Annelida

ANNELIDA

Segmented worms

(Latin.aneleus = little ring)
Annelida have bodies consisting of many essentially similar ringlike segments (somites). This segmentation usually shows in both external and internal features, including muscles, nerves, and circulatory, excretory and reproductive organs.
This phylum is divided into three classes:
Oligochaeta: Earthworms - Most earthworms and their equals are inhabitants of damp soil and fresh waters.
Hirudinea: Leeches - The leeches are found mainly in fresh water or on moist ground.
Polychaeta: Bristle-worms - The marine worms are found chiefly in close vicinity of the shore.
Some annelids are free-living, many inhabit burrows or dwell in tubes, some are comensals on other aquatic animals (few are ecto- or endoparasites), and many of the leeches attach to vertebrates.

Characteristics:

1. Symmetry bilateral. Triploblastic. The body is elongate and usually conspicuously segmented both internally and externally.

2. Appendages are minute rodlike chitinous setae, little to many per somite.Polychaeta have fleshy tentacles on its head and has the setae situated on the lateral fleshy parapodia. Most species of the class Hirudinea lack setae.

3. The Body is covered by a thin moist cuticule over columnar epithelium containing unicellular gland cells and sensory cells.

4. Both the body wall and the digestive canal has layers of circular and longitudinal muscles. The body cavity (coelom) is well developed (except in the leeches) and is divided by septa in the in the Olgochaeta and Polychaeta.

5. A complete digestive canal is present in a tubular shape, extending the whole length of the body.

6. A closed circulatory system of longitudinal blood vessels with branches extending laterally in each segment. Dissolved hemoglobin and free amoebocytes are usually present in the blood plasma.

7. Respiration occurs via the epidermis or through gills in some tube dweller species.

8. Excretory system consists of one pair of nephridia per segment(somite). Each of these nephridia removes waste from the coelom and bloodstream directly to the outside world.

9. A nervous system is present with a pair of cerebral ganglia( a brain) and connectives to a solid (double) midventral nerve cord extending the length of the body. The midventral nerve cord is connected to pairs of lateral nerves in each segment. Sensory cells and organs for touch, taste and the perception of light is also present.

10. The sexes are united and the development in the case of Oligochaeta and Hirudinea are direct or the sexes are seperate and the development includes a trocophore larval stage as with the Polychaeta. Some species of Oligochaeta and Polychaeta reproduce asexually by budding.

Here we only look at the Polychaeta :

OLIGOCHAETA

Earthworms

HIRUDINEA

Leeches

POLYCHAETA

Bristle-worms

(Greek.Poly = many + chaeta = spine or bristle )
Bristle-worms are among the most common and most diverse creatures on the shore. They are the largest class in the phylum annelida consisting of over 8000 species. They also show the best adaptiveness in terms of body form, food habits and the habitat they occupy (most species are marine, ranging from the littoral zone to the abyssal zone). Many species are abundant as individuals, particularly in sand and mud bottoms. They are also a ecologically important group in the different marine ecosystems. A few species are pelagic.

A traverse section through a polychaeta at the level of its intestine showing the presence of a pair of parapodia.

Different types of setae and parapodia.