PORIFERA (pore bearing)
Porifera (sponges) are simple, multicellular animals in which there is little specialization of tissue. They get their name from the fact that their unicellular food is not taken into a single mouth. It is filtered out of water that passes through many pores, connected by canals, in their bodies. The food is digested by cells that line the pores and canals. The undigested food and water are expelled through the osculum, a large opening at the top of the animal.
Porifera have a body that is composed of two layers. The outer and inner layers digest food much like the protozoa do. The middle layer is a non-cellular jelly called mesenchyme. This middle layer may secrete hard mineralized structures called spicules that support the body. In some cases, spicules are composed of opal (a form of silica) or calcium carbonate, which can be preserved. Bath sponges lack spicules, and a sulphur-rich protein called sponging is used to produce a flexible skeleton.
Sponges began evolving and are rare in latest Precambrian rocks (pre-543 million years ago). They have existed from that time into the present in marine settings. Some sponges live in fresh water.