Phylum: Annelida (segmented worms), its classification and characteristics

  • 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.

Phylum Annelida Examples and Characteristics with Classification

                                                                                      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)

PHYLUM ANNELIDA | FAUNAFONDNESS | 2022

                                                                       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).

Lumbricina (Earthworm) - Reproductive Systems of Phyla

                                                                                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

Phylum: Annelida (segmented worms), its classification and characteristics