Coated pits and coated vesicles in unicellulars (Tetrahymena, Crithidia) and in Hydra

Coated pits and coated vesicles were observed in the ciliated unicellular Tetrahymena, in the flagellated unicellular Crithidia, and in Hydra. In the unicellulars the coated structures localized for the most part near to the origin of cilia or flagella, and many were present around the Golgi complex, whereas in Hydra they occurred in nonspecific random locations. Since the membrane receptors of the unicellulars serve originally as food receptors, the coated pits and vesicles are presumably involved in food selection, and represent as such the primitive form of the receptor-mediated ligand internalization mechanism operative in higher organisms.