Phylum Annelida (Segmented worms)
MAIN CHARACTERS
NATURE
Annelida are
triploblastic, symmetrical, coelomata and segmented metozoa.
HABIT AND HABITAT
Annelida are mostly
aquatic, marine or fresh water, burrowing or living in tubes, some free living
forms.
EXTERNAL FEATURES
§ The most important feature of annelida is their metameric
segmentation. (External segmentation)
§ Segmentation is indicated externally by circular constrictions
or grooves on the body wall.
§ Outer covering of the body is cuticle secreted by the underlying
epidermis.
§ Appendages, when present are un jointed.
§ Locomotory organs are segmentally arranged, paired setae or
chaetae.
INTERNAL FEATURES
§ Body wall is contractile, consists of an outer epidermis,
circular and longitudinal muscles.
§ The gut, longitudinal blood vessels and the nerve cord extend
throughout the body length, whereas other structures are repeated in each
segment.
§ Important character of annelida is the development of series of
coelomic compartments in their body between the gut and the body wall.
§ The Coelom is a cavity, which develop within the mesoderm and is
lined by mesodermal cells.
§ Segmented musculature plays an important part in locomotion of
Annelids.
SYSTEMS OF BODY
§ Alimentary canal is tube like extending straight from mouth to
anus.
§ Respiration through general body surface, by gills in some
forms.
§ Blood vascular system is closed type.
§ Blood is red due to haemoglobin.
§ Excretory organs are Nephridia usually one pair in each segment.
§ Nervous system consists of dorsal brain and longitudinal ventral
nerve cord.
§ Sexes may be united or separate.
§ Development is direct when sexes are united and indirect when
sexes are separate.
EXAMPLES
Nereis, Earthworm
and Leeches etc.
CLASSIFICATION OF PHYLUM ANNELIDA
Phylum Annelida is
divided into four classes:
1. Polychaeta
2. Oligochaeta
3. Hirudinea
4. Archiannelida
1. Polychaeta
2. Oligochaeta
3. Hirudinea
4. Archiannelida
1.POLYCHAETA
LOCOMOTORY ORGANS
The Polychaetes possess paired parapodia functioning as locomotry appendages, are present only in the class Polychaeta.
The Polychaetes possess paired parapodia functioning as locomotry appendages, are present only in the class Polychaeta.
PROSTOMIUM
Usually there is a distinct head or Prostomium bearing sensory and feeding appendages.
Usually there is a distinct head or Prostomium bearing sensory and feeding appendages.
MODE OF LIFE
The Polychaetes may be carnivorous, scavengers, or filter feeders.
The Polychaetes may be carnivorous, scavengers, or filter feeders.
REPRODUCTION
The sexes are separate and fertilization of eggs takes place outside body. Their free swimming larva is called Trochophore.
The sexes are separate and fertilization of eggs takes place outside body. Their free swimming larva is called Trochophore.
RESPIRATION
The respiration takes place through the body surface in many but in some gills may be present as respiratory organs.
The respiration takes place through the body surface in many but in some gills may be present as respiratory organs.
EXAMPLES
Some well-known examples of marine polychaetes are Nereis, Arenicola and Sabella. Nereis lives beneath stones and in crakes of rocks.
Some well-known examples of marine polychaetes are Nereis, Arenicola and Sabella. Nereis lives beneath stones and in crakes of rocks.
2.CLASS OLIGOCHAETA
LOCOMOTORY ORGANS
The Oligochaetes possess fewer numbers of Setae as compared to the Polychaetes. The setae help the earth worms in crawling.
The Oligochaetes possess fewer numbers of Setae as compared to the Polychaetes. The setae help the earth worms in crawling.
SENSE ORGANS
There anterior end lacks eyes, or sensory appendages.
There anterior end lacks eyes, or sensory appendages.
CLITELLUM
At sexual maturity, all of the oligochaetes develop in several segment, glandular epithelium, called clitellum.
At sexual maturity, all of the oligochaetes develop in several segment, glandular epithelium, called clitellum.
MODE OF LIFE
§ Oligochaetes live either in fresh water or on land.
§ There is no free swimming larval stage in their development
§ Majority of oligochaetes are scavengers, feeding on decomposing
organic matter.
§ Some fresh water species feed on algae.
§ Burrowers like earth worm ingest a large quantity of soil,
digest the organic matter and the living fauna.
RESPIRATION
Respiration takes place through their general body surface. Some aquatic species possess anal gills.
Respiration takes place through their general body surface. Some aquatic species possess anal gills.
ECONOMIC IMPORTANCE
Earthworms increase the fertility of soil by physically over turning it. They ingest the soil, break it down and deposit it in the form of casts. The over turned soil is relatively in proportions of total nitrogen, organic carbon, calcium, magnesium and phosphorus.
Earthworms increase the fertility of soil by physically over turning it. They ingest the soil, break it down and deposit it in the form of casts. The over turned soil is relatively in proportions of total nitrogen, organic carbon, calcium, magnesium and phosphorus.
3.CLASS HIRUDINEA
BODY SEGMENTS
Unlike polychaetes and oligochaetes, the number of body segment in leeches is fixed at 34.
Unlike polychaetes and oligochaetes, the number of body segment in leeches is fixed at 34.
SUCKERS
The anterior and posterior body segments are fused to form suckers.
The anterior and posterior body segments are fused to form suckers.
LOCOMOTION
Leeches either swim or crawl.
Leeches either swim or crawl.
RESPIRATION
Respiration generally takes place through the body surface. Leaf like gills may be present.
Respiration generally takes place through the body surface. Leaf like gills may be present.
PARASITIC NATURE
Most leeches feed by sucking blood of aquatic invertebrates and vertebrates.
Most leeches feed by sucking blood of aquatic invertebrates and vertebrates.
4.ARCHIANNELIDA
§ It is a small group of marine worms.
§ They are not segmented externally and don’t have bristles.
§ They live in the sea and show annelid characteristics to a minor
extent.
§ Their development is also characterized by Trochophore Larva.
EXAMPLES
Nerilla
Dinophilus
Nerilla
Dinophilus
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