Anatomy and morphology

Sponges have several cell types:

* 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 microvilli and are used to filter particles out of the water. The beating of the choanocytes’ flagella creates the sponge’s water current.

* Porocytes are tubular cells that make up the pores into the sponge body through the mesohyl.

* Pinacocytes which form the pinacoderm, the outer epidermal layer of cells. This is the closest approach to true tissue in sponges

* Myocytes are modified pinacocytes which control the size of the osculum and pore openings and thus the water flow.

* 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.

* Sclerocytes secrete calcareous siliceous spicules which reside in the mesohyl.

* Spongocytes secrete spongin, collagen-like fibers which make up the mesohyl.

* Collencytes secrete collagen.

* Spicules are stiffened rods or spikes made of calcium carbonate or silica which are used for structure and defense.

* Cells are arranged in a gelatinous non-cellular matrix called mesohyl

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

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.

Syconoid sponges are similar to asconoids. They have a tubular body with a single osculum, but the body wall is thicker and more complex than that of asconoids and contains choanocyte-lined radial canals that empty into the spongocoel. Water enters through a large number of dermal ostia into incurrent canals and then filters through tiny openings called prosopyles into the radial canals. Their food is ingested by the choanocytes. Syconoids do not usually form highly branched colonies as asconoids do. During their development, syconoid sponges pass through an asconoid stage.

Leuconoid sponges lack a sperm and instead have flagellated chambers, containing choanocytes, which are led to and out of via canals.

It should be noted that these 3 body grades are useful only in describing morphology, and not in classifying sponge species, althought the asconoid and syconoid construction is present in Calcarea only