ANNELIDA
Phylum- Annelida
General Characters
Annelida (= body annulations results in Ring like appearance)
Are very common and include approximately 1200 species
Aquatic- Marine, freshwater and terrestrial habitats
Burrowing, tubicolous
Parasitic and commensal in nature
Elongated body with bilateral symmetry
Metamerically segmented body/ true segmentation
True coelomate/ schizocoelous
Coelom is divided into compartments by transverse septa. Coelomic fluid has cells
Body is divisible into three regions: prostomium, trunk and pygidium
Prostomium is the anterior most end, bears brain and the sense organs
Trunk consists of a longitudinal series of similar body units, the segments
Pygidium is the posterior most segment where anus is situated
Immediately behind the prostomium lies peristomium which is considered to be the first body segment and ventrally surround the mouth
Prostomium and pygidium are not considered as segments because they don’t develop from the segmental growth zone which lies in front of the pygidium
Annelids exhibit teloblastic growth which means successive addition of segments occurs posteriorly, youngest segments lie in front of the pygidium and the oldest (=peristomium) immediately behind the prostomium
Body wall comprises Cuticle (a fibrous collagenous), Epidermis (glandular, has mucus secreting gland cells involved in lining the burrow and protects the body surface) and Dermis (bears connective tissue and Musculature).
Digestive system is straight and complete. Digestion is extracellular
They are very diversified in feeding habits like suspension feeding, deposit feeding and filter feeding They can either be scavengers or herbivorous or carnivorous
Have Closed blood vascular system
Respiration occurs either through skin (=cutaneous respiration) or gills of parapodia and head
Respiratory pigment is either hemoglobin or erythrocruorin dissolved in plasma
Locomotor organs are chaetae which are chitinous bristles, either embedded in the skin or present on the parapodia
The internal structures (except digestive system, nervous system, hemal system and musculature), are segmented and repeated in each segments
Nervous system comprises of a pair of cerebral ganglia (brain) located in prostomium and a ventral pair of longitudinal nerve cords extending throughout the entire length of the body
A pair of circumpharyngeal connectives joins the brain to a pair of ventral subpharyngeal ganglion in the peristomium, which in turn give rise to the nerve cords
Paired nerve cords along with their segmental commissures resemble a ladder and therefore the nervous system looks like a ladder
From the ganglion present on nerve cords are arising lateral nerves
Sensory structures include photoreceptors, chemoreceptors, mechano- receptors distributed head, appendages and on the body surface. Sometimes simple eyes/ ocelli with simple lenses are also present
Bisexual or unisexual
Cleavage is spiral and determinate
Development direct or indirect through trochophore larvae
Regeneration is very common
Indirect source of food- earthworm used as fish bait, to till soil, to convert organic matter to compost- improving soil fertility
Learn more about Annelids:
https://www.youtube.com/watch?v=hIpLIXV4p7Q&feature=emb_title
Nereis
Phylum: Annelida
Class: Polychaeta
Common name: Rag worm / clam worm
Habit and Habitat
They are found in marine water and have burrowing habits.
Geographical distribution
They are cosmopolitan in distribution.
Identifying features
Long and dorso- ventrally flattened body
5-30 cm in length
A very distinct head is present at the anterior end of the body
Head consist of prostomium and peristomium
Prostomium bears dorsally 2 pairs of eyes, one pair of terminal tentacles and ventrally one pair of short palps
Peristomium has four tentacles on both sides and mouth on ventral side
Remaining body segments behind the peristomium bears lateral parapodia (one pair per segment)
Last body segment bears terminal anus and one pair of anal cirri but no parapodia
Sexes are separate
Interesting Facts:
Gonads are temporary and develop only in breeding season
Sexual phase of Nereis is called Heteronereisin, in which eyes become large, prominent and more sensitive to light.
Body of Heteronereis exhibits two distinct regions: non sexual= atoke and sexual = epitoke
Parapodia of epitoke region become enlarge and highly vascular helping in active swimming
Click here: https://www.youtube.com/watch?v=vE9I3c-ic_o