Parapodium
In invertebrates, the termparapodium(Gr.para,beyond or beside +podia,feet;pl.:parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found inannelids,where they are paired, unjointedlateraloutgrowths that bear thechaetae.In several groups ofsea snailsandsea slugs,'parapodium' refers to lateral fleshy protrusions.
Annelid parapodia
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Most species ofpolychaete annelidshave paired, fleshy parapodia which are segmentally arranged along the body axis. Parapodia vary greatly in size and form, reflecting a variety of functions, such as gas exchange, anchorage, protection and locomotion.[1]
General description
[edit]Parapodia inpolychaetescan beuniramous(consisting of one lobe or ramus) but are usuallybiramous(two lobes or rami). In the latter case, thedorsallobes are called notopodia and theventrallobes neuropodia. Both neuropodia and notopodia may possess a bundle ofchaetae(neurochaetae and notochaetae respectively), which are highly specific and greatly diversified. A single stout internal chaeta, called anacicula,may be present in each lobe, which are used to support well-developed parapodia. Notopodia and neuropodia can also bearcirriwhich are tentacle-like projections of the parapodia. In some groups, such as the scale worms (e.g.Polynoidae), the dorsal cirrus is modified into a scale (orelytron).[2]
In most species, the anteriormost segments may be specialised into the head region andprostomium,which can result in the modification of those parapodia, loss of chaetae and elongation of the cirri into anterior-facing tentacular cirri.
Glossary of components of the parapodium
[edit]| Component | Description |
|---|---|
| Dorsal cirrus | Cirrusextending from the notopodium; can be modified into ascale(or elytron) in scale worms. |
| Neuroaciculum | Stout internal supporting chaeta (acicula) for the neuropodium |
| Neurochaetae | Chaetaeof the neuropodium |
| Neuropodium | Ventral lobe |
| Notoaciculum | Stout internal supporting chaeta (acicula) for the notopodium |
| Notochaetae | Chaetaeof the notopodium |
| Notopodium | Dorsal lobe |
| Ventral cirrus | Cirrusextending from the neuropodium |
Gastropod parapodia
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The fleshy protrusions on the sides of some marinegastropodsare also called parapodia. They are particularly well-developed insea butterflies.Some sea hares use their parapodia to swim. Parapodia can even be used for respiration (similar to gills) or for locomotion.
Parapodia are found in the following taxonomic groups of gastropods:
- CladeCephalaspidea
- CladeThecosomata
- CladeGymnosomata
- CladeAplysiomorpha
See also
[edit]References
[edit]- ^Rouse, Greg; Pleijel, Frederick (2001).Polychaetes.Oxford: Oxford University Press.ISBN0198506082.
- ^Hutchings, P. A.; Fauchald, K. (2000).Polychaetes and Allies: the Southern Synthesis - Class Polychaeata: Definition and General Description.Melbourne: CSIRO Publishing. pp. 1–3.