Phylum Porifera contains the simplest and most primitive of animals that have inhabited the earth. These animals include the Sponges. More specifically, the Red Beard Sponge, Glass Sponge and Giant Vase Sponge who are all examples of what you may find in this phyla. These animals are sessile meaning they cannot move. They have no mouth or digestive cavity, as they use the process of filter feeding to obtain their nutrition. In order to move water in and out of their bodies, Sponges have collar cells with flagella to help pump water in and spicules which give the sponge structural support and aid in defense. These creatures inhabit all marine and fresh waters around the world. Their body plan is asymmetrical, meaning one side is identical to the other. Sponges are diploblastic meaning they have two germ or growth layers. An ectoderm and endoderm. Porifera members are acoelomate as they do not have a coelom tissue. In order to reproduce, sponges have two ways. Asexually or sexually. Asexually involves budding where a part of the sponge breaks off of the parent sponge and settles to the hard sea floor and grows into a new sponge. When conditions are tough, sponges will produce gemmules which are groups of archaeocytes surrounded by a tough layer of spicules. They can withstand freezing and unfavorable temperatures. When conditions become more favorable the gemmules will develop into a new sponge. In most sponges a single sponge forms both the eggs and the sperm by meiosis. The eggs are fertilized inside the sponges body by Internal Fertilization. Sponges can also release their sperm into the water and have it carried to another sponge for fertilization. The Zygote will then turn into a larvae and the larvae will then settle to the seafloor to develop into a fully grown adult sponge. The phylum Porifera is so low on the evolution scale that they don't have any specialized tissues or organs for circulation. They can only rely on diffusion of H20 for excretion and respiration, although they do have an excurrent pore that carries away waste.
In contrast to phylum Porifera, the Cnidarians are a little more advanced. Class Hydrozon contains the Hydras, Scyphozon the Jellyfish, and Anthozoa the corals and sea anemones. Every member except for the corals are motile (making the corals sessile). The cnidarians have a mouth and an anus in which food and waste come and go. They utilize alternation of generations as their reproductive cycle, having a polyp stage and a medusa stage. The most effective and original characteristic that the Cnidarians have are their stinger cells which are located around the mouth and tentacles. This is what ultimately separates them from the previous phyla. Similar to Porifera, this phyla call marine waters and fresh waters home as they rely on the diffusion of H20 to excrete of waste, transport nutrients and respire. In contrast, the Cnidarians start to show signs of evolution as a vey simple and primitive nervous system starts to show. This phyla contains a nerve net that detects certain stimuli like the touch of a foreign object. They also have statocysts which help determine the direction of gravity, The third piece of the "nervous system" this phyla contains are their ocelli which are eyespots that detect light. In order to obtain food and energy, the Cnidarians put their advanced tentacles and stinger cells to work. These features sting an unsuspecting prey with its Cnidocytes which inject poison into the animal. This paralyzes and/or kills its prey which is now helpless. The Cnidarians can now take the prey in its mouth and remove any of the non-digestible pieces. The food that is digested enters the gastrovascular cavity which breaks down and absorbs the nutrients from the food. The body plan of these creatures consists of radial symmetry which differs from the asymmetrical Poriferans. But both phylas have a sac-plan and a 2 germ layer. In order to reproduce their offspring to the next generation, the cnidarians can use asexual budding or produce motile sexual gametes into the water just like the previous phyla. But somewhere along the line, the way a cnidarian reproduces changes. They can utilize the alternation of generations phase including a medusa and polyp. Fertilization will occur in the open water producing many diploid zygotes, the zygotes will then develop into a larvae which then settles to the hard seafloor surface and turn into a polyp. The polyp will bud to form a young medusa. Over time that medusas will mature into an adult medusa which will reproduce sexually by releasing gametes into the water to start the cycle again.