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In this article we will discuss about Annelida:- 1. History of Annelida 2. General Characters of Annelida 3. Classification 4. Metamerism 5. Coelom 6. Segmental Organs 7. Reproduction.
Contents:
- History of Annelida
- General Characters of Annelida
- Classification of Annelida
- Metamerism in Annelida
- Coelom in Annelida
- Segmental Organs of Annelida
- Reproduction in Annelida
1. History of Annelida:
The annelids were by early zoologists included with other worms in the group Vermes but were separated by Cuvier in 1798 from the un-segmented worms. The name Annelida was first of all used by Lamarck in 1809 for the higher segmented worms.
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Annelida (L., annellus = little ring or F .anneler = to arrange in rings) are triploblastic, bilaterally symmetrical, coelomate and segmented Metazoa. Body is covered by a thin cuticle having chitinous setae in most. Body wall is covered with glandular epidermis, below which are muscles forming an outer layer of circular muscles and an inner layer of longitudinal muscles.
The body is divided for the first time in the animal series into metamerically arranged segments or metameres arranged in a linear series, but the segments are integrated into a single functional unit. Perivisceral cavity is a schizocoelic coelom between two layers of mesoderm. The coelom contains a coelomic fluid which is incompressible, consequently it acts as a hydraulic skeleton.
There is a single pre-oral segment called prostomium and a similar post-segmental region posteriorly known as a pygidium. The nervous system has a pair of pre-oral ganglia or brain, and paired ventral nerve cords ganglionated in each segment. There is a closed circulatory system.
The digestive tract is more or less straight but differentiated into well-defined regions; digestion is entirely extracellular. Organs of excretion are metameric ectodermal nephridia, besides which there are tubular mesodermal coelomoducts used for passage of reproductive cells. The larva, if present, is a trochosphere.
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The members of the phylum are modified for sedentary, active, or ectoparasitic life, they occur on land, in freshwater, or in the sea.
The phylum contains over 8,600 known species and is divided into four classes called Polychaeta, Oligochaeta, Himdinea, and Archiannelida. But in some systems of classification, Echiuroidea, Sipunculoidea, Priapulida, and Myzostomaria have also been included as classes, while in other systems these classes have been considered as appendix to phylum Annelida or to some of its classes.
2. General Characters of Annelida:
1. Annelida are mostly aquatic; marine or freshwater, some terrestrial, bun-owing or living in tubes, sedentary or free-living. Some are commensal and parasitic also.
2. Body triploblastic, bilaterally symmetrical, elongated and vermiform.
3. Body metamerically segmented; externally by transverse grooves and internally by septa into a number of divisions; each division is called a segment, metamere or somite.
4. Organ-system grade of body organisation.
5. Outer covering of the body is cuticle secreted by the underlying epidermis.
6. Body wall is contractile, consisting of an outer epidermis, circular and longitudinal muscles.
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7. Appendages when present are un-jointed.
8. Locomotory organs are segmentally arranged paired setae or chaetae in most of the cases.
9. Presence of a true schizocoelous coelom usually divided into a large number of compartments by inter-segmental septa.
10. Alimentary canal is tube-like, complete, extending from mouth to anus.
11. Respiration occurs through general body surface, in some cases by gills also.
12. Blood vascular system is closed type, blood is red due to the presence of haemoglobin or erythrocruorin found dissolved in the plasma.
13. Excretion by segmentally arranged nephridia which usually communicate the coelom to exterior.
14. Nervous system with a pair of cerebral ganglia, the brain and a double nerve cord having segmentally arranged ganglia.
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15. The tactile organs, taste buds, statocysts, photoreceptor cells and eyes are the receptor organs.
16. Usually monoecious, i.e., hermaphrodite; dioecious or unisexual forms also present.
17. Development direct in monoecious forms but indirect in dioecious forms.
18. A free-swimming trochophore larval stage is characteristic in case of indirect development, while in others this stage is passed during the development.
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19. Asexual reproduction also occurs in some forms.
3. Classification of Annelida:
Class 1. Polychaeta
(Gr., poly = many + chaete = bristles):
1. Polychaeta are marine and carnivorous.
2. Body is elongated and segmented.
3. Head consists of prostomium and peristomium and bears eyes, tentacles, cirri and palps, etc.
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4. Setae are numerous and are borne up on lateral prominences of the body wall known as Para podia.
5. Clitellum is absent.
6. Cirri or branchiae or both may be present for respiration.
7. Coelom is spacious usually divided by inter-segmental septa.
8. Alimentary canal is provided with an eversible buccal region and protrusible pharynx.
9. Excretory organs are segmentally paired nephridia.
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10. Sexes separate, fertilisation external, free swimming larval stage trochophore.
11. Asexual reproduction by lateral budding
Order 1. Errantia:
1. Free-swimming, often pelagic, while some living in tubes.
2. All body segments are similar except at anterior and posterior ends.
3. Parapodia, provided with cirri, are equally developed throughout.
4. Head with distinct prostomium which is provided with eyes, tentacles and palps.
5. Pharynx is usually protrusible and armed with chitinous jaws and teeth.
Examples:
Nereis, Aphrodite, Glycera, Polynoe, Syllis.
Order 2. Sedentaria:
1. Burrowing and tube dwelling forms.
2. Body is divisible into two or more regions with unlike segments and Para podia.
3. Head is small or much modified, without eyes and tentacles, prostomium indistinct.
4. Pharynx is non-protrusible devoid of jaws or teeth.
5. Gills, when present, localised to the anterior segments.
6. Feeding on plankton or organic detritus.
Examples:
Chaetopterus, Terebella, Sabella, Arenicola, Serpula, Amphitrite, Spirorbis.
Class 2. Oligochaeta:
(Gr., oligos =few + chaete = setae):
1. Mostly terrestrial or some freshwater forms.
2. Body with conspicuous external and internal segmentation.
3. Distinct head, eyes and tentacles are absent.
4. Parapodia are absent.
5. Setae are usually arranged segmentally.
6. Clitellum is usually present.
7. Pharynx is not eversible and without jaws.
8. Hermaphrodite, i.e., sexes united.
9. Development is direct and takes place within cocoons secreted by clitellum; no free larval stage.
Order 1. Archioligochaeta:
1. Mostly freshwater forms.
2. Body consists of few segments.
3. Setae are present in bundles.
4. Gizzard is poorly developed, non- muscular or absent.
5. Clitellum is simpler, consists of single layer of cells and situated far forwards.
6. Eye spots are frequently present.
7. Male reproductive openings lie in front of the female reproductive openings.
8. Reproduction asexual and sexual.
Examples:
Tubifex, Aelosoma.
Order 2. Neooligochaeta:
1. Usually terrestrial forms.
2. Body is large and many segmented.
3. Setae are arranged in lumbricine manner.
4. Gizzard is well developed.
5. Clitellum is composed of two or more layers of cells and never begins before twelfth segment.
6. Female genital apertures are always on the fourteenth segment and the male pore lies a few segments behind them.
7. Vasa deferentia are elongated extending over the three or four segments.
8. Eye spots are never developed.
9. Reproduction sexual. Asexual reproduction is not known
Examples:
Pheretima, Eutypheus, Megascolex, Lumbricus.
Class 3. Hirudinea:
(L., hirudo = a leech):
1. Mostly ectoparasitic and freshwater forms, while few are marine feeding upon fishes and other animals.
2. Body is elongated usually flattened dorsoventrally or cylindrical.
3. Body consists of definite number of segments, each segment breaks up into 2 to 4 rings or annuli.
4. Parapodia and setae are absent.
5. Body is provided with an anterior and a posterior sucker, both situated ventrally.
6. Mouth opens on the ventral surface in the anterior sucker, while anus opens dorsal to the posterior sucker.
7. Hermaphrodite, i.e., sexes united; reproduction sexual.
8. Asexual reproduction is not known.
9. Eggs are usually laid in cocoons.
10. Development is direct without free swimming larval stage.
Order 1. Acanthobdeilida:
1. Mostly parasitic on the fins of salmon fishes.
2. Body comprises thirty segments only.
3. Anterior sucker is absent but posterior sucker is well developed and composed of four segments.
4. Anterior five segments are provided with double rows of setae.
5. Proboscis is short.
6. Body cavity is spacious and incompletely divided by septa.
7. Vascular system consists of dorsal and ventral vessel.
8. Nephridial opening situated on the surface between the segments.
9. Acanthobdeilida forms a connecting link between Oligochaeta and Hirudinea.
Example:
Acanthobdeilida.
Order 2. Rhvnchohdellida:
1. Parasites on snails, frogs and fishes, marine and freshwater forms.
2. Each typical body segment consists of 3, 6 or 12 rings.
3. Mouth is small median aperture situated in the anterior sucker.
4. Proboscis is prostrusible; jaws are absent.
5. Blood is colourless.
6. Goelom is reduced to sinuses without botryoidal tissues.
Examples:
Pontobdella, Glossiphonia, Branchellion, Piscicola.
Order 3. Gnathobdetlida:
1. Freshwater and terrestrial forms.
2. Each typical body segment consists of five rings or annuli.
3. Anterior sucker with three jaws, one median dorsal and two ventro-lateral.
4. Proboscis is absent.
5. Blood is red coloured.
6. Botryoidal tissue present.
Examples:
Hirudo, Haemopis,
Hirudinaria, Herpobdella.
Class 4. Archiannelida
(Gr., archi = first):
1. Exclusively marine forms.
2. Body elongated and worm-like.
3. Setae and Para podia are usually absent.
4. External segmentation is slightly marked by faint, while internal segmentation is marked by coelomic septa.
5. Prostomium bears two or three tentacles.
6. Unisexual or hermaphrodite.
7. Larva is typical trochophore.
Examples:
Polygordius, Protodrillus, Nerilla, Saccocirrus.
4. Metamerism in Annelida:
Development of a coelom is bound up with the formation of a series of gonadial coelomic sacs lying on both sides of the gut, but in most coelomate animals the coelom is a large perivisceral cavity, yet traces of the segmental nature of coelom are evident, it is divided into compartments by inter-segmental septa and many other systems are also segmentally arranged, thus, in Annelida the body consists of linear series of segments built, more or less, on the same plan and resembling each other.
Animals whose bodies conform to such a plan are said to be metamerically segmented or to show metamerism. In metamerism not only is there a serial repetition of homologous parts (e.g., nephridia, blood vessels, nerves, reproductive organs, muscles), but each of these parts works in cooperation with all the others, the segments being integrated into a single functional unit, the segmental structures are interdependent.
Annelida show metameric segmentation of the main organs and of the entire body which is divided into a series of segments by transverse partitions. The youngest segments occur at the posterior end, and new segments are formed in front of the last segment or pygidium.
5. Coelom in Annelida:
The coelom in Annelida is a perivisceral cavity between the body wall and alimentary canal, it is formed from segmental vesicles of the mesoderm, it is lined on the outer side by a parietal layer of mesoderm and on the inner side by a visceral layer of mesoderm, these mesodermal layers form the peritoneum.
The walls of the coelom give rise to reproductive cells and to coelomoducts which carry sperms or eggs from the coelom to the exterior. Excretory organs lead from the coelom to the outside. In some Polychaeta the coelomic peritoneum gives rise to excretory yellow cells. Coelom contains a coelomic fluid having amoeboid corpuscles, this fluid absorbs nourishment and permits transport of materials in solution.
The coelom in Polychaeta and Oligochaeta is a well developed space. In Polychaeta the coelom is perivisceral, but it is divided by a series of transverse septa lying inter-segmentally from the body wall towards the gut. A septum is a double fold of peritoneum enclosing muscle fibres. The coelomic chambers are serial and they communicate through spaces around the alimentary canal where the septa are not complete.
In Arenicola there are only first three septa and some at the posterior end so that the coelom is almost an uninterrupted space. In Aphrodite there is a spacious coelom lined with cilia which bring about circulation, it has developed at the expense of the blood system.
In Oligochaeta the large perivisceral coelom is divided into compartments by septa lying inter-segmentally from the body wall to the alimentary canal. The first septum in Pheretima lies between segments 4 and 5, so that the coelom of the first four segments is continuous.
The next eight septa have no apertures so that their coelomic chambers are shut off from the others, from the fourteenth segment to the end the septa have many apertures with sphincter muscles, hence, all these coelomic chambers communicate with each other.
In Hirudinea the coelom has been obliterated as a perivisceral cavity by formation of botryoidal tissue. Only one primitive leech (Acanthobdella) has a perivisceral coelom in the anterior region with septa. In Hirudinaria it is represented by four longitudinal haemocoelomic channels, their branches, and by spaces enclosing gonads and vasa deferentia.
The coelomic fluid in the longitudinal channels and their branches is red due to dissolved haemoglobin, but it is not coloured in other coelomic spaces. Archiannelida have a large coelom divided into chambers by transverse septa.
6. Segmental Organs of Annelida:
Annelid have segmentally repeated tubes called segmental organs, they are nephridia and coelomoducts.
Nephridia:
Nephridia are coiled tubes formed by invagination of ectoderm, they lie in the coelom, their ciliated lumen is intracellular.
A nephridium opens into the coelom by a ciliated funnel or nephrostome either in the same segment in which it lies or in the segment just in front; at the other end the nephridium opens to the exterior by a nephridiophore. Nephridia remove waste from the coelom, but their original function was probably removal of water from the body.
Coelomoducts:
Coelomoducts are segmentally repeated mesodermal tubes which open at one end into the coelom by a wide ciliated funnel (which is quite different from a nephrostome), and at the other end to the exterior, their lumen is intercellular.
Coelomoducts may be excretory, or combine excretion with the function of conducting germ cells to the exterior, or only to conduct germ cells to the exterior which was perhaps their original function.
Nephridia and Nephromixia:
Some Polychaeta have nephridia of closed tubes, their blind ends project into the coelom, this is a primitive condition. The blind end is fringed with solenocytes lying separately or in groups. The solenocytes are round ciliated cells connected to the nephridium by a thin tube each, in the lumen of the tube is a long vibratile flagellum; solenocytes resemble flame cells of Platyhelminthes.
Such nephridia are known as protonephridia, e.g., Phyllodoce, Vanadis. But in many Polychaeta and Oligochaeta the nephridia are of the open type, each having a ciliated nephrostome opening into the coelom, they are called metanephridia, e.g., Neanthes, Lumbricus. Some Polychaeta have compound excretory organs formed by the union of nephridia and coelomoducts, they are called nephromixia.
In these the functions of excretory organs and genital ducts become combined.
They are of three types:
(a) Protonephromixium, the coelomoduct has combined with a closed protonephridium, e.g., Aliciopidae, Phyllodoce.
(b) Metanephromixium, coelomoduct is attached to an open metanephridium, e.g., Hesione.
(c) Mixonephrium or Nephromixium, coelomoduct is attached to a nephridium in such a way that they form a single organ, the funnel being coelomoduct and the duct is nephridial, e.g., Capitellidae, Arenicola. There is no hard-and-fast boundary between metanephromixium and mixonephrium.
In some Polychaeta, e.g., Neanthes, a part of the coelomoduct separates from the metanephromixium and becomes attached to the dorsolateral muscles as a dorsal ciliated organ, this keeps the coelomic fluid in circulation.
In some tubicolous worms, e.g., Serpula there is a division of labour, the nephridia of the anterior region are large and carry on excretion, and those of the posterior region are small and act as genital ducts exclusively.
In Oligochaeta and Hirudinea nephridia and coelomoducts are separate. There is generally one pair of metanephridia in each segment, but coelomoducts are restricted only to a few reproductive segments. Their nephridia may open to the outside of the body and are called exonephric nephridia (e.g., Lumbricus) or they may open into the gut and are known as enteronephric nephridia (e.g. Pheretima).
In a majority of earthworms there is a pair of original large-sized metanephridia in each segment, they are called holonephridia or meganephridia, e.g., Lumbricus. But in Pheretima there is large number of small-sized nephridia in each segment, they are called meronephridia or micronephridia. It is assumed that the original pair of holonephridia has broken up to form a large number of meronephridia.
In Pheretima there are three types of meronephridia:
(a) Those of the anterior segments are many enteronephric meronephridia opening into the pharynx, they may have taken on the function of digestive glands and are called peptonephridia.
(b) In every segment behind the sixth are integumentary exonephric meronephridia.
(c) In all segments after the fourteenth there are enteronephric meronephridia which open into supra-intestinal excretory ducts having segmental opening into the intestine.
In Hirudinea the nephridia are generally like metanephridia of Oligochaeta, with a ciliated nephrostome opening into a coelomic space, e.g., Hirudo. In Hirudinaria the nephridia are coiled tubes opening into a bladder which leads to the exterior by a nephridiophore, the other end lies in a coelomic space but it has no nephrostome.
In some Rhynchobdellida, e.g., Pantobdella, there is a complex network on the ventral surface of the body which gives out a pair of branches in each segment terminating in a ciliated funnel, and a similar pair of branches opening to the exterior. Archiannelida have a pair of nephridia in each segment, they may be closed protonephridia with solenocytes, or they may be metanephridia with nephrostomes opening in the coelom, e.g., Polygordius.
7. Reproduction in Annelida:
In Polychaeta sexes are separate. The gonads are patches of coelomic epithelium and are repeated in most of the segments. The gonads become conspicuous during the breeding season and they proliferate a very large number of germ cells which detach and fill coelom where they undergo maturation in the coelomic fluid.
When ripe the germ cells pass to the exterior either through segmental organs or by rupture of the body wall. Fertilisation occurs in sea water. In many forms a phenomenon of swarming occurs, the crawling or burrowing worms rise to the surface to discharge their sex cells, then they sink to the bottom.
Swarming habit is an adaptation for securing fertilisation of the greatest possible number of eggs. Swarming is usually during definite periods and often coincides with lunar periods. Discharge of gametes is nearly always followed by death of the sexual individuals. Fertilised egg gives rise to a trochosphere larva.
In syllids gonads are usually confined to the posterior part of the body which is detached as a free-swimming zooid which develops a head but no jaws or pharynx, it lives for a time to produce gametes. Many annelids have the ability to regenerate lost parts, this is accompanied by a capacity to reproduce asexually.
Some forms reproduce asexually by budding, but in Autolytus there is a proliferating region at the end from which a chain of sexual zooids is budded off which detach one by one. Syllis forms many branches by budding, some of which form a head, develop sex organs, notopodia are formed to reconstruct the parapodia, these sexual forms may remain attached to the parent for long or they may separate from the colony.
In Oligochaeta certain features of reproductive organs are salient characters, they are almost all hermaphrodite.
The sex cells are discharged into the coelom or into seminal vesicles which are special parts of the coelom separated from the rest, they are large coelomic sacs varying in number in different genera; often a pair of seminal vesicles may coalesce to form a median sperm reservoir into which ciliated funnels of vasa deferentia open.
Testes may be several but ovaries are never more than two. Spermathecae are usually present to receive spermatozoa of another worm during copulation.
The clitellum is a glandular development of the epidermis for formation of cocoons and albumen for nourishment of the embryo.
The clitellum may be permanent, as in earthworms, or it may develop only during the breeding season. Some Oligochaeta possess special copulatory setae. In some Oligochaeta asexual reproduction occurs, e.g., Nais and Chaetogaster multiply by proliferation of segments at the posterior end forming a chain of zooids which eventually separate and acquire sex cells.
Hirudinea are hermaphrodite with several pairs of testes but only two ovaries, gonads are completely shut off in closed coelomic vesicles, but they are continuous with their ducts in distinction from other Annelida. The spermatozoa unite in bundles to form permatophores.
Generally copulation occurs, though in some hypodermic impregnation takes place. The clitellum appears during breeding season, and eggs are laid in cocoons formed by clitellar glands.
Archiannelida are generally hermaphrodite, the ovaries occurring in anterior segments and testes behind them, so that gonads are restricted to a few segments. In Polygordius sexes are separate, the ovaries or testes develop in a few posterior segments, there are no ducts.
