Help me describe the characteristics of members in Phylum Porifera, their method of reproduction etc., and explain why they are sessile.
Members of Porifera contain two layers of cells with a jelly-like Mesophyl between the layers. They also contain spicules which give sponges a framework and support. Sponges lack digestive, circulatory and nervous systems. There are several ways a Poriferan can reproduce. Sometimes, when conditions are right, a bud forms on the side of the sponge. This bud can break off and grow independently into another sponge. This type of reproduction is asexual, involving one parent. When food supply becomes scarce or conditions become difficult for their survival, sponges form gemmules. This cluster of cells can survive until the time is right and conditions change. The gemmule will then grow into another sponge. Sponges also reproduce sexually, releasing sperm and eggs into the water. They unite to form larvae which will eventually develop into adult sponges. Sometimes, the sperm enters another sponge and fertilizes the egg internally. The larvae are released into the water when mature. Sponges are generally filter feeders and their diet consists of bacteria and other small organisms in the water. They maintain a constant flow of water which brings a supply of oxygen and food particles into their body. Adult sponges are sessile which means that they don't swim about and live attached to rocks, shells, etc.