Planaria

=Classification/Diagnostic Characteristics =

Planaria are a family (a subset) of the larger phylum of platyhelminthes, which are commonly known as flatworms. There are 25,000 species of flatworms, all of which are invertebrate animals. Many flatworms, including well-known tapeworms, are internal parasites of vertebrates. Planaria specifically are part of the class Turbellaria, which includes only those flatworm groups that are not parasitic, so planaria are free-living animals.

 As flatworms, planaria are flat, of course, and possess relatively simple animal bodies that look like little more than wavy, colored slices. Naturally, they are bilaterally symmetrical, meaning that they could be divided into two roughly mirror image slices along the center, allowing for streamlined movement. They possess only one opening, a mouth at the center of their underside, and are acoelomates, meaning that they do not have any body cavities; cavities are not necessary given the simple internal structure of planaria which would otherwise require protection and cushioning. However, planaria are cephalized, meaning that nervous tissue is concentrated in one end of the body to create a center for sensor organs and a brain to process their information. Planaria specifically have a head with a chemoreceptor (which senses using chemical signals in the water), two simple eyes, and a small brain.

=Relationship to Humans =

Some parasitic species of flatworms have humans as host species, causing diseases such as schistosomiasis, which is common in parts of Asia, Africa, and South America. The disease is developed by contaminated water. Symptoms of this disease include skin rashes, chills, itching, diarrhea and abdominal pains. Planaria, however, are not parasitic and have limited direct interaction with humans. Flatworms can also serve as indicators because they are very sensitive to water quality and they show things like reduced oxygen and depleted resources in the water being tested. Planaria (in the Turbellaria class) also form the basis of the "food chain" and they form links between the microscopic things they consume and their own predators which are visible to the naked eye. Scientists also use planaria to study reproductive patterns and responses to certain stimuli.

=Habitat and Niche =

Planaria are purely aquatic animals, though they live in both saltwater and freshwater ecosystems. Most planaria are found under rocks in these aquatic environments. As bottom feeders, planaria suck up small invertebrates as they glide along the bottoms of their ponds or rivers. Therefore, planaria are considered both secondary consumers since they feed on low-level autotrophs, organisms which fix their own energy, and detritivores since they eat decaying biomatter that has drifted to the bottom of their body of water.

=Predator Avoidance =

As stated before, the sensory system of planaria, used to detect predators as all else, consists of a chemoreceptor, two simple eyes, and a tiny brain. If a planarian detects a predator using the former, it attempts to flee. Planaria use their eyes to detect light and move towards darker areas, which also serves to avoid predators.

=Nutrient Acquisition =

Also as stated before, planaria are bottom feeders: they glide along the bottom of their ecosystems, sucking up nutrients. Specifically, a planarian’s food includes small vertebrates and the decaying biomatter that lines the bottom of its river or pond. Planaria suck up food using a mouth located at the center of their body’s underside; the mouth is attached to the pharynx, the tube which connects the mouth to the digestive tract. Nutrients from the food are absorbed by the digestive tract and diffused to the rest of the body.

=Reproduction and Life Cycle =

Planaria possess a variety of reproduction mechanisms. Some species reproduce sexually, while others reproduce asexually; some species can use both methods. If a planarian is cut, it is capable of regenerating parts to create two individuals. It is estimated that a part as little as 1/279 of the original planaria can regenerate like this. Regeneration consists of a blastema forming at the point of serverance. Over time, the stem cells in the blastema will grow and differentiatie, adjusting the existing cells for symmetry, until a new hydra has reformed. Typically, asexual reproduction occurs via pinching at the individual’s midbody. This is fragmentation when the planaria splits into two new individuals which grow back their missing parts. Sexual reproduction is favored as it boosts genetic diversity. Those planaria that reproduce sexually are hermaphrodites, possessing both male and female sex organs. Eggs are grown inside the body; they mature into adults in a matter of weeks. These eggs can not be fertilized by the same planaria, they must be fertilized by another planaria.

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=Growth and Development =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Planaria are triploblastic, meaning that their cells differentiate in the embryo stage into three types that give rise to divisions in structure that are common to higher species. As stated before, planaria mature from eggs to adults in a matter of weeks.

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Planaria have the ability to regenerate body parts. The speed and ability of regeneration is indicative of where the neoblast stem cells reside. If a low number of neoblast cells are present, then they have to either migrate from other parts of the body, or reproduce by cell division. A low count of neoblast cells means a slower rate of regeneration.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Integument =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Planaria possess only a single layer of epithelial tissue as covering so as to ensure free exchange of gases between cells and the surrounding water (see Gas Exchange).

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Movement =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Like other free-living flatworms, planaria use cilia for movement. Cilia are projections of the cell membrane lined with long, linear proteins to increase drag. Broad bands of cilia beating in a coordinated fashion propel the planarian by pushing against the water. Given that the planarian cannot propel itself vertically, it is resigned to the bottom of its body of water. The movement seems to be gliding due to the way the cilia move. In order to find food and nutrients, planaria must either glide or creep (methods of movement). Gliding involves the beating of the cilia (located on lateral and dorsal surface), while creeping is muscular activity (contractions of muscles under epidermis) of the planaria.

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=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Sensing the Environment =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">As stated before, the sensory system of planaria consists of a chemoreceptor, two simple eyes(ocelli), two auricles(earlike projections), a mouth, and a tiny brain. The eyes are useful mostly only to determine the brightness of the environment. Again, the planarian tries to move towards darker areas, which tend to be safer. Auricles are sensitive to the presence of certain chemicals and aid in sensation. A mouth is found on the underside of the body and is covered in cilia which aids in movement. The brain is little more than an especially thick knot of nerve cells, equipped only to make use of the little information given by the sensor organs.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Gas Exchange =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Flatworms do not possess specialized gas exchange organisms. Since there is no organ system to bring oxygen to cells, individuals cell must receive oxygen gas through diffusion directly with the surrounding water, hence why the flatworms are flat: all cells must be close to outside water. The flatworms flat characteristic increases its surface area to volume ratio, allowing it to exchange gasses at a fast rate.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Waste Removal =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">As stated above, each cell receives oxygen through direct diffusion with the environment. Waste products, including carbon dioxide and nitrogenous waste in the form of ammonia, similarly diffuse out of individual cells. Diffusion is assisted in planarians by the earliest form of a specialized waste removal system: small “flame” cells lining the edges of the organism flick cilia back and forth to help push out nitrogenous wastes through pores in its back. Although planaria do not have respiratory or circulatory systems, they break down their food with enzymes in a gastrovascular cavity.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Environmental Physiology =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">The planarian is ectothermic: it does not attempt to regulate its internal temperature directly, so metabolism tracks environmental temperature. <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">The control of internal water and salt concentration is necessary since a planarian's environment is often hypotonic and water levels could rise dangerously without regulation. This regulation is carried out via the waste removal means discussed above. Flame cells push waste fluids towards a network of tubules called the protonephridium, which is similar to the kidney. The cells of the <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif; line-height: 1.5;">protonephridium, which is shaped something like a tube itself, selectively reabsorb useful molecules to desired concentrations, and the remainder passes out through the perforated end of the tube. The selective reabsorption of salt regulates both salt and water concentration, since a certain salt level corresponds to a certain equilibrium water level: water flows into cells when the cells are more salty than the surrounding water, and water flows out when the cells are less salty.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Internal Circulation =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Planaria do not possess a closed circulatory system, that is, one with blood vessels and a heart. They have a simpler open circulatory system: nutrients sucked up from the mouth through the pharynx flow into a highly branched digestive tract that leads to each cell in the planarian’s body. The tract is highly branched so as to ensure great surface area, allowing for efficient diffusion of nutrients into cells.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Chemical Control (Endocrine System) =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Given the simplicity of the planarian’s internal systems, there is little need for complex coordination inside the body. A coordinated endocrine system and use of associated hormones is thus limited. As noted above, planaria do control internal water and salt levels in addition to expelling nitrogenous wastes.

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Review Questions =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">1) What are cilia? How does the ability of planaria to use cilia to move affect the way that they acquire nutrients? <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">2) What is the integument of planaria composed mainly of? Why is it important to maintain this requirement? How does this affect the functioning of other body systems? <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">3) How do planaria interact with their surrounding environments?

=<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Citations =

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Principles of Life <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Hillis, Sadava, Heller, Price <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">2013, Sinauer Associates, Inc. <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Sunderland, MA <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">p. 467

<span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Emigration Creek Project <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">Westminster College <span style="font-family: 'Lucida Sans Unicode','Lucida Grande',sans-serif;">[]

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