Annelida
Anellifa, or segmented worms, come in a variety such from leeches to marine worms. Annelida are tripoblastic, meaning they have three primary germ layers ectoderm, mesoderm and endoderm, are bilatteraly symmetrical, capable of division into symmetrical halves by only one longitudinal plane passing through the axis, and coelomate, meaning that they have a hydroskeleton. The internal organs of annelids are well developed. They include a closed, segmentally-arranged circulatory system. The digestive system is a complete tube with mouth and anus. Gases are exchanged through the skin, or sometimes through specialized gills or modified parapodia. Each segment typically contains a pair of nephridia. The nervous system includes a pair of cephalic ganglia attached to double nerve cords that run the length of the animal along the ventral body wall, with ganglia and branches in each segment. Annelids have some combination of tactile organs, chemoreceptors, balance receptors, and photoreceptors; some forms have fairly well developed eyes, including lenses.
Characteristics of Annelida
1. Segmented bodies.
2. Bilateral symmetry.
3. True body cavity (coelom) is partitioned by structures called septa.
4. The organs of the digestive, nervous, and circulatory systems penetrate the septa and run the length of the body.
5. The digestive tract is divided into several well developed organs: pharynx, esophagus, crop, gizzard, and intestine.
6. 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.
7. 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.
8. 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.
9. Earthworms are hermaphroditic. They contain both male and female reproductive organs in the same body. These animal usually cannot fertilize themselves.
10. 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.
11. The worms contain a thin water proof layer called the cuticle.
12. They have no skeleton other than the hydrostatic one formed by the fluid filled body cavity.
13. There are no respiratory organs and the worms breathe through its moist skin.
Characteristics of Annelida
1. Segmented bodies.
2. Bilateral symmetry.
3. True body cavity (coelom) is partitioned by structures called septa.
4. The organs of the digestive, nervous, and circulatory systems penetrate the septa and run the length of the body.
5. The digestive tract is divided into several well developed organs: pharynx, esophagus, crop, gizzard, and intestine.
6. 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.
7. 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.
8. 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.
9. Earthworms are hermaphroditic. They contain both male and female reproductive organs in the same body. These animal usually cannot fertilize themselves.
10. 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.
11. The worms contain a thin water proof layer called the cuticle.
12. They have no skeleton other than the hydrostatic one formed by the fluid filled body cavity.
13. There are no respiratory organs and the worms breathe through its moist skin.