The cytosol of Paramecium contains many vesicles of various sizes, shapes and contents as well as complex particles such as ribosomes and glycogen granules. The rough endoplasmic reticulum exists as membrane cisternae and tubules which bear ribosomes on their cytosolic surface. Mitochondria and peroxisomes both have tubular membrane inclusions. Large vesicles with an electron transparent center are acidosomes. Lysosomes have a heterogeneous content but always have a heavy polysaccharide lining on their luminal membrane face. Flattened vesicles, called discoidal vesicles, form a pool of membrane from which phagosome membrane is derived. TEM taken on 10/30/68 by R. Allen with Philips 300 operating at 60kV. Neg. 20,500X.