Monday, 12 August 2013
Phylum Annelida: General Characters
Phylum Annelida
Phylum Annelida (L., Annelus = a ring) includes about 12,000 species of segmented coelomate worms. They range from a deep-sea species measuring less than 1 mm in length to giant tropical earthworms (of Australia) Which measure upto 4 m in length. Most annelids are aquatic found in the sea as swimmers, crawlers, burrowers and tube dwellers or in freshwater where they usually burrow. Terrestrial annelids are mostly burrowers and some are parasites. Phylum Annelida comprises three main classes: the Polychaeta (marine), the Oligochaeta (terrestrial and freshwater) and the Hirudinea (terrestrial, marine and freshwater leeches).Annelids are soft-bodied, elongated, cylindrical, bilaterally symmetrical, metamerically segmented coelomate worms having a thin covering of cuticle, a dermomuscular body wall and often chitinous setae.
General Characters of Annelida:
- Annelids are triploblastic and bilaterally symmetrical coelomates.
- Body is soft, elongated, vermiform (worm-like) and metamerically segmented (i.e, body divided into similar parts or segments which are arranged in a linear series along the antero-posterior axis). The segments are marked externally by transverse grooves and internally by transverse septa. Each septum comprises of a layer of connective tissue sandwitched in between two layers of peritoneum.
- The head or acron is represented by prostomium and contains the brain. It is pre-oral body part. Terminal part of body is called pygidium and contains anus. Mouth exists in peristomium .
- Body wall consists of a fibrous collagenous circle, a glandular epidermis in which the nerve fibres are situated and a connective tissue dermis of varying thickness. Muscles of body wall exists in outer continuous layer of circular muscles(just inner to dermis) and inner layer of longitudinal muscles. The latter is lined by parietal peristoneum (coelomic epithelium). Cuticle is secreted by ectoderm (or epidermis).
- The paired appendages if present are not joined.
- Locomotory organs are segmentally arranged, paired, chitinous lateral bristles called setae or chaetae. They are absent in leeches.
- Body cavity is a true coelom or periviseeral coelom lying between the two layers of mesoderm. Coelom is a schizocoel and divided into segmental compartments by septa.
- Alimentary canal is a straight tube running from the anterior mouth to the posterior anus. Digestion is extracellular.
- Respiration occurs through general body surface or gills.
- Circulatory system or blood vascular system is well developed and closed type. Blood is usually red and contains haemoglobin (a respiratory pigment). Respiratory pigment is often dissolved in the plasma.
- Excretory organs are segmental, ectodermal in origin, ciliated, paired and called nephridia (metanephridia and protonephridia).
- The nervous system consists of an anterior, dorsal ganglionic mass or brain; a pair of anterior connectives surrounding the gut and a long double or single solid ventral nerve cord with ganglionic swellings and lateral nerves in each segment.
- A giant fibre system is used for rapid conduction and evokes fast startle contraction.
- Aphrodite and Chaetopterus have no giant fibre system but other annelids including Lumbricus have giant fibre system.
- Gonads develop from coelomic epithelium.
- Sexes may be united (monoecious or hermaphrodite) or separated (dioecious).
- Development is direct (without any larval stage) in monoecious annelids and is indirect (with trochophore larva) in dioecious and aquatic annelids.
- Cleavage is spiral and determinate.
- Asexual reproduction occurs in some annelids. Power of regeneration of their lost parts occurs in many annelids.
- Aquatic or Terrestrial
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