Phylum Annelida – Class 11 | Chapter – 4 | Short Notes Series PDF

Phylum Annelida includes segmented worms. These animals are found in marine, terrestrial, and freshwater habitats, but a presence of water or humidity is a critical factor for their survival, especially in terrestrial habitats. The name of the phylum is derived from the Latin word annellus, which means a small ring. Animals in this phylum show parasitic and commensal symbioses with other species in their habitat. Approximately 16,500 species have been described in phylum Annelida. The phylum includes earthworms, polychaete worms, and leeches. Annelids show protostomic development in embryonic stages and are often called “segmented worms” due to their key characteristic of metamerism, or true segmentation.

Characteristics of Phylum Annelida

• Annelida are aquatic [marine and fresh water] or terrestrial; free-living, and sometimes parasitic.
• Their body surface is distinctly marked out into segments or metameres [metamerically segmented] and, hence, the phylum name Annelida (Latin, annulus: little ring).
• They exhibit organ-system level of body organization.
• They are coelomate [true body cavity]. This allows true organs to be packaged in the body structure.
• They are bilateral symmetric and triploblastic.
• They possess longitudinal and circular muscles which help in locomotion.
• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.
• A closed circulatory system is present.
• Nephridia (sing. nephridium) help in osmoregulation and excretion.
• Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.
• Nereis, an aquatic form, is dioecious [Sexes are separate], but earthworms and leeches are monoecious [having both the male and female reproductive organs in the same individual].
• Reproduction is sexual.

Classification of Phylum Annelida

Following are the different classification of Annelida:

• Polychaeta

• Oligochaeta

• Hirudinea

• Archiannelida

Polychaeta

• The body is elongated and divided into segments.

• They are found in the marine environment.

• These are true coelomates, bilaterally symmetrical worms.

• They excrete through metanephridia and protonephridia.

• Fertilization is external.

• They have a well-developed nervous system.

• The circulatory system is closed type.

• They are hermaphrodites.

• They might possess fin-like appendages called parapodia.

• The organisms belonging to this group lack clitellum and are dioecious.

• Eg., Nereis, Syllis

Oligochaeta

• They are mostly freshwater and terrestrial organisms.

• The body is segmented metamerically.

• Head, eyes and tentacles are not distinct.

• They are hermaphrodites, but cross-fertilization takes place.

• Fertilization is external.

• Cocoon formation occurs.

• Setae are segmented.

• They do not possess parapodia but clitellum is present.

• The organisms belonging to this class are monoecious.

• They exhibit no free larval stage and the development takes place inside the cocoons.

• Eg., Pheretima, Tubifex

Hirudinea

• Most commonly found in freshwater. Some are marine, terrestrial, and parasitic.

• The body is segmented.

• The tentacles, parapodia, and setae are not present.

• The animals are monoecious.

• The body is dorsoventrally or cylindrically flattened.

• They have an anterior and posterior sucker on the ventral side.

• The organisms lay eggs in cocoons.

• There is no larval stage during the development of the organism.

• The mouth is located ventrally in the anterior sucker, while the anus is present dorsally in the posterior sucker.

• Fertilization is internal.

• They are hermaphrodites.

• Eg., Hirudinaria

Archiannelida

• They are found only in the marine environment.

• The body is elongated without setae and parapodia.

• They are unisexual or hermaphrodite.

• Tentacles are present on the prostomium.

• Eg., Dinophilus, Protodrilus

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