Phylum: Annelida (segmented worms), its classification and characteristics
March 13, 2018
- multicellular, triploblastic, bilaterally symmetrical with system grade of body organization.
- mostly aquatic, found in sea water or fresh water, some are terrestrial and burrowing, few are ecto-parasitic.
- body is elongated, cylindrical and metamerically segmented (body divided into many similar segments).
- body is covered by non-chitinous cuticle secreted by ectoderm.
- complete and straight alimentary canal.
- coelomate.
- excretion by nephridia.
- nerve ring and double ventral nerve cord present.
- respiration through general body surface.
Image source: examplespedia
- closed type circulatory system (blood flows within blood vessels). Respiratory pigment is haemoglobin.
- unisexual or bisexual.
- development is direct or indirect.
- locomotion by the help of setae, parapodia, sucker or absent.
Classification
Annelids are divided into 4 classes on the basis of locomotory organs. They are:
Class 1: Polychaeta
- mostly marine, carnivorous worms.
- body elongated and segmented.
- both external and internal segmentation of body.
- distinct head with tentacles.
- clitellum absent.
- no coccon formation.
- locomotion by parapodia.
- a free swimming trochophore larval stage present.
- examples: Nereis (sand worm/ calm worm), Aphrodite (sea mouse)
Image source: faunafondness
Class 2: Oligochaeta
- mostly terrestrial, found in moist soil and burrowing.
- both external and internal segmentation of body.
- tentacles absent.
- clitellum present.
- coccon is secreted by clitellum.
- locomotion by setae embedded inside body wall.
- development is direct.
- free swimming larval stage is absent.
- examples: Pheretima posthuma (earthworm), Lumbricus (European earthworm).
Image source: weebly
Class 3: Hirudinea
- ectoparasitic and feeds on blood (sanguivorous).
- only external segmentation of body, have 33 fixed segments.
- locomotion by suckers.
- setae or parapodia absent.
- clitellum present, distinct during breeding season.
- eggs are usually laid in coccons.
- cross fertilization and is internal.
- development is direct and larval stage absent.
- examples: Hirudo (medicinal leech), Hirudinaria (cattle leech), Haemopis
Class 4: Archiannelida
- exclusively marine.
- only internal segmentation of body.
- distinct head with tentacles.
- clitellum absent.
- setae, parapodia and suckers are absent.
- sexes are usually separate and rarely they unite.
- trochophore larva is present.
- examples: Polygordius, Dinophilus, Protodrilus, etc.
References:
i) https://animaldiversity.org/accounts/Annelida/
ii) https://nhpbs.org/wild/annelida.asp