Choanocytáik

Choanocytáik, commonly known as collar cells, are specialized flagellated cells found in sponges and some other lower metazoans. These cells are primarily responsible for feeding and respiration within the organism. Each choanocyte is characterized by a single flagellum that beats rhythmically, creating a water current. Surrounding the base of the flagellum is a collar of microvilli, which are short, finger-like projections.

The beating of the flagellum draws water into the sponge through small pores. As water passes through the collar of microvilli, microscopic food particles, such as bacteria and plankton, are trapped by the sticky mucus secreted by the collar cells. The food particles are then phagocytosed by the choanocytes and digested. Undigested waste is expelled back into the water current.

In addition to their role in feeding, choanocytes also play a crucial part in gas exchange. The constant water movement facilitated by their flagella ensures a continuous supply of oxygen to the sponge's tissues and the removal of carbon dioxide. The collective beating of thousands of choanocytes throughout the sponge's body is what drives water flow and sustains the organism's life processes. Choanocytes are considered to be morphologically and functionally similar to choanoflagellates, a group of single-celled eukaryotes, which has led to theories about the evolutionary origin of multicellular animals from colonial choanoflagellates.