Phylum Annelida - Segmented Worms

Phylum Annelida - The Segmented Worms

• Bilateria, triploblastic, coelomates
• These worms are segmented
• Exhibit metameric segmentation (serial repetition) which aids in flexibility and movement
• Each segment is called a metamere or somite
• The coelom is a true body cavity which develops by splitting of mesoderm
• Advantages of the coelom: definite shape, protects from mechanical shocks, cushions internal organs and acts as a hydrostatic skeleton for locomotion in soft-bodied animals
• Exhibit a complete digestive system with a mouth and anus
• Have a closed circulatory system with blood that is red and contains hemoglobin
• Have a well-developed nervous system. The nervous system consists of a simple brain, a ventral nerve cord and a series of ganglia.
• Their respiratory system uses diffusion through the skin into the blood capillaries.
• The excretory system typically consists of a pair of metanephridia per segment
• Some are predators and some are ectoparasites
• Some species are aquatic, some are terrestrial and some are burrowers
• Examples of annelids include: Nereis, Earthworms (Lumbricus terrestris and other species), leeches.

Classification of Annelids

• Polychaeta: mostly marine, free living, many setae on fleshy, lateral outgrowths called parapodia, well-developed appendages, separate sexes, free-swimming trochophore larva. Examples: Sabella (peacock worm), Nereis
• Oligochaeta: usually terrestrial, burrowers, few setae without parapodia, no distinct head appendages, hermaphrodites, eggs deposited in a cocoon, development is direct. Example: Pheretima (earthworm)
• Hirudinea: ectoparasites, some are free living predators, dorso-ventrally flattened, no setae, clitellum present only during reproduction, two suckers (anterior and posterior). Example: Hirudinaria (common Indian leech).

Importance of Annelids

• Soil health and agriculture: Earthworms burrow through the soil, improving aeration and water infiltration, and increasing soil fertility.
• Nutrient recycling: Earthworms break down organic matter, facilitating nutrient cycling. Their castings are rich in nutrients which are good for organic farming.
• Fisheries: Annelids are used as bait in fishing.
• Medicinal uses: Medicinal leeches (e.g., Hirudo medicinalis) are used in medical treatments, particularly in plastic and reconstructive surgery, due to their ability to promote blood circulation and produce anticoagulant chemicals that prevent blood clots.

Specialized Features in Annelids

• Crop: store food
• Gizzard: grind food
• Pharynx: connects the mouth with the digestive tract
• Intestine: where nutrients are absorbed.
• Septa: walls that partition the coelom
• Bristles/seta: used for locomotion in some species
• Parapodia: lateral outgrowths of the body wall found in some species
• Clitellum: a thickened region of the body which is involved in reproduction
• Specialized glands are present , and organs such as the salivary glands, also in medicinal leeches for example

Explore the characteristics and biological systems of Phylum Annelida, the segmented worms. This quiz covers their unique features such as metameric segmentation, coelomic structure, and circulatory system. Test your knowledge on their physiology, anatomy, and ecological roles.