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In this article we will discuss about the General Characters and Classification of Phylum Annelida.
General Characteristics of Phylum Annelida:
Some of the general characters of Phylum Annelida are listed below:
1. Habitat:
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Annelids occur in fresh water, sea water or moist soil. Some are free living, some are burrowing and a few are parasitic.
2. Metameric Segmentation:
The annelids are characterised by metameric segmentation, viz., the body is divided externally by ring like grooves the annuli (Latin, annulus: little ring) and internally by transverse septa. The segments are called metameres. The first segment is called peristomium. An outgrowth, known as prostomium, arises from the peristomium.
3. Setae and Parapodia:
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Except leeches, un-jointed chitinous setae are often present. Some annelids such as Nereis have un-jointed, locomotory structures, the Para podia (Para parallel, podia- feet).
4. Body wall.
The body wall consists of thin, and moist non-cellular cuticle, single layered epidermis and circular and longitudinal muscles. The muscles are smooth which are highly contractile and help in locomotion.
5. Coelom (Body Cavity):
A true coelom is present. From evolution point of view, annelids are, perhaps, the first animals to have a true schizocoelic coelom. In most annelids coelom is divided by septa into compartments. The coelom is filled with coelomic fluid which contains cells.
6. Hydro skeleton:
The coelomic fluid serves as a hydro skeleton.
7. Digestive Tract:
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It is complete.
8. Respiratory Organs:
Exchange of gases usually occurs through the skin (cutaneous respiration). In some annelids gaseous exchange also occurs through gills (branchial respiration), e.g., Terebella.
9. Blood Vascular System:
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It is usually closed type. Blood is red due to the presence of respiratory pigment haemoglobin or erythrocruorin, found dissolved in the blood plasma. Free amoeboid blood corpuscles are present, but there are no red blood corpuscles.
In leech, there is no true blood vascular system, the coelomic space and fluid have been modified to form the circulatory system. It is called haemocoelomic system and red coelomic fluid is called haemocoelomic fluid.
10. Excretory System:
It consists of coiled tubular structures, called nephridia which help in osmoregulation and excretion. Ammonia is chief excretory waste.
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11. Nervous system:
The nervous system consists of a nerve ring and a solid, double, mid ventral nerve cord with ganglia. A ganglion is an aggregation of nerve cells.
12 Receptors:
Tactile receptors (sensitive to touch), gustatoreceptors (receptors of taste) and photoreceptors (sensitive to light) are usually found. Some forms have statocysts (balancing organs).
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13. Sexes:
Both unisexual (e.g., Nereis) and bisexual (e.g., Earthworm, Leech) forms are found.
14. Development:
It is mostly direct. If there is indirect development (e.g.. Nereis), it includes a trochophore larva.
Unique Features.
(i) Metameric segmentation,
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(ii) Haemoglobin in the plasma,
(iii) Nephridia for excretion and osmoregulation,
(iv) Setae.
Advancement over Roundworms:
(i) Metameric segmentation,
(ii) Circular and longitudinal muscles are present in both body wall and the wall of the alimentary canal,
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(iii) True coelom,
(iv) Blood vascular system contains red blood.
Classification of Phylum Annelida:
On the basis of number and presence or absence of setae phylum Annelida is divided into five classes.
Class 1. Polychaeta (Gk. polys-many + chaete = hair):
They have well developed Para podia with numerous setae. The development is indirect, undergoing metamorphosis with a free swimming trochophore larva.
Examples: Nereis, Aphrodite (Sea mouse) Polynoe (Scale worm), Chaetopterus (Paddle worm), Sabella (Peacock worm), Arenicola (Lug worm), Amphitrite, Terebella and Serpula (Fan worm). Both Polynoe and Chaetopterus are bioluminescents. Arenicola, Amphitrite, Terebella and Serpula have gills.
Class 2. Oligochaeta (Gk. Olivos— few + chaete — hair):
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The body has few sectae. There is no metamorphosis, e.g., Pheretima, (Indian ’Earthworm), Lumbricus, (European Earthworm) Tubifex, Magascolex (common earthworm of South India).
Class 3. Hirudinea (L. hirudo- leech):
They are ectoparasitic leeches. There are no Para podia or setae. Beneath the muscular layer of the body wall and surrounding the alimentary canal is found botryoidal tissue (peculiar connective tissue). Leeches have haemocoelomic system.
Examples: Hirudinaria (Blood sucking leech), Pontobdella, Hirudo, Acanthobdella.
Class 4. Archiannelida:
External segmentation is faint but internal segmentation is by septa and is complete. Examples Polygordius, Dinophilus.
Class 5. Echiurida:
These annelids are without external and internal segmentation. Setae are rare. Examples Bonellia, Echiurus.
Neanthes (= Nereis):
It is commonly called clam worm or sand worm or rag worm which is found on the sea shore in the tubular burrows. Nereis is unisexual and its reproductive phase is called Heteronereis.
Fertilization occurs in sea-water. During development, a trochophore larva is present. Except the peristomium (first segment) and last anal segment, each segment bears laterally one pair of fleshy projections, the parapodia, used in swimming.
Hirudinaria granulosa— The cattle leech:
It is an ectoparasite on cattle. Hirudinaria feeds on blood and hence called sanguivorous. The saliva of the leech contains an anticoagulant, called hirudin which prevents clotting of blood during blood meal. It is hermaphrodite, but cross-fertilization occurs. It is important to note that the segments 9, 10, 11 develop a temporary clitellum during breeding season.
The leech bears two suckers: anterior and posterior. There are present five pairs of eyes on the dorsal surface. Some leeches are used as fish bait. Some leeches are used as surgical agents specially for curing diseases such as piles, baldness, etc., which is not correct.
