Phylum Annelida

General Characteristics:

  • Segmented bodies.
  • Bilateral symmetry
  • True body cavity (coelom) is partitioned by structures called septa. The organs of the digestive, nervous, and circulatory systems penetrate the septa and run the length of the body.
  • The digestive tract is divided into several well developed organs: pharynx, esophagus, crop, gizzard, and intestine.

 


 

  • The circulatory system is closed. It contains a dorsal and ventral blood vessel, connected by 5 aortic arches just above the esophagus. The dorsal blood vessel carries blood from the tail to the head, while the ventral one carries the blood back toward the tail.

 

  • The brain is small and is located under the pharynx. A ventral nerve cord extends from the brain and runs the length of the body. In each segment it swells to form a structure called a ganglion.

1. brain, 2. ventral nerve cord, and 3. ganglia

 

  • The nitrogenous waste is removed from the body by structures called nephridia. Each segment contains 2 nephridia except the first 3 and the last one.
  • Earthworms are hermaphroditic. They contain both male and female reproductive organs in the same body. These animal usually cannot fertilize themselves. Their fertilized eggs develop into trochophore larvae. These larvae are also found in the phylum Mollusca.
  • Their muscular system contains both circular and longitudinal muscles. As the circular muscles contract they elongate the body, while the longitudinal muscles shorten it. Setae, small bristle-like hairs, extend from the body walls and act as anchors as the worm moves. There are 4 pair of setae per segment.
  • The worms contain a thin water proof layer called the cuticle.
  • They have no skeleton other than the hydrostatic one formed by the fluid filled body cavity.
  • There are no respiratory organs and the worms breathe through its moist skin.
  • The three classes of Annelids are: Oligochaeta (earthworms); Polychaeta (sand worms); Hirudinea (leeches). Leeches are parasitic and are used by the medical profession to withdraw blood from humans in certain situations.

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