Complete

Characteristics of Annelida: Plesiomorphies and Other Features

Greg W. Rouse and Fredrik Pleijel

1. Symmetry and Size

Annelids are all bilaterally symmetrical animals. They range in size from much less than 1 mm in length to more than 3 m. The smallest annelids described to date would appear to be Neotenotrocha (Dorvilleidae, Eunicida) reaching only a few hundred microns in length (Eibye-Jacobsen and Kristensen, 1994). Also within Eunicida may be found the largest of annelids. A preserved specimen of Eunice (Eunicidae) at the Museum of Natural History in Washington D. C. is close to 3 m in length and around 5 cm in diameter in its preserved and contracted condition. Some earthworms, such as the Giant Gippsland Earthworm (Megascolides australis), can reach 1 m in length. The largest leech in the world, Haementeria ghilianii, is found in South America and reaches more than 40 cm in length.

2. Coelom

Nearly all annelids have a fluid-filled cavity between the outer body wall and the gut, and this is referred to as a coelom (Figure 1). The coelom is often used as a storage area for gametes and acts as a hydrostatic skeleton for locomotion. The coelom (formed via schizocoely) is generally organized as a series of compartments divided by intersegmental septa. This compartmentalization means that if the worm is damaged the coelomic contents will only be lost from a few segments, and locomotion can be maintained. In other annelids there may only be a few septa dividing the coelom. Under these circumstances much more coelomic fluid is lost with injury, and the worm may be severely affected. A number of groups, particularly leeches and a number of small polychaetes, have little or no coelomic space.

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

coelom of an annelid

Figure 1. Amphicorina mobilis (Sabellidae). Bondi,
Australia. Copyright © 1996 Greg Rouse.

3. Body wall

Annelids have a body covered by an external cuticle that is never shed
or molted. Epidermal microvilli secrete a network of fibers that are
in part collagenous and also contain scleroprotein. Chaetae are also
cuticular structures, but contain large amounts of chitin. Beneath the
epidermis and its basal lamina lies a layer of circular muscle. The
circular muscle layer forms a nearly continuous sheath around the body,
except in polychaetes with well-developed parapodia. Beneath the circular
muscle layer lie thick longitudinal muscles. In many annelids the longitudinal
muscles are present as four distinct bands (Fig. 2).

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

cross section of capricornia body wall

Figure 2. capricornia (Hesionidae) (Pleijel and Rouse, 2000). One Tree island, Australia. Copyright
© 2000 Greg Rouse.

4. Parapodia

Parapodia are unjointed segmental extensions of the body wall found in many polychaetes,
though many do lack these features. They are also absent in Clitellata
and Echiura. Parapodia are equipped with musculature derived mainly
from the circular muscle layer and usually carry chaetae. Parapodia
vary in structure but basically can be considered to consist of two
elements; a dorsal notopodium and a ventral neuropodium (Fig. 3). In
addition to bundles of chaetae, noto- and neuropodia can also have a
variety of cirri and gills. They are most elaborate in actively crawling
or swimming forms where they form large fleshy lobes that act as paddles.
Parapodia of burrowing or tubicolous polychaetes can simply be slightly
raised ridges carrying hooked chaetae called uncini.

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

parapodia of an annelid

Figure 3. Ophiodromus flexuosus (Hesionidae). Banyuls, France. Copyright © Fredrik Pleijel.

5. Nervous system

Annelids have a brain or cerebral ganglion that originates and usually resides in the head. The brain varies in structure, with mobile active forms having the most complex brains, and sessile or burrowing forms having simple brains with little differentiation. It is connected to the ventral nerve cord by the circumpharyngeal connectives, which run down each side of the pharynx. The ventral nerve cord, is usually made up of a pair of cords that are bound together and runs the length of the body (Fig. 4). It varies in thickness and dilates into a ganglion in each segment, from which pairs of segmental nerves pass out to the body wall, muscles and gut.

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

Ventral view of a scaleworm showing its double nerve cord

Figure 4. Ventral view of a scaleworm (Sigalionidae) showing its double nerve cord. This worm is unusual in having red pigmentation associated with the cord, making it easily seen. Misaki, Japan. Copyright © 1998 Greg Rouse.

6. Sense organs

There are six major kinds of sensory structures found in annelids. These include palps, antennae, eyes, statocysts, nuchal organs and lateral organs (Fig. 5). Palps and antennae are located on the head of many polychaetes. In some groups they are both sensory while in others the palps are used for feeding. Nuchal organs are ciliated, paired, chemosensory structures, innervated from the posterior part of the brain. They are present in nearly all polychaetes, and Rouse and Fauchald (1997) suggested that they may represent an apomorphy for Polychaeta. This has been challenged by other authors who suggest that nuchal organs may be an apomorphy for Annelida as a whole and have been lost in Clitellata/Oligochaeta (Purschke et al., 2000). Annelids also have a variety of epidermal sensory cells that may be responsive to light or touch (such as lateral organs).

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

eyes, palps, antennae of an annelid

Nuchal organ and lateral organs of an annelid

Figure 5. Left, Eumida (Phyllodocidae). Bundeena,
Australia. Copyright © 1999 Greg Rouse. Right, Proscoloplos (Orbiniidae). Bondi, Australia.
Copyright © 2001 Greg Rouse.

7. Circulation and respiratory structures

A closed circulatory system is present in most polychaetes (Fig. 6),
Echiura and many clitellates. It is reduced or absent in leeches where
it may be replaced by coelomic canals. A limited circulatory system
in which some of the major blood vessels are present but the distal
capillary vessels are missing is found in a number of polychaete groups.
A circulatory system is absent in many small polychaetes.

Click on an image to view larger version & data in a new window

Click on an image to view larger version & data in a new window

Dorsal blood vessel of an annelid

Figure 6. Australonereis ehlersi (Nereididae). Bundeena,
Australia. Copyright © 1999 Greg Rouse.

8. Segmental organs

In most annelids there are usually two fluid systems, the coelom and
the circulatory system, and both (if present) are involved in the excretion
of waste products. To achieve this excretion there must be ducts to
the exterior, and
these are generally referred to as nephridia. Ducts, known as gonoducts
or coelomoducts, are also required for the transfer of gametes that
develop in the coelom to the outside of the body on maturity. The two
different kinds of ducts are often simply referred to as segmental organs
since determining what kind of duct is present is problematic.

References

Eibye-Jacobsen, D., and R. M. Kristensen. 1994. A new genus and species of Dorvilleidae (Annelida, Polychaeta) from Bermuda, with a phylogenetic analysis of Dorvilleidae, Iphitimidae and Dinophilidae. Zoologica Scripta 23:107-131.

Pleijel, F., and G. W. Rouse. 2000. A new taxon, capricornia (Hesionidae, Polychaeta), illustrating the LITU (‘Least-inclusive taxonomic unit’) concept. Zoologica Scripta 29:157-168.

Purschke, G., R. Hessling, and W. Westheide. 2000. The phylogenetic position of the Clitellata and the Echiura – on the problematic assessment of absent characters. Journal of Zoological Systematics and Evolutionary Research 38:165-173.

Rouse, G. W., and K. Fauchald. 1997. Cladistics and polychaetes. Zoologica Scripta 26:139-204.

Rouse, G. W., and F. Pleijel. 2001. Polychaetes. Oxford University Press, London.

About This Page

Greg W. Rouse

Scripps Institution of Oceanography

Fredrik Pleijel

Muséum national d’Histoire naturelle Paris, France

Page copyright © 2002 Greg W. Rouse and


Page: Tree of Life
Characteristics of Annelida: Plesiomorphies and Other Features

Authored by
Greg W. Rouse and Fredrik Pleijel.
The TEXT of this page is licensed under the

Creative Commons Attribution License – Version 3.0 . Note that images and other media
featured on this page are each governed by their own license, and they may or may not be available
for reuse. Click on an image or a media link to access the media data window, which provides the
relevant licensing information. For the general terms and conditions of ToL material reuse and
redistribution, please see the Tree of Life Copyright
Policies .

close box

This page is an article that is attached to a
branch of the Tree of Life.

ToL articles provide more in-depth information about important features
of a given group of organisms.

For a more detailed explanation of the different ToL page types, have a look at the
Structure of the Tree of Life page.

close box

Annelida

Page Content

  • Main Content
  • References
  • About This Page

articles & notes

  • Annelida Branch Page
  • Characteristics of Annelida

Treehouses

  • Investigations
    • Marine Discovery Lab

    • Marine Discovery Slide Show

  • Stories
    • Squirt, The Earthworm

collections

  • Annelida Images
  • Annelida Movies

people

  • Annelida People

options

  • Setting Preferences
  • Show Glossary Entries

Explore Other Groups

other Bilateria

  • Deuterostomia

  • Arthropoda

  • Onychophora

  • Tardigrada

  • Nematoda

  • Nematomorpha

  • Kinorhyncha

  • Loricifera

  • Priapulida

  • Chaetognatha

  • Gastrotricha

  • Rotifera

  • Gnathostomulida

  • Limnognathia maerski

  • Cycliophora

  • Mesozoa

  • Platyhelminthes

  • Annelida

  • Bryozoa

  • Sipuncula

  • Mollusca

  • Nemertea

  • Entoprocta

  • Phoronida

  • Brachiopoda

containing groups

  • Bilateria

  • Animals

  • Eukaryotes

  • Life on Earth

subgroups

  • Phyllodocida

  • Echiura

random page

Skip
to main content

 

go to the Tree of Life home page
Go to quick links
Go to quick search
Go to navigation for this section of the ToL site
Go to detailed links for the ToL site

Site Navigation

  • Home
  • Browse
    • Browse
    • Root
    • Popular Pages
    • Sample Pages
    • Recent Additions
    • Random Page
    • Treehouses
    • Images, Movies,…
  • Help
    • Help
    • Finding Things
    • Navigating
    • Tree Diagrams
    • FAQ
    • Sitemap
    • Contact Us
  • Features
    • Features
    • Preferences
    • Forums
    • Glossary
    • Images, Movies,…
    • Webservices
    • Links
  • Learning
    • Learning
    • Treehouses
    • Building Treehouses
    • Evolution/Phylogeny
  • Contribute
    • Contribute
    • Scientific Content
    • Build Treehouses
    • Images/Media
    • Contributor Log-In
  • About
    • About
    • News
    • Goals
    • ToL/EOL
    • Structure
    • Home Picture
    • History/Future
    • Accessibility
    • Citing the ToL
    • Quality Control
    • Tech Development
    • Policies
    • People
    • Acknowledgments
  • advanced
    • Group Search
    • Text Search
    • Media Search

go to top

  • Home
  • Browse
  • Help
  • Features
  • Learning
  • Contribute
  • About
  • Search

Privacy
Policy — Copyright Policies

Tree of Life design and icons copyright © 1995-2004
Tree of Life Project. All rights reserved.

top


250 thistle logo

ENCYCLOPÆDIA BRITANNICA

250 thistle logo

START YOUR FREE TRIAL

Advertisement

Log In · Join

  • Demystified
  • Quizzes
  • Galleries
  • Lists
  • On This Day
  • Biographies
  • Newsletters

START YOUR FREE TRIAL

Advertisement

Search Britannica

What are you looking for?
Browse popular topics:
  • Chile earthquake of 1960
  • Ming dynasty
  • Chemical reaction
  • Boxer Rebellion
  • Pompeii

Bring fact-checked results to the top of your browser search.
Learn More.

chrome store logo

Annelids

Annelid, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. The coelom is reduced in leeches, and…

Displaying Featured Annelids Articles
  • Feces

    Earthworm

    Earthworm, any one of more than 1,800 species of terrestrial worms of the class Oligochaeta (phylum Annelida)—in particular, members of the genus Lumbricus. Seventeen native species and 13 introduced species (from Europe) occur in the eastern United States, L. terrestris being the most common.…

  • The structure of polychaetes. (Left) Free-moving polychaetes. (A) Neanthes, (B) Nereis. (Right) Tube-dwelling (sedentary) polychaetes. (C) Amphitrite, (D) Sabella.

    Annelid

    Annelid, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. The coelom is reduced in leeches, and…

  • European medicinal leech (Hirudo medicinalis).

    Leech

    Leech, (subclass Hirudinea), any of about 650 species of segmented worms (phylum Annelida) characterized by a small sucker, which contains the mouth, at the anterior end of the body and a large sucker located at the posterior end. All leeches have 34 body segments. The length of the body ranges…

  • lugworm

    Lugworm

    Lugworm, (genus Arenicola), any of several marine worms (class Polychaeta, phylum Annelida) that burrow deep into the sandy sea bottom or intertidal areas and are often quite large. Fishermen use them as bait. Adult lugworms of the coast of Europe (e.g., A. marina) attain lengths of about 23 cm (9…

  • medicinal leech

    Medicinal leech

    Medicinal leech, any of certain leech species (phylum Annelida), particularly Hirudo medicinalis, H. verbana, and H. orientalis, once used in the treatment of human diseases and used at present as a source of anticoagulants following certain surgical procedures. See…

  • earthworm

    Oligochaete

    Oligochaete, any worm of the subclass Oligochaeta (class Clitellata, phylum Annelida). About 3,500 living species are known, the most familiar of which is the earthworm (q.v.), Lumbricus terrestris. Oligochaetes are common all over the world. They live in the sea, in fresh water, and in moist soil.…

  • Palolo worm (Eunice)

    Palolo worm

    Palolo worm, any of various segmented marine worms of the families Eunicidae and Nereidae (class Polychaeta, phylum Annelida). The palolo worm exhibits unique breeding behaviour: during the breeding season, always at the same time of year and at a particular phase of the Moon, the worms break in…

  • sea mouse

    Sea mouse

    Sea mouse, (Aphrodita), any of a genus of marine worms of the class Polychaeta (phylum Annelida), named for their mouselike appearance and behaviour. Sea mice are usually 7.5–15 centimetres (3–6 inches) long; however, some attain a length of 30 centimetres (12 inches). The slightly arched back is…

  • Feather-duster worm (Sabella crassicornis)

    Feather-duster worm

    Feather-duster worm, any large, segmented marine worm of the family Sabellidae (class Polychaeta, phylum Annelida). The name is also occasionally applied to members of the closely related polychaete family Serpulidae. Sabellids live in long tubes constructed of mud or sand cemented by mucus,…

  • tube worm

    Tube worm

    Tube worm, any of a number of tube-dwelling marine worms belonging to the annelid class Polychaeta (see polychaete; feather-duster worm; tentacle worm). Other tube-dwelling worms include the horseshoe worm (phylum Phoronida) and the beardworm (phylum…

  • fireworm

    Fireworm

    Fireworm, any of certain segmented marine worms of the class Polychaeta (phylum Annelida), including species of the genera Hermodice and Eurythoe. Fireworms produce a stinging sensation if touched. The body of H. carunculata, found in the coral reefs of the Caribbean Sea, is covered with fine, …

  • Proboscis worm (Glycera dibranchiata)

    Bloodworm

    Bloodworm, any of certain bright red, segmented, aquatic worms of the phylum Annelida. Included are worms of the freshwater genus Tubifex, also known as sludge worms (class Oligochaeta, family Tubificidae), which are used as a tropical-fish food. The marine proboscis worm Glycera (class Polychaeta,…

  • Polychaete

    Polychaete, any worm of the class Polychaeta (phylum Annelida). About 8,000 living species are known. Polychaetes, which include rag worms, lugworms, bloodworms, sea mice, and others, are marine worms notable for well-defined segmentation of the body. Unique among annelids, most polychaete body…

  • Rag worm

    Rag worm, any of a group of mostly marine or shore worms of the class Polychaeta (phylum Annelida). A few species live in fresh water. Other common names include mussel worm, pileworm, and sandworm. Rag worms vary in length from 2.5 to 90 cm (1 inch to 3 feet); they are commonly brown, bright red,…

  • Parchment worm

    Parchment worm, (genus Chaetopterus), any of several species of segmented worms of the class Polychaeta (phylum Annelida), especially C. variopedatus of the Atlantic and Pacific oceans. They live on the sea bottom in U-shaped tubes that are lined with parchmentlike material. Parchment worms grow to…

  • Peacock worm

    Peacock worm, (Sabella), any of a genus of segmented marine worms of the class Polychaeta (phylum Annelida). This type of fanworm lives in a tube about 30 to 40 centimetres (12 to 16 inches) long that is open at one end and constructed of mud particles cemented together by mucus. All but the top…

  • Scale worm

    Scale worm, any member of the superfamily Aphroditoidea (class Polychaeta, phylum Annelida), a group of widely distributed free-moving, segmented marine worms that possess dorsal scales. Scale worms range in size from 0.5 to 25 cm (about 0.2 to 10 inches). The superfamily is made up of several…

  • Tentacle worm

    Tentacle worm, (Thelepus), any of a genus of tube-dwelling segmented worms of the class Polychaeta (phylum Annelida). They are sedentary forms that remain fixed to the sea bottom except as larvae. T. cincinnatus, 5 to 10 centimetres (about 2 to 4 inches) long and pale red, has lacelike markings o…

See All Annelids Articles

250 thistle logo

Menu
  • Home
  • Demystified
  • Quizzes
  • Galleries
  • Lists
  • On This Day
  • Biographies
  • Newsletters

  • Login
  • Join

share

Share
Share

Facebook

Twitter

Google+

LinkedIn

Email

Email this page

×

Download our free Chrome extension, Britannica Insights.

Britannica Insights screenshot

Facts matter and Britannica Insights makes it easier to find them.

Install for Chrome Now

Learn more