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.

                                                                                      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

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