File:  <annelida.htm>                                <Index to Invertebrates>             <Bibliography>              <Glossary>                      Site Description                <Navigate to Home>


            Introduction                                                                                                                                        Onychophora


Invertebrate Zoology

Kingdom:  Animalia, Phylum: Annelida



Phylum:  Annelida -- ringed worms

     & earthworms


    Class:  Polychaeta
    Class:  Clitellata
:  Oligochaeta - earthworms, etc.
:  Branchiobdellida
:  Hirudinea - leeches


Class:  Myzostomida
    Class:  Arachianelida (Haplodrili)

Bibliography      Citations


Sample Examinations          Invertebrate Classification


CLICK on underlined file names and included illustrations to enlarge:


          The Annelida include the two-segmented worms, earthworms, and ringed worms.  Arrangement of the various groups on Annelida is in a constant state of flux as specialists continue to disagree on where to place the members.  As of July 2010 the classification show here tends to have the majority of adherents.


          They are the first phylum with segmentation.  Their body is divided into a linear series of metameres.  They have bilateral symmetry; a true coelom and one pair of nephridia are located in most segments.  Nephridia reach their highest point of development in this group.  They possess a non-chitinous cuticle but they have chitinous setae. 


          The nervous system consists of a dorsal brain, a ventral, double and solid nerve cord and a ganglion in every segment


          Circulation is via a well-developed blood system, which is usually closed, and a dorsal blood vessel or heart.


          Most species occur in the marine environment, but some are terrestrial and others are found in freshwater.  Their size varies from microscopic to over four meters in length. Marine forms have trochophore larvae and thus there is a common ancestry with the Mollusca.




           The Class Polychaeta, meaning "many bristles" includes those species that possess tufts of chitinous setae on every segment of the body.  All characteristics of the phylum are present.  There is a well-developed head with sensory structures, such as tentacles, palps and eyes.  There are lateral outgrowths called parapodia, which occur on every segment and are equipped with setae.  Transient gonads appear only during the breeding season and occur in many segments of the body.  There are no definite openings for gametes to exit the body, so they escape via temporary openings or by rupturing of the body wall.  Most are marine organisms, but some do occur in fresh and brackish water.


          An example Genus is Nereis (= Neanthes).  The species occur on coastal shores where they live in tubes in mud or sand, which are lined by a mucous secretion.


          Body Plan. -- The species are long and cylindrical with a series of segments that are more or less identical except for the posterior and anterior segments, which have homomous metamerism.  There is an iridescent cuticle, which is caused by fine striations.  The mouth is anterior and the anus posterior.  The head consists of two parts: (1)  prostomium that bears palps, tentacles and eyes and (2) peristomium that bears cirri and a mouth.  The posterior segment bears the anus and anal cirri.



          Body Wall. -- There is a cuticle, which is permeated with small holes through which mucous pores.  There is a glandular epidermis, circular muscles, feathery longitudinal muscles, a peritoneum and a coelom.  The coelom is divided into partitions, or septa, which correspond to each segment.  Pores through the septa allow for inter-communication.



          Food & Digestion. -- All species are primarily predaceous, their food consisting of small crustaceans and other worms.  The food is captured by means of an eversible pharynx that bears powerful jaws on the everted tip.



          There is a short esophagus and a long intestine where digestion and absorption occurs.  The wall of the intestine is well developed with circular and longitudinal muscles and it is covered by a peritoneum.


          Circulation. -- The dorsal blood vessel is contractile having peristaltic contractions.  It pumps the blood anteriorly.  Paired  branches lead to each segment, and the blood moves forward.


          The ventral blood vessel collects blood from the segments and carries it posteriorly.  It is usually not contractile.


          Commissures connect the two vessels at the anterior and posterior ends.  The blood is found in capillaries or vessels, but there are no sinuses.  The blood is dissolved in the plasma or haemoglobin.



          Respiration. -- The whole body surface functions in respiration, but the greatest respiration occurs in the parapodia.  The oxygen carrying capacity of the blood is high due to the haemoglobin.


          Excretion. -- One pair of nephridia occurs in all segments except the last few and the first segments.  The nephridium is composed of a funnel, a tubule and nephridiopore.  The funnel portion lies in one segment while the tubule and pore lie in the adjacent segment.  The funnel is called the nephrostome.  The tubule is well supplied with blood vessels and both secretions and excretions pass through its walls.



          Movement. -- There is a worm-like movement, which includes crawling, burrowing and swimming.  Nereis is primarily burrowing.


          Sense Organs. -- The eyes include a lens, pupil and retina.  Tentacles, palps, cirri, ventral portions of the parapodia and the general body surface all are sensory.


          Nervous System. -- Included here are a dorsal brain or cerebral ganglia, a ventral nerve cord, ganglia in each segment and circumpharyngeal connectives.



          Reproduction. -- Nereis has transient gonads, which are found in posterior parts of the body and seem to develop from the peritoneum.  There are no genital ducts and the gametes are shed through temporary openings in the body wall.  Fertilization occurs in the open water; however, males are attracted to the female while she is shedding the eggs.


          The mating season is predictable, being regulated according to phases of the moon.  In the South Seas a worm, Palolo living in coral burrows has posterior segments bearing gametes that break off and indulge in mating only.


          The sequence of development is shown in the following diagram Inv112:



          Asexual reproduction occurs which involves budding.


          Economic Importance. -- The Polychaeta have no direct economic importance but they may serve as food for other marine animals.




Please see following plates for Example Structures of the Polychaeta:


Plate 54 = Phylum: Annelida -- Class: Oligochaeta & Polychaeta: Lumbricus terrestris & Nereis sp.

Plate 69 = Phylum: Annelida -- Class: Polychaeta: Segmental structure of Nereis showing segmental organs

Plate 70 = Phylum: Annelida -- Class: Polychaeta: Blood system of Pomatocerus

Plate 71 = Phylum: Annelida -- Class: Polychaeta: Coelomic circulation in Aphrodite

Plate 72 = Phylum: Annelida -- Class: Polychaeta: Filter feeding in Sabella

Plate 73 = Phylum: Annelida -- Class: Polychaeta: Nephridia & coelomoducts in polychaetes

Plate 74 = Phylum: Annelida -- Class: Polychaeta: Nephridia in tubicolous polychaetes




          The Class Clitellata includes Subclasses Oligochaeta, Branchiobdellida and Hirudinea.


          The SubClass Oligochaeta, meaning "few-bristled" have members with no well developed head.  There are few bristles such as in the earthworm where there are only four pairs per segment.  Permanent gonads are located in definite segments of the body.  These are hermaphroditic animals with no parapodia.  They occur primarily in freshwater and terrestrial environments, but there are a few marine species.


          An example species is the earthworm Lumbricus terrestris.


          Body Plan. -- The head is absent and there are no parapodia.  Setae are few in number, with four pairs on each segment:  two ventrally and two ventro-laterally.  A swollen structure, the Clitellum is present on the epidermis, which secretes a cocoon in which the eggs are deposited.  Permanent genital openings occur for both sexes on the same animal.  The male opening is on segment 15 and the female opening on segment 14.  There are also dorsal pores on each segment that open from the coelom to the outside of the body, but their function is not completely understood.  The last segment bears the anus.


          Body Wall. -- This is similar to the Polychaeta except that the epidermis is very glandular and secretes mucous from one-celled gland cells.  The longitudinal muscle bundles are distinct, however.


          Food & Digestion. -- Food is obtained from organic debris and the earth generally.  The pharynx is highly muscular and sucks food into the body.  The esophagus is equipped with calciferous glands, which neutralize highly acid soils.  An enlarged part of the esophagus is the crop, which is thin-walled and used for storage.  A thick-walled gizzard is present that grinds up food material.


          There is an invagination of the intestine called the typhlosole, which increases the surface area for absorption and secretion of enzymes.



          Circulation. -- This is similar to Nereis except that aortic loops that encircle the esophagus are present, which connect dorsal and ventral vessels.  These loops are not the main pumping organs but they are the principal mechanism to maintain blood pressure.  The blood is made up of haemoglobin in plasma.


          Respiration. -- There is diffusion through the entire body surface, and there are many capillaries in the epidermis.  Hemoglobin in the blood also carries oxygen.


          Excretion. -- Paired nephridia are present.  Chloragogue tissue accumulates wastes.  This is situated around the intestine and typhlosole.  Pieces of the cells break off into the coelom and are picked up by nephridia.


          Locomotion. -- Contractions of the body and setae that enhance traction allow the animal to move about.


          Nervous System. -- There are no visible sense organs, but sensory cells occur that are light and taste sensitive.  Light sensitive areas are situated in the anterior and posterior regions, which taste or gustatory-sensitive areas are anterior.


          Reproduction. -- Lumbricus is hermaphroditic and only cross-fertilization occurs where each animal acts each sex during copulation.


          The organs involved in reproduction are the testes, of which there are two pair on segments 10 and 11.  Seminal vesicles enclose the testes and are used for storage and maturation of sperm.  There are three pair joined at the base.  The vas deferens joins to two pores.  Male genital pores are on the 15th segment.  One pair of ovaries occur on the 13th segment under the intestine.  The oviduct is connected to a pore on segment 14.  Two seminal receptacles are on each of segments 9 and 10.  They open to the outside by separate ducts and are used to store sperm received from male organs of other animals.  A clitellum secretes mucous to hold worms together during copulation and also to secrete a cocoon on segments 33-37.


          During copulation each worm exchanges sperm to the seminal receptacle of the other.  The clitellum secretes a cocoon around the body.  As the worm moves the cocoon begins to move interiorly, picking up eggs as it passes over the female pore and sperm and albumen as it passes over the seminal receptacle pore.  The ends of the cocoon are then closed.


          Fertilization is external in the cocoon.  Development in the cocoon involves no larval stage and a miniature worms hatch out.


          Asexual reproduction occurs which involves budding.


          Economic Importance. -- Earthworms are valuable in their ability to aerate and fertilize soils.



Please see following plates for Example Structures of the Oligochaeta:


Plate 53 = Phylum: Annelida: Oligochaeta: Lumbricus terrestris -- Dorsal View of Earthworm

Plate 54 = Phylum: Annelida -- Class: Oligochaeta & Polychaeta: Lumbricus terrestris & Nereis sp.

Plate 75 = Phylum: Annelida -- Various heads found in Oligochaeta




Comparison of Polychaeta with Oligochaeta


          In Polychaeta a parapodium is present, the sexes are separate and gonads are temporary.  There is a head with distinct sensory organs.  There is a free-swimming trochophore larva and all members occur in marine environments.  Asexual reproduction is by budding.  Haemoglobin is present.  Some species have a green pigment with iron called chlorocruorin.


          The Oligochaeta do not have a parapodium.  They are hermaphroditic with permanent gonads.  The head is indistinct and there are no sense organs.  Development is direct with no larval stage.  They inhabit terrestrial and freshwater environments.  There is also a budding asexual stage.  Haemoglobin is present and Chloragogue tissue accumulates wastes.




         The SubClass Branchiobdella has only about 150 species of small animals that are mainly parasites or commensals on crayfish),




          The SubClass Hirudinia includes the leeches, which are blood-sucking parasites of other animals.  They are primarily freshwater animals, but terrestrial and marine members exist.  Their characteristics bear a closer resemblance to Oligochaeta than Polychaeta.


          Body Plan. -- There is a higher degree of flattening than in the other members of the Annelida.  They possess conspicuous suckers on both ends of the body, and there are a definite number of segments for each species in the adult form.  Segments are subdivided into "annuli" which are a secondary segmentation. A cluster of simple eyes is situated on each side of the head (Inv114).



          Food & Digestion. -- There are some predatory species but most are parasitic on other organisms.  They have three chitinous jaws that are able to make incisions.  Salivary glands prevent coagulation of a host's blood.  The pharynx sucks the host's blood into the body.  The crop is very large with many lateral pouches that store food.  There is a stomach, intestine, rectum and anus.


          Circulation. -- There is a modified circulation where the blood goes into sinuses, which are remnants of the true coelom.  These are lined with gelatinous tissue called botryoidal tissue.  This reduced and modified coelom differs from all the other Annelida.  Part of the coelom is used as a haemocoel, and is thus thought of as a partially open system.


          Respiration. -- Capillaries are present in the epidermis and oxygen diffuses through the skin.


          Excretion. -- Nephridia perform excretion and bladders are present for storage.


          Motion & Locomotion. -- There is swimming and a measuring worm movement that is facilitated by suckers.


          Nervous System. -- Sense organs are the eyes and papillae on the anterior end.  Nerves are present as in other Annelida.


          Reproduction. -- This is similar to the Oligochaeta as they are hermaphroditic and cross-fertilization occurs.  The clitellum is not visible externally as there is no swelling.  Development is direct.  In some species the sperm are deposited on the body and they make their way through the body wall and into the cavity.


          Economic Importance. -- Hirudinia have been implicated in the transmission of some diseases.  Their attachment to swimmers in lakes causes fright and minor irritation.




          The Class Myzostomida includes a group of small parasitic worms that live on crinoids, a type of echinoderm. They were first discovered in 1827. Some species, as Myzostoma cirriferum, move around on the host.  Myzostoma glabrum remains motionless with its pharynx inserted in the mouth of the crinoid. Myzostoma deformator produces a gall on the arm of the host, one joint of the pinnule growing around the worm enclosing it in a cyst while Myzostoma pulvinar lives in the alimentary canal of a species of Antedon


          A typical animal has a flattened rounded shape, with a thin edge drawn out into tiny radiating hairs or cirri. The dorsal surface is smooth, with five pairs of parapodia on the bottom.  Parapodia are armed with hooked setae, by means of which the worm adheres to its host. Past the parapodia are four pairs of organs, called suckers. These organs are believed to be sensory  and are comparable to the lateral sense organs of Capitellids. The mouth and cloacal opening are usually at opposite ends of the bottom surface. The former leads to a protrusible pharynx, from which the oesophagus opens into an intestinal chamber with branching lateral diverticula. There is no observed vascular system. The nervous system consists of a circumoesophageal nerve, with a slightly differentiated brain, joining below a large mass of ganglia. The dorsoventral and the parapodial muscles are developed, but the coelom is reduced mostly to branched spaces in which the genitalia mature.


          Full-grown myzostomids are hermaphroditic. Their internal organs consist of a branched sac that opens to the exterior or each side of the animal. Paired ovaries discharge eggs into a median chamber with side branches, referred to as the uterus, from which the ripe ova are discharged by a mediar dorsal pore into the end of the rectum.




          The Class Arachianelida (Haplodrili) is sometimes considered as an order in the Polychaeta.  It is a small group of marine animals that do resemble the Polychaeta.  However, they are most primitive in some ways and quite advanced in others.  Segmentation is indistinct externally, but very obvious internally.  Parapodia and setae are absent.  Nervous tissue is in contact with the epidermis and there is only one brain ganglion.  Nephridia are present.



          These animals are hermaphroditic or dioecious.  There is a trochophore in the developmental cycle.  They have no known direct economic importance, but like the Polychaeta they may serve as food for other animals (See illustrations  Inv115, Inv116, and Inv117).








            Introduction                                                                                                                                                    Onychophora