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| Hydra |
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| ‹ Cnidarians: Hydra › |
| Hydra organisms are very interesting in the fact that they can reproduce both asexually and sexually. |
| Asexual reproduction occurs through a process called budding (which yeast do too, by the way). In budding, a new individual grows off the body of the parent as a bud. This occurs below the tentacles, on the “body” of the hydra. A small group of cells are formed and eventually differentiate into specialized cells. After the tentacles and mouth form, the bud pinches off and becomes an independent organism. |
| Sexual reproduction occurs only in response to certain environmental stimuli. When this occurs, the Hydra will grow egg and sperm cells. Often, both egg and sperm can be found on a single individual. |
| ‹ Platyhelminthes: Dugesia › |
| Flatworms (Dugesia sp., etc.) have the ability to regenerate. It has been shown that Planarians are able to regenerate so successfully that a very small planarian fragment that is only 1/279 of the organism is still able to form a complete individual. In other words, if you were to cut the flatworm in half (cross section), a new head would grow on one end and a new tail would grow on the other. |
| Links: |
| http://www.cvgs.k12.va.us/research/Final/sresch01/Ocallaghan/introduction.htm |
| Quite similar to its ability to regenerate, Planarians reproduce asexually by architomy. This is a type of fission that occurs when the worm divides into two fragments without any prior cellular differentiation. This leaves one, nearly normal end with a head and most of the gut, and one not-so-normal end—a headless mass of tissues that must produce the missing parts. You are not likely to see this process occur, but it is quite possible to see these headless lumps stuck on the walls of the live specimen container. Keep your eyes out for these strange looking structures! |
| A note about architomy: |
| If you remember from Bio 190, bacteria reproduce asexually by binary fission. This occurs when a parent cell divides into two daughter cells after DNA replication and cellular organization. Although architomy is similar to this process, it is different in the fact that separation into daughter cells occurs before bodily reorganization. |
| Links: |
| http://biotech.icmb.utexas.edu/search/dict-search.phtml?title=binary+fission |
| Another important fact to remember about Dugesia is that they are acoelomates. This means that the area between the gut and the organism’s outer body wall is completely filled with mesoderm. It is a triploblastic organism that does not have an internal body cavity other than the gut. |
| ‹ Annelida: Lumbricus › |
| These segmented worms are coelomates. This means that they have an internal, fluid-filled body cavity, which originated from the mesoderm. A good way to remember which organisms are coelomates, etc. is to think that the more “advanced” the worm, the more likely it is to have a coelom. This could be because there are more organs present, and the fluid filled cavity can be used for protection. What do you think? |
| Do setae really do anything? |
| Of course! Not just for cosmetic reasons, either. Although, when looking at them, they just look like holes in the epidermis of the worm, they are actually small hair-like bristles inside pores. They act as anchors to hold parts of the worm against the substratum (the layer of earth beneath the soil) so that elongation of the organism will result in controlled, usually forward, motion. |
| Setae are retractable and may be withdrawn in some, or even all, of the segments of your worm. Careful examination, possibly with a dissecting microscope or hand lens, however, should reveal some segments with extended setae. |
| Links: |
| http://www.lander.edu/rsfox/lumbricu.html |
| ‹ Roundworms: Nematoda › |
| Nematodes are pseudocoelomates. This means that there is an internal body cavity other than the gut, but it is not derived or completely lined with mesoderm. |
| Nematodes are almost unbelievably abundant. According to one study, around 90,000 individual nematodes were found in a single rotting apple. Another reported 236 different species living in a few cubic centimeters of mud. The number of described species is around 12,000, but little attention is paid to these organisms and the true number of species may be closer to 500,000. Some species are generalists, in that they can live in many different habitats, but some are much more specialized. A good example of the latter is a species of nematodes that is known to exist only in felt coasters placed under beer mugs in a few towns in Germany. |
| Links: |
| http://animaldiversity.ummz.umich.edu/nematoda.html http://www.niaid.nih.gov/newsroom/focuson/bugborne01/filar.htm |
| ‹ Bivalves: Venus › |
| What’s in that chowder? |
| Unlike scallops, another mollusk, where we eat only the large muscle that attaches the halves of it’s exterior together, clams yield their entire body for consumption—muscles, intestines, gonads, and all their other internal organs. Yum! |
| Natural pearls vs. cultured pearls |
| Clams, unlike common belief, can produce pearls just as oysters can (just not as commonly). Since they are created when sand grains get trapped and the mantle secretes pearly layers around it, pearls can be easily “forced”. Some organizations insert very small hard objects between the mantle and the shell and allow pearls to form around them. This may not seem as natural, but they turn out just as beautiful. Maybe you can culture your own pearls for your significant other next Valentine’s Day. |
updated 3-12-06