ANNELIDA
FOR VIDEO https://youtu.be/2jKG8vAnbWE
(annulus-little ring, lidos-form)
Animals belonging to this phylum are commonly called annelids or segmented worms
The term Annelida was introduced by Lamarck in 1809.
There are about 9,000 species of annelids known today.
They exhibit most diverse forms ranging from 1mm to 2.5m in length.
Important characteristic features:
• They are terrestrial or marine or fresh water or parasitic.
• They possess an organ system level of body organization.
• Bilaterally symmetrical, triploblastic, true coelomates.
• Metamerically segmented: body is externally divided into segments called metameres. Remarkable feature of annelids is the true segmentation.
• Body surface is covered by a thick cuticle. Body wall has both longitudinal and circular muscles.
• The organs of locomotion are setae or unjointed lateral appendages called parapodia which helps swimming in aquatic annelids, eg: Nereis.
• Digestive system: it is complete and consists of pharynx, oesophagus, stomach, intestine, etc.
• Typhlosole: A longitudinal fold present in the intestine which increase the surface for secretion and absorption.
• Chloragogen cells: Present in the cavity of typhlosole which store up food material. They synthesize glycogen and store fat. They also perform excretory function.
• Excretory system: Excretory organs are paired segmentally arranged coiled tubes called nephridia. They also help in osmoregulation.
• Circulatory system: it is closed type. Blood flows through closed blood vessel. A heart to move the oxygen carrying blood is found for the first time in the evolution of animals.
• They show skin respiration.
• Nervous system: It consists of a brain, segmented ganglia and a double ventral nerve cord.
• Reproduction: They are either dioecious or monoecious.
They reproduce sexually. In many members a trochopore larval stage is noted.
The common examples of this phylum are:
Earthworms (Pheretima, Megascolex), Nereis, Hirudinaria (blood sucking leeches), Aphrodite (sea mouse), Tubifex,
Chaetopterus, Terebella.
Classification of this phylum:
Class I: Polychaeta (numerous setae on the parapodia. Eg. Nereis, Aphordite, Chaetopterus, etc )
Class II: Oligochaeta (setae are few, scattered over the body. Eg : Earthworm)
Class III: Hirudinea (parapodia and setae are absent. 2 suckers are present. Eg : Leech)
Class IV: Archiannelida (without setae or parapodia. Primitive annelids. Eg: Polygordius, Bonelia, etc ).
0 Comments