Describe the locomotory structures and mechanics of locomotion in annelids.

1. Taxonomic Retrospect of Phylum Annelida:

1. The soft bodied animals including para­sitic arachnids and crustaceans were grouped under a phylum vermes by Linnaeus (1768).

2. Cuvier first separated it into an independ­ent segmented higher group. He united the annelids with the arthropods under the taxon Articulata.

3. Lamarck first coined the term ‘Annelida’ in 1802.

4. Grube (1851) used the taxa-Polychaeta and Oligochaeta respectively.

2. Definition and Fossil Record of Phylum Annelida:

Bilaterally symmetrical, elongated, metamerically segmented eucoelomates and soft bodies covered with thin cuticle containing segmental chitinous setae.

The whole body fossil record of annelids is scanty but well represented of semi-polychaetes have recorded from Middle Cambrian in Canada.

3. Habit and Habitat of Phylum Annelida:

Most polychaetes are marine. Larger forms like Aphrodite, Glycera live in mud while slender and smaller forms live in water spaces between sand grains. Many forms like Polygordius, Protodrilus secrete sticky sub­stances and remain attached on sand.

In some forms the setae are used in attachment to the sand. Nereis vitabunda of Sumatra is a terrestrial form and lives in soil channels. Most polychaetes on being transferred to freshwater die, while a few species like N. diversicolor, Troglochaetus can withstand and live in freshwater.

Oligochaetes are freshwater animals though a few of them like Helodrilus are marine. Most oligochaetes are terrestrial and live on mud-shores, in forests, meadow and cultivated lands. In Borneo, a group of earth­worms under the genus Pheretima, climb trees and live on barks and leaves of trees.

Seventy-five per cent of the members of the class Hirudinea live in freshwater and are very common in marshes and ponds. South American soil leeches (Erpobdellidae) live in moist soil, cow dung and rotten wood and act as predators of worms and insects.

Terrestrial leeches usually live in moist vege­tation of the tropics and feed on the blood of Vertebrates. Piscicola geometra is a brackish water form and remains attached to bottom- dwelling fishes. Branchellion is found attached to skates and rays.

Echiurids are semisessile and marine ani­mals many forms live in burrows, and cavi­ties made by other animals or on empty shells of molluscs.

Annelids vary widely regarding their sizes. Most polychaetes are minute to moderate in size but Neanthes brandti of Californian coast attains a size of about 1.5 metres.

Oligochaetes like Chaetogaster, Aeolosoma are a few cm in length but a few attain giant size. Rhinodrilus fafneri of Ecuador and Australian Megascolides australis grow over two metres in length and 25 mm in diameter. The average length of leeches is between 10 and 200 mm.

4. Generalised Characters of Phylum Annelida:

The annelida as the name implies (annulus = ring) are segmented worms and the segments in many cases are externally recog­nised as ring-like constrictions. In many forms a distinct head consisting of a pre-oral ‘prostomium’ and a post-oral ‘peristomium’ is present.

In most of the forms anterior, posterior, dorsal and ventral surfaces are well-recognised. The food canal is a straight tube with demarked regions and extends from anterior mouth to terminal anus. In many forms the perivisceral cavity or coe­lom is well-developed.

The coelom is lined with mesoderm and communicates to the exterior through paired nephridia. The body wall is muscular having circular and longi­tudinal layers of muscles and many have setae embedded in skin. Typical members have thin and non-chitinous cuticle. The blood-vascular system is of closed type.

The organs of excretion are metamerically ar­ranged tubular nephridia or tegmental or­gans which are closed internally or lead from coelom to exterior. A series of paired ducts called coelomoducts, either united with or distinct from the nephridia may be present to carry the reproductive elements outside.

The central nervous system consists of a pair of pre-oral ganglia connected by commissure with a pair of ventral cords which is ex­panded at each segment to form ganglion. Development may be direct or indirect. The larva is called trochophore and undergoes metamorphosis.

5. Specialised Characters of Phylum Annelida:

In Annelids the triploblastic condition has attained perfection. The body cavity is a true coelom which lies between the body wall and the tubular food canal. The body plan may be best described as a tube within a tube. This structural plan is maintained in higher forms. Definite segmentation is encountered in annelids. The body is composed of numerous distinct segments arranged in a linear series.

Bilateral symmetry is well represented in annelida and this is an evolutionary advance­ment over more primitive radial symmetry. Nervous system of annelids is more organ­ised and consists of a pair of dorsally-placed cerebral ganglia and ventral nerve cord. The nerve cells are distributed in the nerve cord and ganglia.

The tendency of ‘Head’ forma­tion is distinct. Head is usually associated with sense organs and as the sense organs help the animal to keep in touch with the sur­roundings, they are usually housed at that end of the body which remains directed forward during locomotion.

Organs and organ sys­tems performing vital functions as nutrition, excretion and reproduction are highly devel­oped in artnelids. Thus association of differen­tiated tissue groups has occurred in them.

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