General Characteristics
- "Ringed forms"
- triploblastic, coelomate animals whose members metameric (segmented), elongate, and cylindrical or oval in cross-section
- complete digestive tract
- paired, epidermal setae (chaetae)
- dorsal suprapharyngeal ganglia and ventral nerve cord(s) with ganglia
- spiral cleavage, trochophore larvae (when larvae are present), and schizocoelous coelom formation
- closed circulatory system
- metanephridia (usually) or protonephridia
- about 12,000 species
- Examples: Marine worms, soil-building earthworms, and predatory leeches
Relationship to Other Animals
- Lophotrochozoan
- Originated at least as early as Precambrian times, more than 600 million years ago
Metamerism and Tagmatization
- Metamerism- segmental arrangement of body parts in an animal and influences the arrangement of organs
- Each segment has its own excretory, nervous, and circulatory structures
- Primary adaptive features of metamerism: Flexible support and efficient locomotion
- They depend on the metameric arrangement of the coelom and can be understood by examining the development of the coelom and the arrangement of body wall muscles
- Advantage: creation of hydrostatic compartments, which allow a variety of advantageous locomotor and supportive functions not possible in nonmetameric animals that use a hydrostatic skeleton
- Advantage: lessens the impact of injury, where other segments can take over if one is damaged.
- Advantage: permits the modification of certain regions of the body for specialized functions, such as feeding, locomotion, and reproduction
- Each segment can be controlled independently from others
- Tagmatization- the specialization of body regions
- Segmentation- partitioning of coelom by septa