Phylum Annelida - NEET-Biology

 Phylum Annelida

Fig: Leech (Hirudinaria)

Characteristics:

• Triploblastic, coelomate, bilaterally symmetrical & metamorphically segmented animals.

• In earthworm, each segment has a separate coelom which acts as a hydraulic skeleton.

• During muscular contraction, the body wall pushes against each compartment wall. This allows separate regions to contract independently & elongate during locomotion.

• The body is elongated, cylindrical, or flattened.

• It is divided into segments by ring-like grooves, the annuli. This character gives the phylum its name.

• The segmented worms are triploblastic, i.e. they develop from the three germ layers.

• The body-cavity is a true coelom, as it is lined by a mesodermal epithelium.

• It is divided by vertical septa into compartments.

• A closed circulatory system present.

• Segmented nephridia for excretion & osmoregulation.

• Typically, there is a trochophore larva during development.

• Example: Nereis, Pheretima (Earthworm), & Hirudinaria (Bloodsucking leech).

Fig: Earthworm (Pheretima)