''

The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera ( ). They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. However, recent genomic studies suggest they are not the most ancient lineage of animals, but may instead be secondarily simplified.

There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.

Anatomy and morphology

• Archaeocytes (or amoebocytes) have many functions; they are totipotent cells which can transform into sclerocytes, spongocytes, or collencytes. They also have a role in nutrient transport and sexual reproduction.
• Cells are arranged in a gelatinous non-cellular matrix called mesohyll

Sponges have three body types: asconoid, syconoid, and leuconoid.

• Choanocytes (also known as "collar cells") function as the sponge's digestive system, and are remarkably similar to the protistan choanoflagellates. The collars are composed of many microvilli and are used to filter particles out of the water. The beating of the choanocytes' flagella creates the sponge's water current.
• Collencytes secrete collagen.
• Myocytes are modified pinacocytes which control the size of the osculum and pore openings and thus the water flow.
• Pinacocytes which form the pinacoderm, the outer epidermal layer of cells. This is the closest approach to true tissue in sponges
• Porocytes are tubular cells that make up the pores into the sponge body through the mesohyl.
• Sclerocytes secrete calcareous siliceous spicules which reside in the mesohyl.
• Spongocytes secrete spongin, collagen-like fibers which make up the mesohyl.
• Spicules are stiffened rods or spikes made of calcium carbonate or silica which are used for structure and defense.

Asconoid sponges are tubular with a central shaft called the spongocoel. The beating of flagella forces water into the spongocoel through pores in the body wall. Choanocytes line the spongocoel and filter nutrients out of the water.