Opening of osculum is regulated partly in cells
a. Myocytes
b. Porocytes
c. Archaeocytes
d. Choanocytes

Hint: Osculum is a characteristic feature of living sponges where it serves as an excretory structure. It is a large opening through which water (carrying sponge’s wastes) passes out after going through a sponge.

Complete answer:
- Myocytes are small muscular cells that form a circular ring around the osculum, thereby guarding the opening and closing of the osculum. Thus, myocytes act as sphincters.
- Porocytes, as the name indicates, are known for making pores of the sponge which is known as ostia. These cells are tubular in shape. These cells help in controlling the amount of water that enters into the spongocoel.
- Archaeocytes are amoeboid cells present in sponges, hence, also called as amoebocytes. These cells are totipotent and can change themselves into any cell type of sponges. Archaeocytes are responsible for the ingestion and digestion of food and further transporting the nutrients to other cells of sponges. Also, they take part in the regeneration of sponges.
- Choanocytes are flagellated cells that possess a collar of protoplasm at the end of flagellum. Thus, they are also called as collar cells. Because of the presence of flagella, these cells are known to maintain the water flow through the body of sponges. Also, due to the presence of collar cells which are sticky in nature, these cells are known to pick even bits of food particles brought inside through water.
- So, by looking at the characteristics of all the 4 types of cells mentioned in the question, the cells in which the opening of the osculum is partly regulated are the myocytes.

Hence, the correct answer is option (A).

Note: Myocytes are modified amoebocytes that are contractile in nature. They are present in the form of rings around the osculum, thereby acting as sphincters to regulate the opening and closing of the osculum.