Chapter 7: Marine Invertebrates
Bilateral Symmetry and
the Advancements of
the Worms
Oh, to be a Worm!
Adaptive trends exhibited by worm phyla:

Bilateral symmetry

Cephalization –development of a head region

Coelom development

Increasing development of nervous sensory
systems.
Bilateral Symmetry

“Bilateral symmetry refers to a basic
animal body plan in which one plane
of symmetry exists to create two
mirror-image halves.”
Sumich (1999) An Introduction to the Biology of Marine Life
Planaria
gecko.gc.maricopa.edu/.../platyhelminthes/ platyhel.htm
Bilateral Symmetry


Organisms with
bilateral symmetry
have developed an
anterior “head”
region and a
posterior “tail”
region.
In addition they
also display a top
or back side
(dorsal) and a
belly or underside
(ventral).
Worms with Direction

“Animals with a front end [anterior] region
generally move in a forward direction.”
Villee, et. Al. (1989) Biology

Thus the tendency would naturally be to
concentrate sensory organs in this anterior
region to detect changes in the environment.
–
–

Leads to more active predation
More sophisticated behaviors
This process is termed “cephalization”
–
from the Greek for “getting a head”
A Bit About Germ Layers

Early in embryonic development, the structures
of most animals develop from three tissue
layers call germ layers.

Ectoderm – outer layer
Mesoderm – middle layer
Endoderm – inner layer


Digestive cavity
Germ Layers

Ectoderm – outer layer
–

Mesoderm – middle layer
–

outer covering of the body and the
nervous system
gives rise to most of the body
structures
Endoderm – inner layer
–
lines the digestive tract
A Tube-Within-A-Tube


As organisms become more
sophisticated anatomically,
the development of a body
cavity or coelom [see-luhm] is
observed.
The coelom is lined by
mesoderm tissue and is
essentially an open tube
within the organism’s body in
which digestive, reproductive
and other organs arise.
PHYLUM: Platyhelminthes
Flatworms – A Tiny “Inch” Forward





Exhibit bilateral symmetry and cephalization
Acoelomate
Mouth and anus are still shared
Simplest organisms with well-developed organs
Have a simple brain called a ganglia in the head
with two nerve cords that extend the length of
the body.
Anatomy of a Flatworm
Flatworms

Turbellarians
–
–

Trematodes
–
–

Planarians
Marine, free-living
Flukes
Mostly parasitic
Cestodes
–
–
Tapeworms
Parasites that live in the
intestines of vertebrates
(including humans!)
Flatworms –
Another Look
Anatomical diagram of a planarian
– a typical flatworm found in
both fresh and marine waters
as well as terrestrial habitats
Flatworm Media
Planaria
Swimming
Turbellarians
Trematode
infection of
salamanders
Warning:
Colonoscopy
showing
tapeworm !
PHYLUM: Nemertea
Proboscis Worms/ Ribbon Worms



Simplest animals to possess
definite organ systems.
Almost exclusively marine
Possess a proboscis – a
long, hollow, muscular tube
which can be everted from
the head to capture food or
for defense.
Proboscis Worms/ Ribbon Worms





Are truly a “tube-within-a-tube.” The digestive tract is a
complete tube with mouth at one end and anus at the
other.
First example of separate circulatory and digestive
systems
Acoelomates
Non-parasitic, mostly benthic
Claim to fame – one species has been observed up to
30 m long (the longest invertebrate!)
PHYLUM: Nematoda
Roundworms




Most common worms in the world – inhabit
almost every species of plant and animal.
Mostly parasitic, some benthic
Have a tough, outer covering called a cuticle
which keeps them from drying out.
Sexes separate and dimorphic – separate male
and females that look different (male smaller)
Roundworms


Pseudocoelomates
Have a cavity filled with incompressible fluid
which acts as a hydrostatic skeleton.
–
–
–
Cavity is not completely lined by mesoderm.
When muscles in the body wall contract they flex
and squeeze against this fluid causing the shape of
the worm to deform and therefore move.
Excellent technique for sediment burrowing.
Good slide show of various
roundworm images
Marine
roundworm
Roundworm in
cat gut
PHYLUM: Annelida
Segmented Worms


20,000 species including
marine and terrestrial
species (e.g.
earthworms)
Defining characteristics
–
–
Body divided into
segmented units called
metameres.
Chaetae (or setae) – hairlike structures on each
segment
Other Innovations of Annelids


Digestive tract (or gut) extends through all segments.
Coelomates
–
–


Acts as a hydrostatic skeleton
Organism can move each segment individually. This permits
localized and more efficient movement.
Have a closed circulatory system
In aquatic species, respiratory exchange is through gills
Annelid Classes

Polychaeta
–
–

Oligochaeta
–
–

All marine, may be free-swimming or live in benthic
aggregations
Include bloodworms, sandworms, lugworms, bristle worms, fan
worms, feather duster worms, beard worms, etc.
Aquatic or terrestrial, live in mud or sand bottoms’
Include earthworms
Hirudinea
–
–
Mostly freshwater, but some marine species
Leeches
Polychaete Biology

Anatomy:
–

Life History:
–
–

Chaetae emerge from flat parapodia which are stiff extensions on each body
segment
Have a planktonic larval stage called a trochophore
As adults, some crawl on bottom, others burrow, others build tubes and live in
aggregations, while still others remain planktonic
Feeding:
–
–
–
Some are carnivorous, some are suspension feeders, and others are deposit
feeders.
Crawling worms have well developed parapodia, a proboscis, and jaws.
Suspension feeding worms often have tentacles, cilia, or mucus to capture
prey
Serpula vermicularis – reef building tube worm
Common lug worm (Arenicola marina) Plymouth, Devon, England
Lug worm casts on the coast of North Ireland
King Ragworm (Nereis virens)
Tubeworm (Spirorbis tridentatus) Batten Bay, Mount Batten, Plymouth,
Devon.)

Myrianida pachycera, a polychaete (worm) (60x)
Christmas tree worms on coral head
Trochophore larvae of a bristle worm
Note the bristles anchored in the body for swimming and the reddish
eye spots.
Polychaete sandworms - Notice the tubes sticking up from the mud.
Some sandy beaches can contain up to 32,000 polychaete worms/m2
that consume 3 tons of sand/ year.
Feather duster worms, Bimini, Bahamas
Polychaete epitokes swarming . Glover’s Reef, Belize
Pogonophora
beard worms





Deep water species – live near
hydrothermal vents
No mouth or gut
Tuft of tentacles absorbs
dissolved nutrients from the water
Symbiotic bacteria inside the
worm use these nutrients to
make food.
Formerly classified in their own
phylum
Oligochaeta




Found in mud/sand bottoms
Usually deposit feeders
Lack parapodia
Includes the common
earthworm
Hirudinea
leeches




Usually parasitic and bloodsucking
Inject a chemical into prey
that is both an anticoagulant
and an anesthetic.
Have a sucker on anterior
and posterior.
Lack parapodia
Sipuncula
peanut worms







Strictly marine
Unsegmented
Burrow in shallow water soft
bottom sediments
Possess a long anterior
portion that can be retracted
into the body.
Deposit feeder
1-35 cm long
Approximately 320 species
Echiura
innkeepers/ spoon worms

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

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Strictly marine
Unsegmented, though now
classified with annelids
Have a non-retractable, spoonlike proboscis for gathering
organic material.
One species creates a U-shaped
burrow that is often shared with
other organisms.
Deposit feeder
Approximately 135 species
proboscis
Unifying Characteristics of Worms




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

Ubiquitous in marine environment (benthic, parasitic,
free swimming)
Usually small
Responsible for mixing marine sediments.
Recycle bacteria and detritus into the food chain.
Have highly developed feeding appendages and
digestive systems.
Important food for higher invertebrates and some fish.
May have important health effects on marine
vertebrates
Image Citations
Brown, Hugh. “Serpulid polychaete worm” Digital Image. Serpulid reefs. The Scottish Association for Marine Science (SAMS).
5 January 2009. <http://www.sams.ac.uk/research/departments/ecology/ecology-projects/reefecology/researchproject.2007-04-18.1807501867>
Fiege, Dieter. “Glyceridae” Digital Image. Senchenbergische Naturforschende Gesellschaft. 2008. 5 January 2009.
<http://www.senckenberg.de/root/index.php?page_id=2301>
“Leech.” Digital Image. Annelids Live Invertebrates – Niles Biological, Inc. 2006. Niles Biological, Inc. 5 Jaunary 2009
<http://www.nilesbio.com/subcat288.html>
Rouse, Greg. “Chaetae of an Annelid” Digital Image. Annelida 2004. Tree of Life Web Project. 5 January 2009
<http://www.tolweb.org/Annelida>
Rouse, Greg. “Myrianida pachycera, a polychaete.” Digital Image. Nikon Small World – Gallery. 2008. Nikon Small World –
Photomicrography Competition. 5 January 2009.
<http://www.nikonsmallworld.com/gallery.php?grouping=year&year=2003&imagepos=2>
Siddal, Mark. “Medicinal leech” Digital Image. Leech on Me. 2007. Science Friday Newsbriefs. 5 January 2009.
<http://www.sciencefriday.com/newsbriefs/read/120>
“Social feather duster worm close-up” Digital Image. ReefNews. 2001. 5 January 2009.
http://www.reefnews.com/reefnews/photos/bimini/sfdust2.html
“Swarming polychaetes” Digital Image. Rpolychaete epitokes Ryan Photographic. 5 January 2009.
<http://www.ryanphotographic.com/epitoke.htm>
“Trocophore larvae” Digital Image. Bristleworms and their larva. 1995. Mic-UK: Bristle worms. 5 January 2009.
<http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artmar99/poly2.html>
Veitch, Nick. “Lug worm casts” Digital Image. Wikimedia Commons. 2008. 5 January 2009.
<http://commons.wikimedia.org/wiki/File:Lugworm_cast.jpg>