ANNELIDA

Phylum- Annelida

General Characters

  1. Annelida (= body annulations results in Ring like appearance)

  2. Are very common and include approximately 1200 species

  3. Aquatic- Marine, freshwater and terrestrial habitats

  4. Burrowing, tubicolous

  5. Parasitic and commensal in nature

  6. Elongated body with bilateral symmetry

  7. Metamerically segmented body/ true segmentation

  8. True coelomate/ schizocoelous

  9. Coelom is divided into compartments by transverse septa. Coelomic fluid has cells

  10. Body is divisible into three regions: prostomium, trunk and pygidium

  11. Prostomium is the anterior most end, bears brain and the sense organs

  12. Trunk consists of a longitudinal series of similar body units, the segments

  13. Pygidium is the posterior most segment where anus is situated

  14. Immediately behind the prostomium lies peristomium which is considered to be the first body segment and ventrally surround the mouth

  15. 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

  16. 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

  17. 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).

  18. Digestive system is straight and complete. Digestion is extracellular

  19. They are very diversified in feeding habits like suspension feeding, deposit feeding and filter feeding They can either be scavengers or herbivorous or carnivorous

  20. Have Closed blood vascular system

  21. Respiration occurs either through skin (=cutaneous respiration) or gills of parapodia and head

  22. Respiratory pigment is either hemoglobin or erythrocruorin dissolved in plasma

  23. Locomotor organs are chaetae which are chitinous bristles, either embedded in the skin or present on the parapodia

  24. The internal structures (except digestive system, nervous system, hemal system and musculature), are segmented and repeated in each segments

  25. 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

  26. 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

  27. Paired nerve cords along with their segmental commissures resemble a ladder and therefore the nervous system looks like a ladder

  28. From the ganglion present on nerve cords are arising lateral nerves

  29. 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

  30. Bisexual or unisexual

  31. Cleavage is spiral and determinate

  32. Development direct or indirect through trochophore larvae

  33. Regeneration is very common

  34. 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

  1. Long and dorso- ventrally flattened body

  2. 5-30 cm in length

  3. A very distinct head is present at the anterior end of the body

  4. Head consist of prostomium and peristomium

  5. Prostomium bears dorsally 2 pairs of eyes, one pair of terminal tentacles and ventrally one pair of short palps

  6. Peristomium has four tentacles on both sides and mouth on ventral side

  7. Remaining body segments behind the peristomium bears lateral parapodia (one pair per segment)

  8. Last body segment bears terminal anus and one pair of anal cirri but no parapodia

  9. Sexes are separate

Interesting Facts:

  1. Gonads are temporary and develop only in breeding season

  2. Sexual phase of Nereis is called Heteronereisin, in which eyes become large, prominent and more sensitive to light.

  3. Body of Heteronereis exhibits two distinct regions: non sexual= atoke and sexual = epitoke

  4. Parapodia of epitoke region become enlarge and highly vascular helping in active swimming


Click here: https://www.youtube.com/watch?v=vE9I3c-ic_o