Coclusion for.topic kingdom clasification of plants and animals
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Kingdom Animalia has evolved vastly in its complexity, starting with simple sponges, and ending with us. These organisms have been constantly growing in complexity and physical adaptations, and I will now explain exactly what these adaptations are that we have learned throughout this course.
Porifera is the first phylum in Kingdom Animalia, and is by far the most simple. Sponges do not contain any real tissue, but are rather just a series of cells living together. These organisms are sessile filter feeders that have changed very little since they evolved. They reproduce sexually through internal fertilization with eggs, sperm, and larva, as well as asexually through budding via gemmules. They are completely reliant on water, which they use for circulation, respiration, and excretion. They have no body symmetry and no germ layers.
Next to evolve were the Cnidarians, which contain the jellyfish. These organisms are also extremely simple, and haven't changed in millions of years. They have evolved germ layers, and are diploblastic, also displaying radial symmetry. They have also evolved a very simple nerve net type nervous system. They also have an incomplete digestive system, consisting of a gastrovascular cavity. These organisms display the alternation of generations life cycle, having both a poly and medusa form. The medusa is a highly motile predator, reproducing sexually, while the polyp is a sessile filter feeder, reproducing asexually through budding. They also have cnidocytes with nematocysts, which they use for hunting their prey. These organisms are also entirely dependant on water, living in both fresh and salt water bodies.
The next phylum to evolve was Platyhelminthes, which are also known as flatworms. These organisms are also acoelomates, displaying bilateral symmetry for the first time, as well as being triploblastic. This new mesoderm housed muscle for movement. They still have an incomplete digestive system with flame cells for excretion. They are the first organisms to start to show anterior cephalization, housing auricles for hearing, and light spots for sensing light and dark. They finally have a primitive brain in their anterior region, along with a nerve ladder type nervous system. Platyhelminthes can either be free-living like the Planarians, or parasitic like tapeworms or liver flukes.