However, others exist at considerable depths, or even in fresh water. These are the most primitive of the metazoa, with no tissue or organ grade of construction, and little integrated control of individual cellular activities. A number of recent reports have provided evidence of some of the oldest known fossils ever to have been discovered. There have been no recorded extinctions amongst the poriferan phylum, but the greatest threats to their survival must inevitably stem from sedimentation and pollution of waters, coupled to direct destruction of their habitat. The sponges are typified by a body supported by a hard skeleton composed of spicules .
Spicules are needles of hard material, typically Calcium carbonate, or silica and collagen.
The animal cells are embedded in a proteinaceous, jelly matrix. Despite being larger and more elaborate than the Placozoan and Mesozoan fauna, the Porifera are a highly primitive group with no tissue grade of organisation. The stiff body houses numerous channels and pores allowing fresh currents of water to enter the animal. The largest incurrent pores are known as oscula , and the smallest are ostia . Water is encouraged to flow within the sponge by the action of flagellated choanocytes.