Annelids Metameric Body Form



Population explosion of annelids: Millions of Samoan pablo worm (Eunice viridis) are produced in an island of South Pacific. The Samoan pablo worm is a member of the phylum Annelida. Millions of worms transform the ocean into a soup. There can be a high yield of this worm for one week. The people harvest it in nets and buckets. These worms are cooked or wrapped in breadfruit leaves. They spend their entire adult life in coral at the sea bottom. One week after the full moon in each November. they come out of the burrow. They have specialized body segments for sexual reproduction. These segments break and get free. They float on the surface. The remaining worms are safe on the ocean floor. The sexual segments release countless eggs and sperm. The surface water is discolored due to these egg and sperms. The reproductive swarms (egg and sperm). They do not reappear till next year. Other members of this phylum include countless marine worms the earthworms, and predatory leeches.

Characteristics of Phylum Annelida

Characteristics of the phylum Annelida include:

  1. Their body is metameric. bilaterally symmetrical, and wormlike.
  2. They show protostome characteristics include spiral cleavage. trochophore larvae and schiz coelous coelom formation.
  3. They have paired epidermal setae.
  4. They closed circulatory system
  5. They have dorsal supra pharyngeal ganglia and ventral nerve cords with ganglia.
  6. They develop metanephridia or protonephridia.

Evolutionary Relationshios of Annelida


Annelids are protostomes. They show protosotme characteristics:

  1. They have spiral cleavage.
  2. They have mouth derived from an embryonic blastopore.
  3. They have schizocoelous coelom formation.
  4. The develop trochophore larvae. Trochophore larvae are present in most members of the phylum.

This diverse phylum originated in Precambrian times (more than six hundred million years go). Unfortunately, the evidence about its evolutionary pathway is not known. Number of hypotheses is present about annelid origins. These hypotheses are based on the or ,in of the coelom.

1. Schizocoelous origin: It suggests that the coelom of the annelids has schizocoelous origin. It means that annelids have evolved from ancient flatworm stock.

2. Enterocoelous origin: It suggests the coelom origin is Enterocoelous. It means that the ancient diploblastic animals are ancestor of annelids. Similarly, the acoelomate animals have derived form coelomate animals.

Recently a worm Lobatcerebrum has been discovered. It shows both annelid and flatworm characteristics. Therefore, it gives support to the enterocoelous origin hypothesis. Loharcerebrum is classified as an annelid. It has certain annelids characters:

  1. They have segmentally arranged excretory organs.
  2. They have annelid-like body covering.
  3. They have a complete digestive tract.
  4. They have annelid-like nervous system.

But it has a ciliated epidermis and is acoelomate like flatworms. Some zoologists believe that Lobatocerebrum indicates the triploblastic. acoelomate animals can derive from the annelid lineage.


Segmental arrangement of body parts in an animal is called metamerism (meta, after + mere, part). Earthworm bodies are divided into series of ring like segments. The body is also divided internally.

Metamerism influences every aspect of annelid structure and function. It includes anatomical arrangement of organs. This arrangement is associated with metamerism.

There is compartmentalization of the body. Therefore, each segment has its own excretory, nervous and circulatory structures. There are two related functions metamerism: flexible support, efficient locomotion.

These two functions depend on the metameric arrangement of the coelom.



Origin of coelom: The body cavity of annelids arises by segmental splitting of mesoderm during embryonic stage. Mesoderm occupies the region between ectodermic and endoderm on both side of the embryonic gut. Each cavity is enlarged and it forms a double-membrane septum. This septum separates the anterior and posterior margin of each coelomic space. It also separate the dorsal and ventral mesenteries associated with the digestive tract.

Origin of muscles: Muscles also develop from mesodermal layers of each segment. A layer of circular muscles lies below the epidermis. A layer of longitudinal muscles is present just below the circular muscles. These muscles run between the septa that separate each segment. Some polychaetes also have oblique muscles. The leeches have dorsoventral muscles.

Advantages of segmentally arranged coelom and muscles

1. The coelomic spaces and muscles produce hydrostatic compartments. Coelomic hydroskeleton has many advantages. It has locomotory and supportive functions. These functions are not possible in non metameric animals that have a hydrostatic skeleton. Each segment is controlled independently of other segments. The muscles can act as antagonistic pairs within a segment. The coelomic fluid provides a hydrostatic skeleton. The muscles operate against it. These arrangement causes swimming, crawling, and burrowing.

2. A second advantage is that it lessens the impact of injury. If one or a few segments lured, adjacent segments are separated from injured segments by septa. They are o maintain nearly normal functions. It increases the chance of survival.

3.Metamerism causes modification in certain regions of the body. They become lized for feeding, locomotion, and reproduction. The specialization of body regions in a metameric animal is called tagmatization.


Similarities of metamerism in annelids and arthropods

Tagmatization is best developed in the arthropods. But some annelids also display tagmatization. Because of this similarity the annelids and arthropods are closely related. Other common features between arthropods and annelids are triploblastic coelomate organization, bilateral symmetry, a complete digestive tract, and a ventral nerve cord. It means that arthropods and Annelids and have common ancestor. Their ancestor may be a marine, wormlike, bilateral animal.


It may possessed metameric design. Changes in the ancestors gave rise to these two phyla.

Generalized Annelids Body Plan

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