Download coelomate worms

Coelomate Worms
Coelomate Worms:
Sipunculans, Pogonophorans, Echiurans
and Vestimenifera
Ectoderm
Mesoderm
Dt
Endoderm
Coelom
Often dismissed as minor or lesser
Comprise less than 600 species
total
Never abundant, often in extreme
environments
Not ecologically important as some
worms
Sipunculans, echiurans =
protostomes
Pogonophorans, vestimentifera =
protostome/deuterostome
characteristics
Sipunculids
= “little tube”
Peanut worm
Sausage shaped
Retractible introvert
Range between 1-50cm
Benthic
Exclusively marine
Burrowers, inhabit other
shells, tubes etc.
Intertidal - 5000 m
In Vietnam occasionally
consumed as food
Sipunculid Bauplan
Built similar to
annelids
No segmentation
Intestine = twisted Ushaped tube
Introvert = retractor
muscle
Mouth with tentacles
(60-150)
Large, fluid filled
coelom
Hemerythrocytes
No circulatory/
respiratory system
Sipunculid Physiology
Movement with help of
hydrostatic skeleton
! Peristalsis (annelids)
! circular/longitudinal muscles
Feed on particulates in the
sediment
!Introvert and tentacles
Tentacles/oral for gas exchange
Dioecious, no sexual dimorphism
Gametes do not grow or ripen in
gonads = small at all times
Trochophore larva
Sipunculid Taxonomic Conundrum
Sometimes grouped with
annelids
However:
No segmentation
No setae (bristles)
Shared characteristics are
general
Also been linked to
Molluscs:
Early development
Trochozoa, which could
include annelids
Echiurans
= sausage or spoon worms
All marine
Live in U-shaped tubes
Bilateral symmetry
Molecular phylogeny places
them with annelids
Lack metamerism
150 described species
Few mm - 50cm
Diagnostic proboscis = may
be 2 m long i.e. Bonellia
viridis
Males either live on female
or in groups of 20
Echiuran Bauplan
Divided into proboscis
and trunk
Proboscis = large &
flattened
! homologous with
prostomium in annelids
! Not retractible
Few setae = often just
2 ventral setae
Complete digestive
tract
Nerve ring around gut
connecting to cord
Echiuran Physiology
Simple closed circulation = dorsal
and ventral longitudinal bv
= hemoglobin and erythrin
Feed on epibenthic detritus
Mucus coating on proboscis
Metanephridia (1-100 pairs)
Cloaca and hindgut for gas
exchange
Bonellin
Echiuran Reproduction
Dioecious
Some reproductive at 7mm others
at 2m (Japanese)
Most extreme sexual dimorphism
Males often live on or in female =
parasitic
Males with large genital setae
Sex determination environmentally
determined = settlement near
female => male
Trochophore larva
Asexual reproduction unknown
= no regeneration
Pogonophora and Vestimenifera
Pogon = beard, phor = to
bear
Vestimentum = garment,
ferre = to bear
145 species
Strange, vermiform marine creatures
= beard worms
Live in thin tubes buried in substrate, at depths
from 200-10,000m
Most with a long thin body, no segmentation
Less than 1mm diameter, 10-75 cm long
Ristia (Vestimenifera) reaches 3 m in length
Characteristics: Shared and Separate
Bilateral, coelomate
Hind part appears metameric
Lack digestive system in adults
Closed circulation
Marine, benthic tube dwellers
Pogonophora
Anterior cavity developed
Three nerve cords bearing
ganglia
Serially arranged coeloms
Apparently without gut at
all times
Vestimenifera
Anterior cavity not
developed
Single nerve cord
Coeloms serially arranged
and paired
Transitory gut in juveniles
Anatomy
Body divided into 4 regions:
1. Prosoma
= cephalic lobe, 1-200
Tentacles with pinnules
2. Mesosoma
= forepart
3. Metasoma
= trunk, longest body
region, bears annuli,
papillae and ciliary tracts
4. Opisthosoma
= last part to be discovered
prosoma = cephalic lobes
forepart
trunk
Vestimeniferan
anatomy under debate
1. Obturaculum
2. Tentacle bearing
3. Vestimentum = wings
Some Comments on Phylogeny
Evolutionary relationships
= difficult to evaluate
- almost no fossil record
- no agreed upon
ancestor
Embryonic: S & E =
protostomes
! treated as members
of annelids, early
polychaete
P = tripartite structure
= early deuterostome?