Phylum Annelida
Body
Bilateral symmetry.
Triploblastic.
Elongated, Sub-cylindrical body.
Smallest one -less than 1mm.
largest one 3m-4m.
The body is basically divided into 3 regions,
They are,
1.Prostomium
2.Trunk
3.Pygidium
Larvae develop by adding segments one by one in front of the pygidium.
The oldest segment to be located near the head region.
Body wall
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| The body wall of an annelid |
Consists of a thin, Moist Fibrous collagenous cuticle, Glandular epidermis, Dermis.
Cuticle - Resists bulging of the body & toughens the body wall.
Epidermis - Single layer, consists of sensory cells, unicellular gland.
Dermis - Consists of connective tissue.
And they have a well-developed outer circulatory muscle layer & an inner longitudinal muscle layer
in below the dermis.
Coelom
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| Annelid segmentation and anatomy |
Their body cavity completely lined by mesoderm.
That body plan called "A tube within a tube".
It formation by split the mesoderm into 2 layers.
Coelomic fluid functions as a hydrostatic skeleton to affect locomotion.
Metameric segmentation
A longitudinal series of repeating segments.
This repetition of segments & organ systems including,
musculature.
nervous.
circulatory.
excretory.
reproductive system.
This repetition of a segment called Metameric segment or Metamerism.
Their segments separate from each other.
So, their coelomic fluids isolated one segment from that adjacent segment.
Advantage of Metanerism
Each segment can be regulated more independently.
This allows the contraction of the musculature within one single segment.
That enabled them to change their body shape & move forward by peristaltic movement.
Digestive system
Anterior mouth & posterior anus is present.
Most of them are scavengers or predators.
Some of them are filter-feeders.
The digestive tract is a long & straight tube.
It extends the length of the body.
Their gut is differentiated into the buccal cavity, pharynx, esophagus, stomach & intestine.
Circulatory system
Blood circulation only within blood vessels, without mixing coelomic fluid.
They have 2 longitudinal vessels.
Once in located dorsal to the alimentary canal, other is located ventral to the alimentary canal.
Blood follows forward by dorsal vessels & blood backward by ventral vessels.
Blood in the dorsal vessel passes into the ventral vessels through transverse vessels.
Ventral vessels give arise segmental branches, that supply blood to the organs and
then return blood to the dorsal vessels.
There is no specialized heart, instead, circulation is maintained by the contraction of the vessels.
Annelids have 3 respiratory pigments,
there are,
1.Heamoglogin
2.Heamoerythrin
3.Chlorocrunin
Excretory system
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| A metanephridia of Nereis |
Excrete through the metanephridia.
Each segment has a pair of metanephridia.
Metanephridia opes into the coelom via a funnel-like structure, that called the nephrostome.
Nephrostome leads into a coiled tubule. Which turn empties into a bladder.
The bladder finally opens to the outside through a pore called Nephridiopore.
Aquatic annelids excrete mainly Ammonia, Terrestrial annelids excrete urea.
Nervous system
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| The nervous system of Nereis |
Pair of ganglia located anterior, dorsally.
Pair of circum oesophageal connectives.
double solid ventral nerve cord with segmental ganglia.
Annelids have a variety of sensory organs,
touch,taste,light reception.
Eyes with retina and lenses.
Some annelids have a photosensitive cell in their epidermis.
Touch receptors in their prostomial antennae.
Reproduction system
Most of them are separate from the sexes.
Being aquatic forms so, their development occurs through ciliated trochophore larvae.
Being terrestrial form they lack larvae stage & their development is direct.
Some annelids occur asexual reproduction.
Budding or division of the body into 2 or more parts.
Aquatic form trochophore larvae are bilateral, pear shape, the ciliated band around their equator.
That ciliated band helps to locomotion & collecting their food.
More article,
Classification of phylum annelids
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