act as
  hearts)
• some circulation is also accomplished by the
  coelomic fluid

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Annelid Characteristics
Excretion

• excretion is accomplished by organs called
  nephridia (singular nephridium)
• predominantly sexual

Reproduction
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Phylum Annelida
Class Polychaeta Class Clitellata Subclass
Oligochaeta Subclass Hirudinea
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Class Polychaeta
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Class Polychaeta

• most marine about 30 species that are
  freshwater and brackish water in the US.
• this class contains 2/3 of all known Annelids
• have a well developed head with specialized
  sense organs

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Class Polychaeta

• have many chaetae setae (chitonous bristles
  secreted by the epidermis)
• (Poly many, chaeta setae)
• these setae are arranged in bundles on
  paddle-like appendages called parapodia

setae
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Class Polychaeta
Parapodium
chaetae
The parapodia function in gas exchange,
locomotion, and feeding.
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Parapodia are biramous
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Class Polychaeta

• Tagmatization (tagmosis)
• the fusion and specialization of formerly
  metameric segments

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Class Polychaeta
Many are filter-feeders with specialized
structures
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Terebellids with tentacles that creep over the
surface of the substratum and accumulate food
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Class Polychaeta
Many are predatory with specialized structures
Jaws
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Class Polychaeta
Many construct their own homes out of CaCO3 or
sand debris and mucous
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Class Polychaeta

• Reproduction
• usually dioecious
• no permanent sex organs gametes are shed into
  coelom
• fertilization is usually external
• indirect development ? trocophore larvae

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Class Polychaeta

• Epitoky
• In some species, the rear portion of the worm
  (epitoke carries the eggs or sperm), breaks
  off from the body and swims to the surface to
  reproduce. The remaining benthic worm (atoke)
  continues to feed and grow and will eventually
  produce new epitokes.
• To synchronize these spawnings,
• this swarming behavior is usually
• triggered by the full moon.

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Polychaetes Swarming
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Class Polychaeta

• Reproduction
• asexual

Epitokes
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Symbiotic relationships

• Many scaleworms are found near, or in the mouth,
  of brittlestars, starfish, sea urchins, and
  molluscs.
• The scaleworm eats its hosts leftovers and with
  its vicious jaws, it will attack any predator
  trying to eat its host.

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Subclass Oligochaeta
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Subclass Oligochaeta

• terrestrial, freshwater and few marine
• have few setae (Oligo few, chaeta setae)
• usually feed on detritus
• (decaying organic matter)
• have specialized digestive system to obtain the
  maximum amount of nutrients out of the detritus
  (e.g. typhlosole, gizzard, crop)

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Subclass Oligochaeta

• typhlosole-
• infolding of the dorsal side of the intestine
• increases surface area for absorption of
  nutrients

typhlosole
gut
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Subclass Oligochaeta
Reproduction

• usually monoecious
• cross-fertilize by
• exchanging sperm

clitellum
testis
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Earthworms Secret Cocoons Where They Store Their
Eggs!
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Ecology

• Earthworms are essential soil aerators in
  terrestrial and aquatic habitats
• If all the material ever moved through
  earthworms was piled up, the heap would rise 30
  miles , more than 5 times the height of Mount
  Everest!!

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What is the most exciting thing about earthworms?
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What is the most exciting thing about earthworms?
The Giant Gippsland earthworm, Megascolides
australis can reach lengths of nearly 4 meters,
and the Australian earthworm Terriswalkeris
terraereginae can get to 2 meters in length and
the mucin it releases is luminescent!
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What is the most exciting thing about earthworms?
The Microchaetus rappi more commonly know as the
Giant South African Earthworm to over 20 feet
long!