Evolutionary Biology Topics allow us to trace the history of life on Earth.
Introduction to the Annelida
Phylum Annelida (L., annelus – a ring; segmented worms)
1. Body triploblastic, bilaterally symmetrical, soft, elongated, vermiform, and cylindrical or dorsoventrally flattened.
2. Body is metamerically segmented externally by transverse grooves and internally by septa. Some of the anterior body segments concentrate to form the head.
3. Exoskeleton absent; the body is covered by a thin cuticle.
4. Locomotory organs are segmentally arranged paired lateral appendages, parapodia or chitinous setae, or chaetae.
5. Alimentary canal is tube-like, complete, and extends straight from mouth to anus.
6. True coelomate animals (first animals with true body cavity) with closed blood vascular system. Coelom allows true organs to be packaged in the body structure. There is thus, extensive organ ‘differentiation’.
7. Excretion by paired segmental nephridia which removes wastes from coelom and bloodstream directly to the exterior.
8. The nervous system consists of a dorsal “brain” and a ventral nerve cord having ganglia and lateral nerves in each body segment.
9. Reproduction is by sexual means. Sexes may be united (hermaphroditic) or separate.
10. They live in a variety of habitats. Mostly aquatic, marine, or freshwater; some are terrestrial (in moist soil), burrowing in tubes, and some are free-living forms.
Phylum Annelida includes the following three classes:
Class 1. Polychaeta – Locomotion by segmentally arranged parapodia having numerous setae.
Examples: Nereis (clam worm or sandworm), Aphrodite (sea mouse).
Class 2. Oligochaeta – Body without a distinct head and lacks eyes, tentacles, and parapodia.
Examples: Pheretima (earthworm), Eutypheus.
Class 3. Hirudinea – Body with anterior and posterior suckers, Parapodia, and setae are absent.
Examples: Hirudinaria (Indian cattle leech).