In addition it also contains viral proteins forming spikes or peplomers. It functions as the viral antireceptor or attachment protein. An envelope is a common feature in animal viruses but uncommon in plant viruses. In some other viruses the envelope is derived from the nucleus membrane or the golgi body membrane. A detailed examination of spherical viruses shows that they often have icosahedral symmetry or a symmetry based on the icosahedron.
Icosahedral viruses are very common plant and animal viruses.
Some icosahedral viruses e.g. those of the picornaviridae , are not enveloped and do not have a matrix layer. The capsid is the outer layer of the virus. An icosahedron has 20 equilateral triangles arranged around the face of a sphere. Viruses having 20 subunits are not known to exist. Most viruses generally fit 60 x N subunits into their capsids.