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 unjointed.
• 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
LOCOMOTORY ORGANS
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.
MODE OF LIFE
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.
RESPIRATION
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.
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.
SENSE ORGANS
There anterior end lacks eyes, or sensory appendages.
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.
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.
3.CLASS HIRUDINEA
BODY SEGMENTS
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.
LOCOMOTION
Leeches either swim or crawl.
RESPIRATION
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.
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