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Phylum Rhombozoa Dicyemida |
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Many hypotheses have come out to explain why dicyemida has a simple body plan: “While living as a parasite, it discarded complicated structures and transformed into a regressed form”, or “they are keeping the primitive form since they become the multi-cellular organism”. What is the truth? An adult dicyemida attaches to the kidney of squid and octopus to take in nutrients. It is long and slender, approximately 1mm in length. There is no hollow space in the body but a single axial cell extends in the center of the body as a long thin axial rod. Small cells wrap around the axial cell, and they form the body surface. This cell structure is equivalent to the gastrula stage of the initial animal development except that the endoderm consists of only one axial cell which is unique to dicyemida. Dicyemida’s body has anterior and posterior sides. A cap-like part (calotte) is the anterior region where the dicyemida attach to a host organism. A young dicyemida increase the number of individuals by producing many small copies. The animal has a unique reproducing process that embryos are continuously produced within the axial cell. No other animal reproduce the same way as the dicyemida. A newly produced individual is called vermiform larva and has a long and slender body. The vermiform larvae move out of their parent’s body and live as a group of parasites. At that time, the larva already has the same number of cells as the adult dicyemida. Once the host’s kidney is filled with the clones of the vermiform larvae, the parental dicyemida stop producing the vermiform larvae. Instead, the mature dicyemida start producing a completely different type of offspring called infusoriform larvae by self fertilization. The adult dicyemida develops hermaphroditic gonads (having both male and female functions) and fertilizes the eggs and sperms that are produced in the same individual to give birth to an infusoriform larva. The larva looks completely different from the parent as if they are not related. The infusoriform larva swims with cilia to move out from the host to the sea. However, the process from a free swimming larva to becoming a parasite is unknown. Culturing the infusoriform larva in artificial kidney extract has not re-created the life cycle of the dicyemida. As mentioned above, approximately one hundred species of dicyemida, that are known to produce two morphologically different types of larvae, have been identified. According to an expert, the number of registered dicyemida species will increase to approximately 140 in the future. Let’s go back to the first question. Researches on the development and morphology of the dicyemida did not reveal its ancestral animal. However, DNA sequence analysis showed that the dicyemida is closely related to triploblastic organisms. This evidence suggests that the body structure of the dicyemida is regressed due to parasitism. Several hundred million years before the squid and octopus were born and evolved in the Mesozoic era, a free living, bilaterally symmetric animal in the sea started to specialize in parasitism and the dicyemida was born. Because the reproduction is based on self fertilization, new genetic combination cannot be produced. Therefore, it may be a fate that the force of natural selection may eliminate the animal. Before cooking squid or octopus, insert a dropper into the kidney to take some fluid and prepare a slide. Under the microscope, you can see a crowd of vermiform larvae or adult dicyemida having infusoriform larvae inside of their body. It is possible that several species infest in one kidney at the same time. |
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In preparation
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I am sorry.
A song is only Japanese.
Song
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karaoke
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