PHYLUM ANNELIDA WITH SPECIAL REFERENCE TO METAMERISM
TOPIC: GENERAL ORGANISATION OF PHYLUM ANNELIDA WITH SPECIAL REFERENCE TO SEGMENTATION
SUBMITTED TO : Dr. RAKESH KUMAR
SUBMITTED BY : YOGINI SHARMA
COURSE NAME : STRUCTURE AND FUNCTION OF
COURSE CODE : ZOO416
ROLL NO. : CUHP21ZOO33
2. GENERAL CHARACTERISTICS
They may be aquatic (marine/freshwater) or
terrestrial : free living and somertimes parasitic
Body is soft and elongated.
Body is divided into segments or metameres by
ring like grooves called annuli.
They are bilaterally symmetrical animals.
Organ system organisation.
These are triploblastic animals.
Many annelids have simple , unjointed
locomotory appendages called parapodia.
ANNELIDA – SEGMENTED WORMS OR RINGED WORMS
HABIT & HABITAT
LEVEL OF ORGANISATION
3. There is no mineralised skeleton.
Fluid present in coelom serves as hydrostatic skeleton.
True coelomates, coelom is schizocoelic in origin.
It is divided by transverse septa into compartments.
It is filled with coelomic fluid.
It consist of thin , moist cuticle and single layered epidermis and well
Minute bristle like chitinous structures called setae are present.
Muscles are of two types:
4. DIGESTIVE SYSTEM
Tube within tube body plan.
Digestion is completely extracellular.
It occurs through moist skin (cutaneous respiration). Some
annelids (e.g. Amphitrite) have external gills and also
Closed circulatory system.
Blood is red in colour with haemoglobin dissolved in plasma.
Some annelids have erythrocruorin pigment.
5. It consists of coiled tubules called nephridia , which helps in excretion as well as
Excretory material is ammonia in aquatic forms and urea in terrestrial forms.
Nervous system consists of paired cerebral ganglia and a solid mid ventral
Sense organs like taste buds , photoreceptor cells , statocysts are also present.
Sexes may be sepatare or united. Neries is unisexual whereas
earthworm and leech are bisexual.
It is external in some forms (e.g. earthworm) and internal in
others (e.g. leech).
Development is mostly direct, if indirect, the larva is trochophore.
On the basis of locomotory organelles, it has three main classes:
Also known as sand worms.
Each body segment has a pair of locomotory organs called parapodia that bear many
Mostly marine , some are freshwater.
They have well differentiated head with sense organs.
No clitellum on body.
Most of Polychaetes are dioecious but have no permanent sex organs.
Gonads appear as temporary swelling of peritoneum at certain seasons.
Some polychaetes live most of the year as sexually immature individuals called “atokes”.
After living 1 or 2 years as benthic organisms they become sexually mature and swollen
with gametes called “epitokes”.
Excretoty organs are Nephridia .
Examples of polychaeta : Neries , Polynoe , Glycera , Aphrodite
They have no parapodia but few setae are present.
Mostly terrestrial; most abundant worms on land
- burrow in soil.
Also many live in freshwater and a few marine.
No distinct head.
Clitellum is present.
Earthworms are hermaphrodites, but no self
fertilisation, they cross fertilize.
Development is direct.
For excretion and osmoregulation, there are paired
nephridia in each body segment.
Examples of oligochaete : Phertetima posthuma ,
Lumbricus , Tubifex , Megascolex .
They have no parapodia and no setae.
Mainly freshwater species, a few marine and
Many are carnivores and some are parasites.
In parasitic forms anterior and posterior suckers are
They are hermaphrodites.
They cross fertilize during copulation.
Fertilization is internal.
Development is direct.
They do have clitellum
-produce cocoon that receives eggs and
They have 10 - 17 pairs of nephridia for excretion
Examples of Hirudinea : Hirudinaria , Bonellia .
Differentiation of body along the longitudinal axis , into series of similar segments
which are repeated one after other , these body segments are called
“Metameres” or “ Somites”.
Each segment repeats some or all organ systems.
The animal which exhibits such features are called metamerically segmented.
Metamerism was first seen in annelids in animal kingdom.
Segments are externally divided by ring like grooves called “annuli” and internally
divided by transverse partition called “septa”.
In earthworm, there is repetition of homologous parts such as muscles ,
setae , nerves , blood vessels , excretory organs , reproductive organs and coelomic
The segmented structures are interdependent .
Metamerism is also seen in arthropods and vertebrates.
11. TYPES OF METAMERISM
In arthropods, the metamerism is only external.
Internally , the segments are not marked by
In vertebrates , there is only internal
metamerism , which is confined to
the muscular, skeletal and nervous
In annelids , the metamerism is both
externally and internally.
Externally it is marked by the constrictions on
the body wall (annuli) and internally it is
marked by the transverse partitions (septa). INTERNAL
12. COMPLETE METAMERISM
It is found in annelida , where all the segments are alike and there is
repetition of similar structures like muscles , setae , nerves , blood
vessels , excretory organs ,reproductive organs and coelomic sac.
When the segmentation is not seen in all the body parts or segments are
greatly dissimilar to one another is called incomplete metamerism.
It is seen in arthropods and chordates.
In arthropods , embryonic stages show complete metamerism but later
during development, some segments become specialized in their structure
13. THEORIES ON ORIGIN OF METAMERISM
This theory was supported by Hyman (1951) and Goodrich.
It states that metameric organisms were developed from acoelomates and unsegmented
ancestors having serial repetition of body parts.
Metamerism was evolved as a result of accidental appearance of transverse partition
between repeated body parts so that each segment receive a repeated part of each
Metameric segmentation was linked with the evolution of coelom.
CORM OR FISSION THEORY
According to this theory , the segmented animal is a chain of coordinated sub –
individuals formed by repeated transverse fission or by asexual budding of some
non - segmented ancestors.
These sub individualsor zooids remained united end to end and become
physiologically and structurally integrated to form complete complex organism.
14. EMBRYOLOGICAL THEORY
It suggests that mechanical stresses in the mesoderm during elongation of the
embryo or larva resulted in mesoderm fragmentation , leading to segmental
repetition of all mesodermal derivatives in adult.
This theory postulates that metamerism is evolved probably as an adaptation to
locomotion of different kind , e.g. swimming , creeping , burrowing , etc.
CONCLUSION : Not a single theory could explain the origin of metamerism in different
organisms. It probably originated in different way in different organisms.
15. SIGNIFICANCE OF METAMERISM
Metameric segmentation helps the animal in their locomotion.
Coordination of muscular action and fluid coelomic
compartments causes efficient swimming and creeping which is
an advancement over ciliary action of lower invertebrates.
The segments will show high structural development which gave
scope for evolution.