Question

Write four important characters of phylum annelida

Answer 1

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Answer 2

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Characteristics of Annelida:

1. Symmetry and Size

Annelids are all bilaterally symmetrical animals. They range in size from much less than 1 mm in length to more than 3 m. The smallest annelids described to date would appear to be Neotenotrocha (Dorvilleidae, Eunicida) reaching only a few hundred microns in length (Eibye-Jacobsen and Kristensen, 1994). Also within Eunicida may be found the largest of annelids.

2. Coelom

Nearly all annelids have a fluid-filled cavity between the outer body wall and the gut, and this is referred to as a coelom . The coelom is often used as a storage area for gametes and acts as a hydrostatic skeleton for locomotion. The coelom (formed via schizocoely) is generally organized as a series of compartments divided by intersegmental septa.

3. Body wall

Annelids have a body covered by an external cuticle that is never shed or molted. Epidermal microvilli secrete a network of fibers that are in part collagenous and also contain scleroprotein. Chaetae are also cuticular structures, but contain large amounts of chitin. Beneath the epidermis and its basal lamina lies a layer of circular muscle.

4. Parapodia

Parapodia are unjointed segmental extensions of the body wall found in many polychaetes, though many do lack these features. They are also absent in Clitellata and Echiura. Parapodia are equipped with musculature derived mainly from the circular muscle layer and usually carry chaetae. Parapodia vary in structure but basically can be considered to consist of two elements; a dorsal notopodium and a ventral neuropodium

5. Nervous system

Annelids have a brain or cerebral ganglion that originates and usually resides in the head. The brain varies in structure, with mobile active forms having the most complex brains, and sessile or burrowing forms having simple brains with little differentiation. It is connected to the ventral nerve cord by the circumpharyngeal connectives, which run down each side of the pharynx.

4. Ventral view of a scaleworm (Sigalionidae) showing its double nerve cord. This worm is unusual in having red pigmentation associated with the cord, making it easily seen.

6. Sense organs

There are six major kinds of sensory structures found in annelids. These include palps, antennae, eyes, statocysts, nuchal organs and lateral organs

7. Circulation and respiratory structures

A closed circulatory system is present in most polychaetes (Fig. 6), Echiura and many clitellates. It is reduced or absent in leeches where it may be replaced by coelomic canals.

8. Segmental organs

In most annelids there are usually two fluid systems, the coelom and the circulatory system, and both (if present) are involved in the excretion of waste products. To achieve this excretion there must be ducts to the exterior

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