Planarian
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Introduction
Anatomy
The small flatworm, planarian, is tiny, flat(because of no coelom), and leaf-shaped. It ranges from about 3mm to 12mm and .30 to 2.54 in length mostly. The planarian has the rounded head on the dorsal side and two eys-pots which called ocelli. The eye-spots are groups of cells and quite sensitive to the light(presence and direction),so that it can detect changes in light.
The planarian has bilateral symmetrical body. It has no coelom(body cavity), but consists of three cell layers: epidermis, gastroderm, mesoderm. The epidermis is outhermost layer helps the animal to move. The gastroderm is the innermost layer and lines the digestive tract. The mesoderm is between the other layers and developes many organs and systems. [1]
The planarian must move to find the food. There are two ways for locomotion, gliding(beating by cilia) and creeping(muscular activity). The first way, creeping, is through muscular contractions. The contractions of muscle layers which is under epidermis cause large movements. In another way, it glinds smoothly by beating the cilia. Cilia is on the lateral and dorsal surface, but not head side. Moreover, the ventral side of cilia is not beating during the locomotion. [2]
It has a single-opening digestive system with one anterior branch and two posterior branches, very similar to cnidarian's. The digestive system is composed of a mouth, pharynx, and the intestine. The mouth of planarian is at the center of ventral surface and the tubelike pharynx is located between the mouth and a branched intestine. Planarian feeds on the decayed and small animals, because it is zoophagous(even it has no teeeth). This kind of food is sucked up by the mouth and goes throuth pharynx into intestine. The food is broken down into small pieces and digested completely. Then, the food diffuses to body and remained things are expelled by pharynx, because of no exit. The movement and feeding occur at night chiefly. [3]
The excretory system of planarian has many tubes that extends to flame cells. These cells contain cilia.
By diffusion, it respires through its skin.
Reproduction
The planarian is hermaphroditic(it has testicles and ovaries), so it can reproduce both asexually and sexually. It has lots of testicles and only one pair of ovaries. The ovaries are located at the anterior of it and the testicles are connected to penis. [4] In sexual reprocuction, the planarian mates by cross-fertiliation. After mating, some fertilized eggs are grown in the body where the capsule is. Then, the planarian release it and it goes to the plants or a rock in water. When a few weeks pass, the eggs emerge and become adults. In asexual reproduction, the planarain cut pieces of its body and each of these regenerates missing parts. [5]
Ecology
It is found in moist areas like rivers mostly, but some are not. Some of them prefer the dark and cool place instead of moist areas, so that land planarian inhabit under rocks, logs, soil, plants, and in cave.
Other
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References
- Taxonomy-Planarian wikispecies.
- Anaomy-planarian new world encyclopedia.
- reference title author, publisher, date.
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