The nucleocapsid is sometimes surrounded a membrane called an envelope. Enveloped viruses are usually spherical because the envelope, unlike the capsid, is loose-fitting. Envelopes are lipid bilayers that contain proteins. Some proteins incorporate carbohydrates and are thus called glycoproteins. Glycoprottins usually protrude out of the envelope as spikes called peplomers.
Viruses are smaller than the smallest bacteria and larger than macromolecules.
They can be seen with the electron microscope. An introduction to infectious Diseases, 3rd ed. Medical Microbiology, 2nd ed. 1-3C from Tortora et al. Microbiology, 4th ed.