Question

Choanocytes are found in
A. Inner epithelium
B. Mesenchyme
C. Outer epithelium
D. None of the above

Answer 1

Hint: Choanocytes are also referred to as the "collar cells”. Leuconoid, asconoid and Syconoid body type sponges are interiorly lined by these collar cells. The sponges have a central flagellum lined by microvilli collar cells that are joined by a thin membrane.

Complete answer:
Choanocytes (or the collar cells) are one of the major cell types that aid in the process of digestion. Their cell surface comprises a collar of microvilli that help in the entrapment of food particles that are flowing in the organism. These choanocytes form the choanoderm which is the layer of cells present in sponges. Sponges are usually diploblastic, having pinacoderm as an outer layer and choanoderm as an inner layer. Pinacoderm layer is comparable to the outer epidermal layer and is formed of cells known as pinacocytes. A funnel shaped collar and a flagellum, which is a hairlike whip, is present in these collar cells. Choanocytes are the cells lining the interior of the sponge body.
With the help of flagellar movement, collar cells filter the particles from the water into the spongocoel thus moving them outside through the osculum. The digestive as well as the respiratory functions are thus improved. This helps to pull the oxygen as well as the nutrients inside thereby removing the carbon dioxide as well as other waste products from the body. Hence in sponges, the majority of digestion is carried out through choanocytes.

Hence, the correct answer is option (A).

Note: The flagellum present in the center of these cells beats in a continuous manner to pump water actively throughout the boy of the sponges. However, the cylindrical part of the cells helps in nutrient absorption thus contributing to the versatility of these cells.