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  Phylum Annelida, Biology tutorial

Introduction to Phylum Annelida:

Annelida phylum of soft-bodied, bilaterally symmetrical segmented animals, called as segmented, or annelid, worms. More than 12,000 known species are clustered in 3 classes: leeches (hirudineans), marine worms (polychaetes) and earthworms and freshwater worms (oligochaetes). Annelids are located throughout world, from deep ocean bottoms to high mountain glaciers. They dwell in protected habitats like sand, mud, and rock crevices, and in and among other invertebrate animals, like sponges. Several dwell in tubes they secrete around themselves.

Characteristic Features of Phylum Annelida:

i) They are mainly aquatic, few are terrestrial.

ii) They are usually burning animals, few are sedentary or free living, and few are ectoparasites.

iii) Body is bilaterally symmetrical, vermiform, and metamerically segmented.

iv) They contain straight tube alimentary canal, and suffer extra-cellular digestion.

v) Include segmentally arranged Locomotory organs, repeated groups of chitinous setae or chaetae. Leaches do not have setae.

vi) Respiration is usually through body surface or through special projection of parapods.

vii) Has well grown closed kind blood vascular system.

viii) Have Nephridia that are excretory organs.

ix) Nervous system comprises of paired cerebral ganglia or brain, double ventral nerve cord bearing segmental ganglia.

x) Gonads grow from coelomic epithelium.

Categorization of Phylum Annelida:

Phylum Annelids is usually divided into 3 classes. These are:

i) Polychaeta: Mainly marine forms, separate head with eyes and tentacles, segmental with laterial projection of body called as parapodia, don't have clitellum. Sexes are separate. Have no separate or permanent sex organs. Several forms reproduce asexually by budding.

ii) Oligochaeta: Dwells in soil, or in fresh water, body is noticeably segmented but no separate head; parapodia absent; they are hermaphrodites; includes more comples reproductive system with compact ovaries and testes but lesser in number clitellum is present ; no larva, and growth is direct. For e.g.Earthworm.

iii) Class Hirudinea in phylum Annelida (segmented worms) includes leeches, most highly specialized of major annelid groups. Leeches are generally dorsoventrally flattened annelids with suckers at both ends. Many species in North America are located in fresh and marine waters, but several terrestrial species happen in tropical regions. As predators, vectors of parasites, parasites of animals, and as food for semi-aquatic and aquatic animals, leeches are significant parts of food webs. They are hermaphrodites, has direct growth; may be terrestrial fresh water or Marine. For e.g. are Leeches.

Earthworm:

External Characteristics:

i) Body is lengthened almost cylindrical and tapering at both ends.

ii) Mature Earthworm estimates 150mm in length and 3-5 mm in width.

iii) It is brown in colour, dorsal surface darker than ventral.

iv) Metameric organization of body is evidently pointed to on outside by circular groves equivalent to body segments or metameres.

v) First segment where mouth is located is known as peristonium.

vi) Preistomium projects forward a small sensory tube at dorsal side that is called as Prostomium.

vii) In the mature worm there is Clitellum lying about 20mm behind anterior extremity, plays the significant part in reproduction.

viii) About middle of every segment is ring of Chitinous structures known as Setae

ix) First, last and 3 segments of clitellar regions are without setae.

x) Paired male generative or spermiducal apertures situated on ventral surface of segment.

Structural Adaptation of Earthworm:

i) Body wall forms the protective covering for all surrounded internal parts.

ii) Mucus in epidermis underlying body wall keeps skin of earthworm slimy and clean and doesn't hold any foreign germs to settle in it.

iii) Body wall serves as only organ of respiration due to its thin pervious and highly vascularised nature.

iv) Skin serves as the efficient receptor organ.

v) Mucus acts as cement for plastering walls of burrow.

vi) Setae help in locomotion.

vii) Earthworm feeds in decaying leaves and humus present in soil.

viii) Earthworm excretes waste products with help of the organ called Nephridia.

Habits and Habitat of Earthworms:

i) Earthworms are situated in almost all parts of world.

ii) Even in mountains up to the height of approx 3,000 meters.

iii) They consume their way through ground and create burrows which they live in.

iv) They form burrows by simply pushing in body particularly in soft soil their habit of swallowing soil also helps in making of burrows particularly in compact soil.

v) Soil having organic matter is digested and absorbed as food and residual soil is ejected in form called CASTING Earthworms exude the liquid that is antiseptic and protects worm from any unsafe bacterial which may be present in soil or settle down on body of animal. Burrows of earthworm are generally 30-60cm deep and may be partially lined with dead leaves. At bottom of burrows, there is improvement where worm can turn round and on top, creature at times draws over opening of some small pebbles for objective of keeping out water and enemies such as centipede.

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