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.
Choanocytes also constitute the animals filter-feeding apparatus, trapping suspended food particles as they pass along the series of internal channels. Because sponges are essentially sessile, they are heavily reliant upon the effectiveness of their cell-lined channels in trapping food, oxygen uptake and removal of waste products. It is unsurprising that sponges are highly susceptible to pollution and release of suspended sediments which block up their delicate system of tubes and pores, thus preventing basic body functions. Larger sponges have developed more complex body-plans to provide a sufficiently large surface area for food capture. The sponges are diverse in size and shape, ranging from a few millimetres to two metres across.