They also have a tubular body with a single osculum, but their body wall is thicker and the pores that penetrate it are longer, forming a system of simple canals. These canals are lined by collar cells, the flagellae of which move water from the outside, into the spongocoel and out the osculum. The third category of body organization is leuconoid . These are the largest and most complex sponges. These sponges are made up of masses of tissue penetrated by numerous canals.
Canals lead to numerous small chambers lined with flagellated cells.
Water moves through the canals, into these chambers, and out via a central canal and osculum. Sponges are found in virtually all aquatic habitats, although they are most common and diverse in the marine environment. Many species contain toxic substances, probably to discourage predators. Certain other marine animals take advantage of this characteristic of sponges by placing adult sponges on their bodies, where the sponges attach and grow. The chemicals also probably play a role in competition among sponges and other organisms, as they are released by sponges to insure themselves space in the marine ecosystem.