The choanoderm is composed of flagellated collar cells, or choanocyte

Choanocyte

Choanocytes are cells that line the interior of Asconoid, syconoid and leuconoid body type sponges that contain a central flagellum surrounded by a collar of microvilli which are connected by a thin membrane. It is the closest family member to the free-living ancestor called choanoflagellate...

s. The sponge body is mostly a connective tissue

Connective tissue

"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

, the mesohyl

Mesohyl

The mesohyl, formerly known as mesenchyme, is the gelatinous matrix within a sponge. It fills the space between the external pinacoderm and the internal choanoderm...

, over which are applied epithelioid

Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 monolayer

Monolayer

- Chemistry :A Langmuir monolayer or insoluble monolayer is a one-molecule thick layer of an insoluble organic material spread onto an aqueous subphase. Traditional compounds used to prepare Langmuir monolayers are amphiphilic materials that possess a hydrophilic headgroup and a hydrophobic tail...

s of cells

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

, the outer pinacoderm and the inner choanoderm. Most aspects of sponge biology, including feeding, reproduction, and gas exchange, depend on a low pressure flow of water generated by the flagella of the choanoderm. Three grades of organization, asconoid

Asconoid

Asconoid is one of three possible body plans for sponges, which form the phylum Porifera in kingdom Animalia and subkingdom Parazoa which means that they do not have a definite body shape. The asconoid body plan is characterized by a stalk-like spongocoel surrounded by a single layer of choanocytes...

, syconoid, and leuconoid, reflect the degree of elaboration of the choanoderm layer and mesohyl. In the asconoid plan the interior water space, or atrium

Atrium (architecture)

In modern architecture, an atrium is a large open space, often several stories high and having a glazed roof and/or large windows, often situated within a larger multistory building and often located immediately beyond the main entrance doors...

, is large and unpartitioned. In the syconoid plan the periphery of the atrium is divided into numerous small flagellated chambers with increased surface area for choanocytes. In leuconoid sponges the atrium is replaced by a proliferation of mesohyl and a complex network of water channels and flagellated chambers.