Phylum Annelida includes a group of bilaterally symmetrical, triploblastic, and coelomate animals commonly known as segmented worms. The body of annelids is divided into a series of ring-like segments called metameres, which show a high level of organisation. These animals are found in marine, freshwater, and terrestrial habitats.
Classification of Phylum Annelida
Based on the number and presence or absence of setae, the phylum Annelida is divided into three classes.
1. Polychaeta
- They are mostly marine and have highly developed parapodia with numerous setae.
- The development is indirect, undergoing metamorphosis with a free-swimming trochophore larva.
- Examples: Nereis (Clam worm or sandworm or Ragworm), Aphrodite (Sea mouse), Chaetopterus (Paddle worm), Sabella (peacock worm), Arenicola (lugworm), Amphitrite, Terebella, Serpula(Fanworm).
2. Oligochaeta
- They are semi-terrestrial or freshwater annelids.
- They have a few setae.
- Clitellum is present.
- There is no metamorphosis.
- Example: Pheretima, Lumbricus, Tubifex.
3. Hirudinea
- They are ectoparasitic annelids.
- The clitellum is formed only during the breeding season.
- Surrounding the alimentary canal is found botryoidal tissue.
- Example: Hirudinaria, Pontobdella, Hirudo.
Characteristics of Phylum Annelida
The phylum Annelida exhibits the following key characteristics:
- Annelids occur in freshwater, seawater, or moist soil. Some are free-living, some are burrowing, and a few are parasitic.
- Organ-level body organisation.
- The first animal to acquire a true coelom.
- Both longitudinal and circular muscles are present.
- Straight and complete alimentary canal.
- Excretion with the help of nephridia.
- The first animal to have a closed circulatory system.
- Blood is red due to haemoglobin dissolved in plasma.
- The mode of respiration is cutaneous respiration through moist skin.
- Locomotion organs are segmentally arranged paired lateral appendages, parapodia, chitinous setae, or chaetae.
- Sex may be unisexual (nereis) or may be bisexual (Earthworm).
- Fertilisation is external or internal.
- Development is direct or indirect, and there is a free-swimming larval stage(trochophore).
- The nervous system consists of a dorsal “brain” and a ventral nerve cord, having ganglia and lateral nerves in each body segment.
Examples of Phylum Annelida
Some of the Examples of the phylum Annelida are given below:
1. Nereis
- It is commonly called a clam worm, or sandworm, or ragworm.
- Nereis is unisexual, and its reproductive phase is called Heteronereis.
- During development, a trochophore larva is present.
- Each segment bears laterally one pair of fleshy projections, the parapodia, used in swimming.
- They are cylindrical in shape, found not only in sandy areas, and they are adapted to burrowing.
2. Hirudinaria (Bloodsucking leech)
- It is a facultative ectoparasite of cattle.
- It is sanguivorous.
- Its saliva contains an anticoagulant called hirudin.
- The body cavity is filled with a mesodermal botryoidal tissue formed of branching tubular cells
3. Pheretima (Earthworm)
- It is found in wet soil containing rich organic matter.
- It is omnivorous, fossorial, nocturnal, hermaphrodite, and protandrous.
- Furthermore, it possesses a great power of regeneration.
- Earthworm is brown or clay-colored due to the pigment porphyrin.
- The body shows metamorphic segmentation.
- The digestive system consists of the alimentary canal and digestive glands.
- The blood vascular system of earthworms is a closed type.