Classification of Annelida

On the basis of the number and presence or absence of setae, the phylum Annelida is divided into the following classes:

Polychaeta

• They are mostly marine and have highly developed parapodia with numerous setae.
• The development is indirect, undergoing metamorphosis with a free-swimming trochophore larva.
• Examples: Nereis (Clam worm or sandworm or Ragworm), Aphrodite (Sea mouse), Chaetopterus (Paddle worm), Sabella (peacock worm), Arenicola (lugworm), Amphitrite, Terebella, Serpula(Fanworm).

Oligochaeta

• They are semi-terrestrial or freshwater annelids.
• They have few setae.
• Clitellum is present.
• There is no metamorphosis.
• Example: Pheretima, Lumbricus, Tubifex.

Hirudinea

• They are ectoparasitic annelids.
• The clitellum is formed only during the breeding season.
• Surrounding the alimentary canal is found botryoidal tissue.
• Example: Hirudinaria, Pontobdella, Hirudo.

Archiannelida

• External segmentation is absent.
• Internal segmentation is present.
• Parapodia and setae are absent.
• Examples are Polygordius, and Dinophilus.

Echiurida

• These annelids are without external and internal segmentation. 
• Setae are rare. Usually, they have long prostomium.
• Examples are Bonellia, and Echiurus.

Animals have definite shapes and sizes. They are unbranched except for the sponges. Animals’ organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc. The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They lack chlorophyll and are heterotrophic in their mode of nutrition. Animals have distinct excretory organs. Asexual reproduction occurs only in the lower animals. Animals are classified according to some basics like the number of germ layers present in the embryo; the symmetry of the body of the organism, mode of origin of mouth. The broad classification of Animalia is based on common fundamental features. The phyla of the animal kingdom are Porifera, Cnidaria, Ctenophora, Platyhelminthes, Aschelminthes, Annelida, Arthropoda, Mollusca, Echinodermata, Hemichordata, and Chordata. Hereunder is a detailed explanation of the phylum Annelida.