What is Phylum Annelida - Segmented Worms?

Members of this Phylum are commonly known as the segmented worms. The body of a segmented worm is made of many individual ring-like segments. The Latin word for "little ring" is "annellus," hence the name "Annelida."

Segmentation here and in the arthropods represents a distinct and significant departure from the Mollusk body plan. In terms of evolution, segmentation provides a built-in redundancy of systems. The embryo of segmented animals can develop identical segments, so that if one segment becomes damaged, other segments can take over the damaged segment's functions. Body movement is also improved, because many independent segments offer a wider range of motion than one segment by itself.

Most of the segments that form the body of annelids are for the most part identical. The anterior segments are modified to perform specific functions, depending on the type of annelid. Special functions found here would include the brain, light-sensitive eyes or antennae. Each individual segment is compartmentalized by cross walls called septa (singular: septum).

Even though each segment is partitioned, the digestive system, the nervous system and blood vessels extend along the length of the annelid body. The digestive system takes full advantage of a coelom and reflects the segmentation, in that it is divided into specialized parts called the pharynx, esophagus, crop, gizzard, and intestine. Each segment contains a pair of ganglia, or cluster of nerve cells that connect to a ventral nerve cord, providing coordination. Annelids have closed circulatory systems that use small muscular "hearts" toward the anterior of the body. A pair of organs called nephridia in each segment carries out waste excretion.

Annelids have two types of muscles. One muscle group runs longitudinally along the body from the anterior to the posterior. These muscles serve to shorten the length of the worm when they contract. The other muscle groups circle the cylindrical body rings. These circular muscles make the worm smaller in diameter when they contract. The two groups of muscles work together to move the worm's body.

Annelids can exchange gases with their surrounding medium either through gill-like structures, or directly through their skin. The digestive tracts of annelids is a complete tube that opens through a mouth and empties through an anus at the rear.

There are three classes of Annelids: Oligochaeta, Hirudinea, and the Polychaeta.