Sponges (Porifera) are a group of animals that includes about 10,000 living species. Members of this group include glass sponges, demisponges, and calcareous sponges. Adult sponges are sessile animals that live attached to hard rocky surfaces, shells, or submerged objects. The larvae are cilliated, free-swimming creatures. Most sponges inhabit marine environments but a few species live in freshwater habitats. Sponges are primitive multicelluar animals that have no digestive system, no circulatory system, and no nervous system. They do not have organs and their cells are not organized into well-defined tissues.
There are three subgroups of sponges. The glass sponges (Class Hexactinellida) have a skeleton that consists of fragile, glass-like spicules that are made of silica. The demosponges (Class Demospongiae) are often vibrantly colored and can grow to be the largest of all sponges. The demosponges account for more than 90 percent of all living sponge species. The calcarious sponges (Class Calcarea) are the only group of sponges to have spicules that are made of calcium carbonate. Calcarious sponges are often smaller than other sponges.
The body of a sponge is like a sac that is perforated with lots of small openings or pores. The body wall consists of three layers:
- an outer layer of flat epidermal cells
- a middle layer that consists of gelatinous substance and amoeboid cells that migrate within the layer
- an inner layer that consists of flagillated cells and collar cells (also called choanocytes)
Sponges are filter feeders. They draw water in through the pores located throughout their body wall into a central cavity. The central cavity is lined with collar cells which have a ring of tentacles that surround a flagellum. Movement of the flagellum creates current that keeps water flowing through the central cavity and out of a hole at the top of the sponge called the osculum. As water passes over the collar cells, food is captured by the collar cell's ring of tentacles. Once absorbed, food is digested in food vacuoles or transferred to the amoeboid cells in the middle layer of the body wall for digestion.
The water current also delivers a constant supply of oxygen to the sponge and removes nitrogenous waste products. Water exits the sponge throuhg the large opening at the top of the body called the osculum.
Sponges are divided into the following basic groups:
- Calcarious sponges (Calcarea)
- Demosponges (Demospongiae)
- Glass sponges (Hexactinellida)