Tube Within A Tube Body Structure example essay topic
361 words
EARTHWORM: Kingdom Animalia Phylum Annelida Class Oligochaete The earthworm is the best-known member of the phylum Annelida, the segmented worms. Annelids are bilaterally symmetrical, and their bodies are divided into segments both externally and internally. They have a tube-within-a-tube body structure. The outer tube is the body wall, while the inner tube is the digestive tract.
The cavity between the outer and inner tubes is the coelo m. CIRCULATORY SYSTEM As food is digested, the blood in the circulatory system picks it up for distribution to all the cells of the body. In simpler we have studied so far, the digested food had only to diffuse a short distance in order to reach all cell of the body. But in higher forms, the distances are greater, and more food material is needed by the many specialized and active cells of the body.
The blood of the earthworm moves through a series of closed tubes, or vessels. It flows forward to the anterior end in a dorsal blood vessel and moves to the posterior end in a ventral blood vessel. Small tubes connect the dorsal and ventral vessels throughout the animal, except in the segments 7 and 11. Here there are five pairs of connecting tubes which are large and muscular.
By means of alternate contraction and relaxation, they keep the blood flowing. Not true hearts, they are called aortic arches. RESPIRATION AND EXCRETION The earthworm absorbs oxygen and gives off carbon dioxide through a thin skin. This skin is protected by a thin cuticle secreted by the epidermis and kept moist by a slimy mucus also produced by epidermal cells. A moist surface is necessary for oxygen to be absorbed and carbon dioxide to be given off. If the worm is dried by the sun, it will die because the exchange of gases can no longer take place.
Nitrogen-containing waste materials from cell activities are removed to the outside of the body by little tubes. There are two such structures, called nephridia, in each segment except the first three and the last..
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