- Free-living, terrestrial or aquatic form (freshwater or marine).
- Body is long, cylindrical and metamerically segmented.
- Body wall consists of cuticle, epidermis and musculature.
- Body cavity is a true coelom, lined by epithelium.
- Digestive system is complete.
- Respiration is either through skin or through gills.
- Excretion with the help of characteristic nephridia.
- Presence of a circulatory system (closed type). Hemoglobin dissolved directly in the plasma.
- Nervous system consists of a nerve ring and a double ventral nerve cord with segmental ganglia.
- Locomotion with the help of setae or parapodia.
- Reproduction by sexual methods. Either unisexual or bisexual development may be direct or indirect.
- Body has a true coelom.
fig. 10.13 - Examples of Annelida
The phylum is divided into four classes
Characteristics | Class Polychaeta | Class Oligochaeta | Class Hirudinea | Class Archi annelida |
---|---|---|---|---|
Habit and habitat | Free living marine | Free living forms found in most soil | Fresh water forms Temporary ectoparasites | Marine forms |
Locomotor Structure | Parapodia | Setae | Absent | Absent |
Lifecyle | Unisexual with indirect development | Bisexual with direct development | Bisexual with direct development | Bisexual with indirect development |
Examples | Nereis (reg worm) | Pheretima, Megasolex (earth worms) | Hirudinaria (leech) | Polygrdius |
fig. 10.14 - Anterior Segments of Nereis
fig. 10.15 - Earthworm Anatomy
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