Phylum Annelida - Practice Questions & MCQ

• The term ‘Annelida’ is derived from the latin term annulus, that implies small rings. This phylum contains the animals that are also called ‘Ringworms’.
• The body of annelids is truly segmented because these segments or metameres are externally visible as annuli and internally as septation. 
• Segmentation improves the movement and enhances the chances of survival by providing redundancy.
• They are triploblastic and bilaterally symmetrical. 
• They have a true coelom, i.e., they are coelomate. The mesoderm lines the cavity on both sides. 
• Examples : Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech)

Body Plan of Annelida:

• The outermost layer of epidermis is protected by an acellular cuticle. 
• There are two layers of muscles - circular and longitudinal. 
• There are hair-like projections coming out through the epidermis called setae/chaetae. 
• These setae are present in every segment. 

Digestion in Annelids:

• They have a complete digestive system with a pharynx, gizzard, stomach and long intestine.
• The first segment called peristomium contains the buccal cavity.
• The buccal cavity is covered with prostomium.
• Annelids can be filter-feeders or predators.

Excretion in Annelida:

• The excretion is carried out by metanephridia.
• Annelids are ammonotelic animals, i.e., they excrete ammonia as the nitrogenous waste.
• The following diagram shows working of metanephridia in earthworms:

Nervous System and Sense Organs in Annelida:

• The nervous system is well developed in annelids. They bear a nerve ring made up of fused ganglia around the pharynx. They have a ventral nerve cord. The nerve cord shows the branching of ganglia in each segment. 
• In marine free-living annelids, tentacles, chemical receptors, statocysts and eye-spots are present.

Circulatory System in Annelida:

• The annelids have a closed circulatory system, i.e., blood flows in vessels. There is a system of ventral and dorsal blood vessels that run parallel to the alimentary canal. These blood vessels supply to each segment through transverse loops. 

Reproduction in Annelida

• Annelids like earthworms and leeches are hermaphroditic with permanent gonads while polychaetes show seasonal gonads.