Phylum Annelida

Dr. Elizabeth A. Bergey and Dr. Eric G. Bright, University of Oklahoma

Modified, with permission, from Invertebrate Anatomy OnLine
copyright 2003 by Richard Fox (Lander University)

Annelida Overview

Annelida consists of the segmented worms in the major taxa:

  1. Polychaeta (polychaetes or bristleworms)
  2. Clitellata (Oligochaeta = earthworms and relatives and Hirudinea = leeches)
  3. Echiura (marine spoon worms)

There is a total of about 12,000 known species in marine, freshwater, and terrestrial environments.  The segmented body is composed of an anterior prostomium, a linear series of similar segments, and a posterior pygidium.

The body wall consists of a secreted cuticle and epidermis. A connective tissue dermis lies beneath the epidermis. Many annelids have bristles, or chaetae, projecting from the body.  The coelom is large, segmentally compartmented, lined by peritoneum, and well developed in polychaetes and oligochaetes but reduced in leeches.  Successive coelomic spaces are separated by septa which consist of double layers of peritoneum with connective tissue in between.   

The gut is a straight, regionally specialized tube that penetrates each septum.  The nervous system consists of a dorsal brain, a pair of circumpharyngeal connectives around the anterior gut, and a double, ventral nerve cord with paired segmental ganglia and nerves.  The circulatory system of most annelids is a set of tubular vessels, some of which are contractile and serve as hearts.  The circulatory system is absent or greatly reduced in leeches.  The system includes a dorsal longitudinal vessel above the gut in which blood moves anteriorly, a ventral longitudinal vessel below the gut, in which blood moves posteriorly, and paired segmental vessels that connect the dorsal and ventral vessels. The digestive, circulatory, and nervous systems are continuous and pass through the segments. 

Respiration is accomplished in a variety of ways.  In some, the general body surface is sufficient but gills may be present, especially in polychaetes.  Excretory organs are best developed in freshwater and terrestrial species, where they are typically present in each segment. The sexes are separate in polychaetes but oligochaetes and leeches are hermaphroditic.

Class Polychaeta

Polychaeta is a large (about 8000 species) and diverse taxon of marine annelids thought to be the most like the ancestral annelid.  The body of a typical polychaete is divided into segments, each of which bears a pair of fleshy appendages, or parapodia.  The head is often equipped with abundant, well-developed sense organs.  The anterior gut is muscular, sometimes eversible, and frequently equipped with chitinous jaws.  Polychaetes are dioecious and gametes ripen in the coelom from which they are shed through ducts or by rupture of the body wall. 

We will not dissect the polychaetes. Whole specimens will be used for external anatomy; a cross-section will be examined to look at the general structure of the internal organs.
Nereids are considered as examples of basic annelid anatomy because they are like the ancestral polychaete.

Nereis virens, the clamworm or sandworm, is a large species that may reach almost a meter in length but is usually considerably smaller, around 20-40 cm.  It is a shallow-water, benthic species occurring from the shoreline to depths of about 150 m.  It occurs from Newfoundland to Virginia in North America and in northern Europe from Norway and Iceland to France.

Examine a preserved Nereis in a dissecting pan with tap water. 

Observe the elongate, vermiform shape and the bilateral symmetry of the worm.  The worm consists of a small anterior prostomium, a very long trunk, and a tiny posterior pygidium. The body is slightly depressed dorsoventrally and is divided into numerous (up to 200) segments.  Each segment bears a pair of fleshy, lateral appendages, the parapodia.

The body is covered by an iridescent, collagenous cuticle secreted by the epidermis beneath it. The iridescence is caused by microscopic striations which diffract light.

The head, with the mouth and an abundance of sense organs, is at the extreme anterior end.  Study the head with low power (5-10X).  It is composed of a dorsal, anterior prostomium with which several anterior segments have fused (Fig 1).  The prostomium itself is a smallish, flattened, triangular (in dorsal view) plate on the dorsal surface of the anterior end of the worm.  It is situated over (dorsal to) the very large, puckered, ventral mouth but does not extend around the sides of the mouth.       

Dorsal view of the head of Nereis virens.

The prostomium bears a pair of small, short antennae attached to its anterior margin, on either side of the midline.  A pair of much larger, fleshy palps extends anterolaterally and slightly ventrally from the sides of the prostomium.  Four black or reddish eyes are located on the posterior dorsal surface of the prostomium at the four corners of an imaginary trapezoid.

If the head of your specimen seems to differ markedly from this description or from, it is probably because the pharynx is partially or wholly protruded from the mouth. 

Under normal circumstances with the pharynx retracted, the mouth is located ventral to the prostomium as described earlier and no part of the body is anterior to the prostomium. When feeding, the anterior end of the gut (the pharynx) can be turned inside out and everted from the mouth.  This dramatically changes the anterior end of the worm.  The inner walls of the pharynx bear teeth and jaws.

The segment behind the prostomium bears four pairs of whip-like tentacular cirri and lacks parapodia.

The remaining segments resemble each other and have paired lateral parapodia which bear fleshy lobes with bristle-like setae. Look at an anterior parapodium from the region of about segment 20. The parapodia function in locomotion and gas exchange.  They have well-developed musculature and are heavily vascularized.  A typical polychaete parapodium is composed of two major branches, or rami, and accordingly is said to be biramous.  Each ramus bears clusters of chitinous bristles, chaetae, or setae.

Anterior view of the right parapodium of Nereis virens.

The posterior end of the worm is the pygidium, which is fused with the last regular segment to form a tiny tail.  The pygidium has two long anal cirri.  The anus is a large opening dorsal to the anal cirri.  The pygidium, cirri, and some posterior segments are often lost when the animals are collected or handled.

Cross section of Nereis

The body is covered by an outer integument consisting of the extracellular, cuticle secreted by the underlying, monolayered epidermis. 

A layer of circular muscle encircles the section just under the integument. The longitudinal muscles are divided into four conspicuous bundles, two dorsolateral muscle bundles and two ventrolateral muscle bundles.  A layer of oblique muscles covers the ventrolateral muscles. Try to determine the orientation of the long axis of the muscle fibers in the muscle bundles.  In a cross section the fibers would be cut in cross section in the longitudinal muscles and longitudinally in the circular and oblique muscles.

Cross section of Nereis virens.

The cavity surrounded by the muscles is the coelom. The gut (intestine) is visible near the center of the section surrounded by coelom.  The gut wall encloses the gut lumen. Mesenteries hold the gut in place and, like the septa, are double sheets of peritoneum.

The dorsal longitudinal blood vessel is in the dorsal mesentery.  The ventral longitudinal blood vessel and the ventral nerve cord are in the ventral mesentery.  In addition, also depending on the plane of section, you may also see segmental nerves, metanephridia, acicula, and parapodia or parts of parapodia. 

Some other polychaetes covered in lab

The sea mouse, genus Aphrodita, is a predatory marine polychaete (eating mostly polychaetes) that looks different from most polychaetes. In addition to having a relatively wide body (up to 8 inches long and 2 inches across), the sea mouse is covered with a pelt of iridescent hairs (chaetae). Light shining at different angles produces a variety of colors, ranging from red to green and blue. This iridescence seems a mismatch for an animal that often lives in deep water, where light much attenuated. Scientists have examined these hairs, looking at cross sections with an electron microscope and found holes in a hexagonal pattern with a period of less than a micron. This pattern resembles that found in photonic crystal fibers and scientists think that studying chaetal morphology can help with improvements in fiber optics and other technologies (like color advertising displays!).

Chaetopterus is called either the "parchment worm" because of the parchment-like tube in which it lives or the "inkeepter worm" because of the commensal crabs that share its tube. The adults have a unique feeding mechanism and are bioluminescent. They are an excellent model system for the study of regeneration.

The lugworm (genus Arenicola) is a burrowing marine polychaete that is common in muddy estuaries and sandy beaches from the middle shore downwards. Its presence is often indicated by holes and casts which mark the entrances to its U-shaped burrow. The lugworm is an important food for birds such as curlew and godwits which have longish beaks that probe the sand.

The clamworm or sandworm, Nereis virens, inhabits sandy and gravelly sediment types of the littoral and sublittoral zones in both marine and estuarine habitats. This species does not appear to do well in muddy sediment, at least under laboratory conditions.

Amphitrite are polychaetes that live in soft sediment on the ocean floor. They are selective deposit feeders. Special head structures extend over the benthic substrate, and move back and forth. Materials stick to the mucous found on these head structures, and is brought to the mouth and eaten. The worm does not have to leave it's burrow to feed.

Class Clitellata (Oligochaeta, leeches)

Clitellata includes earthworms and their allies, and the leeches.  The head is reduced and its sensory functions minimized. Clitellates lack the parapodia characteristic of polychaetes, although chaetae may be present, and also lack head and pygidial appendages.  The clitellum, a girdle-like band of secretory epidermis near the anterior end of the worm is present.  These worms are hermaphroditic and the reproductive system is restricted to a few specialized segments.  Development is direct, without a larva. 


Most of the 3500 known species of oligochaetes are small worms found in freshwater benthic habitats, although about 200 species live in the sea.  The group also includes terrestrial earthworms. 

The Earthworm

Lumbricus terrestris, the night crawler, is a well- known earthworm - mostly because it is used as fishing bait.  Its availability and large size make it good subject for study.

Examine the external features of the worm.  The anterior end is usually larger than the posterior end and the posterior end tends to be flattened.  The dorsum is darker than the venter.

Dorsal view of the anterior end of Lumbricus (segments are numbered).

A band of thickened secretory epithelium, the clitellum (clitell = saddle), girdles the body near the anterior end.  The clitellum secretes a mucus cocoon, into which gametes and albumen to nourish the developing embryos are released, and where fertilization occurs.

The body is segmented and each segment is separated by a groove. The prostomium (pro = before, stome = mouth) is a small dorsal lobe and is not counted as a segment. The segments are numbered anterior to posterior. 

Count the preclitellar segments in your specimen.  ____________ How many segments contribute to the clitellum?  _____________ Compare your counts with those made by other students.  Does the number of segments in each region appear to be constant or variable? ____________

The segments posterior to segment 1 are complete rings.  The posteriormost division of the body is the pygidium, which encircles the anus at the posterior tip of the worm.  Like the prostomium, the pygidium is not considered to be a true segment.

The eight small chaetae on each segment are usually visible with adequate magnification (25X).  The chaetae are arranged in four pairs.  The chaetae are used as anchors during peristaltic movement, so that elongation of the animal results in controlled, usually forward, motion.  The chaetae are retractile.

Rinse a live worm with tap water and place it on a damp paper towel on the stage of the dissecting microscope.  Watch how the segments change shape as the animal moved. Focus on the lateral body wall and watch the chaetae.  You may see the animal retract or protract some of its chaetae while you watch.

Earthworms are simultaneous hermaphrodites and each individual has complete female and male reproductive systems including separate external gonopores.  The genital pores are on the venter of segments 14 (female) and 15 (male). The male genital pores are flanked by two low ridges, so are more easily seen.

Segments posterior to the twelfth have a tiny coelomic pore on the dorsal midline in the groove between adjacent segments.  These are very difficult to find.  They are used to leak coelomic fluid onto the surface of the animal to keep it moist.  Each pore has a sphincter to prevent unnecessary fluid loss.

Slightly tilted ventral view of Lumbricus.

While holding the preserved worm, use your finest scissors to make a shallow incision a little to one side of the dorsal midline.  Start by pinching the body wall with forceps and then cut through the pinch with the scissors.  Extend the dorsal incision anteriorly to the prostomium and posteriorly to about segment 30-40, keeping the incision a little to the side of the dorsal midline.  Position the worm in a wax pan and pin the worm open with pins angled at about 45degrees (to keep the pins out of the way as much as possible).

The body cavity, or coelom, is partitioned by septa, which are transverse sheets of thin tissue that extend from the body wall to the gut tube. A thick and conspicuous layer of circular muscle lies inside the epidermis.  Inside the circular muscle layer is a thick layer of white or gray longitudinal muscle.

Digestive system

The digestive system is a straight tube extending from mouth to anus.  The gut is regionally specialized to perform the several functions, such as food procurement, storage, grinding, digestion, absorption, and feces formation. 

The mouth opens into the short, thin-walled buccal cavity in segments 1-3.  The pharynx is posterior to the buccal cavity in segments 3-5.  The pharyngeal wall is thick and muscular.  Numerous small radial muscles run from the pharynx to the body wall.  Contraction of these muscles dilates the pharynx.  The conspicuous, white cerebral ganglia (brain) is located atop the junction between the buccal cavity and the pharynx. 

Posterior to the pharynx, the gut narrows to become the thin-walled esophagus in segments 6-12.  The posterior end of the esophagus is hidden by six large, creamy white seminal vesicles that arch over it.  The esophagus narrows and joins the large, bulbous, thin-walled crop in about segment 12.  The crop is a food storage organ.   Posterior to the crop is the gizzard. The gizzard has thick, heavy, muscular walls for grinding food into smaller fragments. The buccal cavity, pharynx, esophagus, crop, and gizzard form the foregut

A nearly complete anterior end of a dissected earthworm.
The gut and reproductive of a dissected earthworm.

Posterior to the gizzard, the gut narrows again and becomes the intestine (or midgut), which is the region for chemical digestion and absorption.   The intestine extends from the end of the gizzard almost to the anus and its dorsal wall is invaginated to increase surface area.

The extreme posterior end of the gut is the rectum, or hindgut.  It opens to the exterior via the anus.

Circulatory system

The basic plan of the circulatory system includes dorsal and ventral longitudinal vessels, which are connected in each segment by paired segmental vessels.  The dorsal vessel and five pairs of segmental vessels in the region of the esophagus are contractile and function as hearts.  Blood flows anteriorly in the dorsal vessel (where one-way valves prevent backflow) and posteriorly in the ventral vessel.

Find the large dorsal blood vessel on the midline of the dorsal surface of the gut.
Follow the dorsal vessel anteriorly to the esophagus and find the five pairs of large, contractile, segmental blood vessels which function as hearts.  They pump blood ventrally, which is the reverse of the other segmental vessels. The hearts may be hidden by the septa and nephridia of their segments.  Remove these as necessary to reveal the hearts.

The ventral blood vessel is attached to the ventral side of the gut by the narrow mesentery. 

The blood of earthworms is red because of the hemoglobin, which is in solution.

Earthworms have no specialized respiratory structures and gas exchange takes place across the general epidermis which can be moistened if necessary with fluid from the coelomic pores.  The integument is heavily vascularized to serve its respiratory function. 

Organ systems

The earthworm nervous system is consists of a dorsal, anterior brain, circumpharyngeal connectives, ventral subpharyngeal ganglion and a ventral nerve cord with segmental ganglia, commissures, connectives, and segmental nerves.

The brain, consists of a pair of cerebral ganglia above the anterior pharynx in segment 3.  If your previous incision exposed the buccal cavity, the brain is visible without further dissection.  Each ganglion is white and pear-shaped, and the two are connected across the midline.  The brain is the primary center for coordination of sensory and motor functions.

A large circumpharyngeal connective exits the side of each cerebral ganglion and runs ventrally around the pharynx to join the subpharyngeal ganglion ventral to the pharynx.  (We will not dissect out this ganglion, which is a major center for motor control). 

The double, solid ventral nerve cord extends posteriorly, on the ventral midline, for the length of the worm.  The nerve cord swells in each segment to form a segmental ganglion.

There are no special sense organs in earthworms but the body surface, especially that of the head, bears receptor cells for taste, touch, light, and vibration.

Almost all segments of the earthworm possess a pair of complex metanephridia on the sides of each segment. These may be difficult to see in preserved specimens. 

Oligochaetes are simultaneous hermaphrodites and each individual contains complete and simultaneously functional male and female systems.  The female system produces eggs and receives and stores sperm from the partner.  The male system produces sperm and delivers it to a partner during copulation.  The reproductive system is restricted to a few preclitellar segments (9-15). You will need to recognize the reproductive system, but do not need to know its parts. 

FYI. reproductive system in segments 9-16 of Lumbricus.  The second right seminal receptacle has been removed.  The sperm reservoirs are drawn as if transparent so the testes and sperm funnels are visible. 

Earthworm Cross Section

Look at a slide of an earthworm cross section using the compound microscope.  Look at the section first with low power (40X) and orient yourself.  Find the body wall, coelom, gut wall, and gut lumen.

Look at the body wall with high power (400X) beginning on the outside of the worm and work inward.  The outermost layer is the thin, noncellular cuticle. The cellular epidermis lies immediately below the cuticle and is a monolayered epithelium containing abundant secretory and sensory cells. The next layer is fairly thick and is the circular muscle of the body wall.  The muscle fibers of this layer are seen here in longitudinal section.  Their nuclei are clearly visible.  Setae (chaetae) may be visible.

Cross section through the intestinal region of Lumbricus.  The section is slightly oblique. 

The thickest layer of the body wall is the longitudinal muscle layer, which lies inside the circular muscle.  Its fibers run parallel to the long axis of the worm and you see them here in cross section.  The featherlike appearance of these fibers when in cross section is distinctive.   The large open space is the coelom.  Depending on the location of the section you may also see other features. 


True leeches are specialized annelids.  Most leeches live in freshwater; a few are amphibious or terrestrial.  Leeches are predatory or parasitic; the parasites consume blood - usually that of vertebrates.

Leeches have anterior and posterior suckers.  The underlying segmental organization of these annelid worms is obscured by superficial rings around the body.  Parapodia, chaetae, and head appendages are absent.       

The circulatory system is reduced or absent and its role in fluid transport has been taken over by coelomic channels.  The coelom lacks septa or mesenteries and is reduced to four longitudinal channels.

The excretory system is metanephridial.  The nervous system is typically annelidan.  Like oligochaetes, leeches are hermaphroditic and have a clitellum.  Eggs are enclosed in a hard proteinaceous cocoon.  Development is direct.

This exercise is written for Haemopis marmorata, the predatory American horse leech and Hirudo medicinalis, the blood-feeding European medicinal leech.

The use of Hirudo for medical purposes has regained favor recently.  Leeches are now used with considerable success to maintain arterial blood flow to traumatized areas with impaired venous drainage.  They are used by plastic surgeons in the repair of skin flaps or severed digits.  The leech provides the drainage necessary to sustain and encourage arterial flow to the traumatized region.  The leech anticoagulants hirudin and hementin are administered to cardiac patients to inhibit clot formation and to destroy existing clots respectively.  At least two companies, Leeches USA and Biopharm UK, have been established to provide the medical profession with medicinal leeches and pharmaceuticals derived from leeches. Unfortunately for the leeches, those used in medicine are considered biohazards and are disposed of.

Haemopis is an opportunistic predator and scavenger feeding on a wide variety of small animals, both living and dead.  Among its prey are earthworms and other oligochaetes, other leeches, insects, small crustaceans, snails, slugs, and amphibian tadpoles and eggs. Haemopis is amphibious and at night crawls many meters from the water to feed on small terrestrial animals and carrion.  One species is semiterrestrial.

Our examination of leeches will be restricted to external anatomy.

The leech body is elongate and dorsoventrally flattened.  The shape is highly variable from species to species and sometimes it may be nearly cylindrical or short and fat.  The body is covered by a thin cuticle and the integument is pigmented, often with bright colors but sometimes it is drab gray or black. 

The posterior sucker is larger than the anterior sucker.  Both are equipped with gland cells that secrete adhesive.  The large preoral chamber occupies the interior of the anterior sucker; the mouth is a small opening in the anterior sucker.   

The body of all leeches consists of the prostomium plus 33 segments of which 26 are body segments and seven form the posterior sucker.  The first five segments are the head, which can be seen dorsally but is ventrally obscured by the sucker.  

Most segments are superficially subdivided into false segments, or annuli, which make it difficult to recognize the true segments.  Most segments have five annuli but some have fewer.  It is not necessary that you recognize the boundaries of segments.

Haemopis has five pairs of small black eyes that form an arc around the anterior end, but the eyes may be difficult to see against the darkly pigmented integument.  Each eye points in a different direction.  The eyes have different arrangements in other genera and the arrangement is used in identification.

Ventral view of the leech, Haemopis marmorata.  The insets are dorsal views of the anterior and posterior ends.

Leeches are hermaphroditic with male and female systems opening independently of each other.  The two relatively large gonopores are on the ventral midline in the anterior quarter of the body. The leech clitellum is present only during periods of reproductive activity and even then it is inconspicuous.  Gland cells of the clitellum secrete a hardened, proteinaceous cocoon into which fertilized eggs are deposited.

© Copyright by Elizabeth Bergey and Eric Bright 2016

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