Small invaginations of the plasma membrane, characterized by glypiated proteins among others, that provide binding sites and points of entry of small molecules into the cytoplasm via trancytosis or potocytosis. Trancytosis is the pinching off of the ligand-bearing vesicle into the cytoplasm. Potocytosis is the putative closure of the narrow neck of the invagination and consequent isolation of the vesicular space from the extracellular fluid; the high local concentration of the ligand allows diffusion of trapped molecules into the cytoplasm.