Phylum : Annelida General Characters & Example

Defination

Annelids are first Coelomate, Triploblastic, bilateral symmetrical, metamerically segmented animals with organ system level of organisation.

General features

• They are aquatic and found in marine and fresh water and also live in  terrestrial habitat.

• They are free-living some are parasite like Pontobdella, Acanthobdella etc.

• They have organ-system level of body organisation and bilateral symmetry. They are triploblastic, metameric segmented and coelomate animals. 

• Their body surface is distinctly marked out into segments or metameres. Hence the phylum name Annelida has been given.

• They possess longitudinal and circular muscles which help in locomotion. 

• Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming. 

• A closed circulatory system is present in which blood is flown in network of blood vessels.

• Nephridia  help in osmoregulation and excretion. 

• Nervous system consists of paired ganglia  connected by lateral nerves to a double ventral nerve cord. 

• Nereis is dioecious, but earthworms and leeches are monoecious. 

• Reproduction is sexual.

Examples of Annelida  

1. Nereis

• It is commonly called as Calm worm or Rag worm.

• Prostomium and Peristomium are two parts of Head of Nereis. First one is triangular and second one is ring like.

• Mouth is located at anterior of Peristomium with fine tentacles.

• It use parapodia for locomotion and swimming.

• It usually live in burrow of sand and carnivores and noctural in nature.

• It can maintain its internal body environment with respect to surrounding.

• Alimentary canal is in form of straight tube and include mouth to rectum.

• The number of body segments are 200 

• It has four pairs of simple and round eyes.

• It reproduce sexually because it is dioecious. Male and female are different.

2. Pheretima 

• It is commonly called as Earth worm.

• Earthworms have microscopic  fine hair like structure  called setae that help in locomotion and Anchorage. Setae are present in body except first, last and clitellum region.

• Earthworms have a closed circulatory system with a heart, capillaries, and vessels. Their blood is red and contains hemoglobin which is dissolved in plasma.

• Earthworms have a dark band of glandular tissue called the clitellum, which is located between 14th and 16th segment. Clitellum has  mature eggs of Earthworm.

• Earthworms have blood glands in segments 4-6 that produce hemoglobin and blood corpuscles.

• Earthworms are hermaphrodites or Monoecious . They have both male and female sex organs.

• Earthworms have nephridia that help them excrete nitrogenous waste and regulate osmoregulation.

• Alimentary canal is in form of straight tube and include mouth to rectum.

3. Hirudinaria

• It is commonly called Leech.

• Leeches has  an anterior sucker at the front of the body and a posterior sucker at the back of the body

• The number of body segments are 33.

 

• Leech are hermaphrodites or Monoecious . They have both male and female sex organs.

• Skin is kept moist and slimy due to abundant mucus secretion

• Body is divided into cephalic, pre clitellar, clitellar, middle, caudal and posterior sucker.

• Anterior sucker is oval and contains ventral tri radiate mouth formed by the fusion prostomium and few anterior segments. 

• The posterior sucker is highly muscular and acts as powerful locomotory and adhesive organ

• Eyes 5 pairs dorsally. 

• It is sanguivorous ectoparasite and release anti coagulant called Hirudin.