Annelida | Additional Study Material for NEET PDF Download

Phylum- Annelida

–    Lamarck coined the term Annelida.  

–    Free-living found in moist soil, fresh water, sea or few are parasite.  

–    Body is Soft, elongated, cylindrical or flattened divided into segments or metameres by ring like, grooves called Annuli. 

–  Symmetry – Bilateral, Triploblstic, Organ-system level having a tube within tube plan, metamerically segmented.

– Anterior end forms a distinct head with sense organ in few annelids.

– Appendages are simple unjointed, and locomotory having Chitinous Setae and Parapodia with setae.

– Body wall consists of  

(i)   Cuticle – Thin moist albuminoid cutical allow free exchange of gas.

(ii)  Epidermis – Single layered epidermis made up of supporting cell, sensory and glandular cell.

(iii)  Muscle layer – (1) Circular layer, (2) Longitudinal layer. Muscle are smooth/unstriated.

Body wall may have minute chitinous setae.

–   Locomotion by means of setae or parapodia or both. Absent in leeches

–    First Protostomi animals.

Body cavity is true coelom lined by mesodermal coelomic epithelium (Schizocoel.)

It is divided by transverse septa into compartment. It is filled with coelomic fluid that contains cells.

– As such there is no Skeleton. Fluid-filled coelom serves as a hydrostatic skeleton.

–    Digestive tract is complete,  straight and extends through entire body. The gut has both circular and longitudinal muscles. Few annelids are sanguivorous. Digestive gland are developed for the first time in Annelida.

– Respiration is through moist skin i.e. Cutaneous respiration. Some have gills (branchial respiration).

– Circulatory system is closed. Some blood vessels enlarge to act as pumping heart.

(Heart appear first time in annelids)

The blood is red with haemoglobin dissolved in plasma (Erythrocruarin). It has amoeboid corpuscles only. 

–    Few Annelids like Sabella have Chlorocruarin as a respiratory pigment.

–    Hirudinaria has circulatory system with haemocoelomic system.                                 

–    Excretory organ is Nephridia. Coiled tubules also helps in osmoregulation.

Excretory matter (1) Ammonia in aquatic form  (2) Urea in land form   

–    Nervous system consist of a circumentric nerve ring,   double, midventral, nerve cord with ganglia.

–    Sense organ chemoreceptor, photo-receptor  & tentacle, palp, eyes may be present.

–    Sexes may be separate or united. Asexual reproduction by budding or fission. In some cases. Atoke  (asexual), Epitoke (sexual) phenomenon also found (Nereis).

–    Cleavage is spiral and determinate unequal & holoblastic. Regeneration is usually found. Life history  includes a trochophore larva in few annelids.   

Nereis 

Inhabits in sea - shore between tide mark, burrower, nocturnal, carnivorous, gregarious, fertilization - in sea.  

Parapodia in each segment except first & last. During breading body divides in two parts.

Anterior asexual part - Atoke and posterior sexual portion Epitoke. This change is known as epitoky.         

Annelida

Based on presence or absence/of Parapodia, Setae and Sense organs

Polychaeta

1. Almost all are marine

2. Cephalisation is more distinct. Head with well developed eyes,  tentacles and olfactory palps.

3. Setae numerous and  are present in parapodia helps in locomotion and also in respiration. Suckers are absent.

4. Clitellum absent

5. Animals unisexual and gonads are formed only during breeding season.

6.  Development is indirect. Larval stage is called Trochophore.

e.g.*Nereis – Sandworm/clamworm

Aphrodite - Sem mouse

Arenicola - Lugworm

Seballa - Peacock worm

Chaetopterus -  paddle worm

Shows bio- florescence, great power of  regeneration.

Eunice - Palalo worm

Polynoe - Scale worm

Terebella -  respire by gills

Glycera – Smooth bloodworm

Oligochaeta

1.  Most of the members are terrestrial, but some are aquatic.

2. Cephalisation absent. No distinct head, eyes, tentacles and olfactory palps.

3. Setae for locomotion. Number of setae is limited and situated in setal sac present in body wall, a single setae is present in a bag. Parapodia & sucker are absent.

4.  Clitelium is present permanently for cocoon formation. Fertilization is external and is held in cocoon.

5.  Bisexual or hermaphrodite cross fertilisation & external.

 6.   Development is direct. No larva.

 e.g.

Pheretima

Eutyphaeus 

Lumbricus –European earthworm

Dravida

Megascolex – Largest earth worm (S. India)

Dero – Fresh watered  

Nais – Fresh watered

Tubifex – Blood worm(Fresh water)

Indicator of organic Hirudinea

1. Aquatic, terrestrial, ectoparasite  and sanguivorous.

2.  Cephalisation absent. No distinct head, eyes, tentacles and olfactory palps.

3. Parapodia and seate are absent.  Suckers at both the ends.

4.  Clitellum (9-11 segments) develop  only in  breeding season.

5. Animals of this class are bisexual. Fertilization is external.

 6.  Development is direct.

 Number of segments are fixed i.e.  33 segments. Each segment subdivided into numberous rings. called ''Secondary   external annulation''

–   Anticoagulant Hirudin – in the saliva

–   Saw like chitinous teeth in buccal cavity Circulation with haemocoelomic system.

–  Coelom is divide in tubes having coelomic fluid and haemoglobin. A special mesodermal tissue Botryoidal tissue  made up of adipose tissue for fat storage / supposed to be excretory.

e.g. Hirudinaria – Fresh  water leech

Pontobdella – Skate sucker.

Hirudo – Medicinal   leech (Highly modified)

Glossiphonia –   Fresh water leech  

Haemadipsa  –   Terrestrial leech

Haemopis –   Horse  leech

Acanthobdella –  Extroparasite with setae connecting-link between oligochaeta – Hirudinea  

Archiannelida

1.    All Marine

 2.  Cephalistion absent.No distinct head,  eyes, tentacle present and olfactory palps.

 3.   Parapodia and setae  are absent.

 4.   Clitellum absent

5.   Animals are unisexual

6.  Development is indirect. Larval stage is called Trochophore.

–  Segmentation is Mainly intermal

 e.g Polygordius –       Connecting-link between Annelida & Mollusca (living  fossil) Larva  known as Loven's Larva

– without external and internal segmentation.

– Seate are rare and usually have long prostomium or proboscis.

e.g., Bonelia, Echiuris