There are two types of collar in the choanocytes of adult Tetilla serica: one type is a continuous cytoplasmic tube and the other consists of discontinuous microvilli. The former is found in the small flagellated chamber and is considered to belong to a young choanocyte in the process of differertiation. To confirm this idea, very young choanocytes which are about to differentiate the collar were examined during embryogenesis.
The youngest choanocytes are noticed forming aggregations of small cells in 3-day larvae. Around the flagellum in each choanocyte, there is a depression which will become wider. At first, the collar is observed as a ring of cytoplasm; next this extends outward and becomes thinner, and finally it divides into microvilli. The microvillous collar is formed by the opening of vesicles and fusion of their membranes. These vesicles are considered to be derived from the Golgi complex. The process of collar formation through fusion of vesicles is discussed.