The protein shell that encloses the nucleic acid genome of phages along with genomes of viruses in general.
The capsid is sometimes called a capsule instead. In tailed phages – order Caudovirales the capsid is combined with a tail.
Tails are are responsible for adsorption as well as translocation of the phage nucleic acid into adsorbed bacteria. Note, though, that the tail is an appendage to the capsid rather than strictly a constituent of the capsid.
The more complex a virion, the more types of proteins that make up its capsid. Regardless of the number of types of capsid proteins, a.k.a., capsomeres, the capsid itself consists of multiply repeating multi-protein motifs.
The capsids of most phages takes on an icosahedral shape which is either symmetrical or elongated. The prominent exception are the filamentous phages which possess helically shaped capsids rather than icosahedral.
From the glossary of Hershey (1971), p. 773, "The proteinaceous 'shell' or 'coat' of a phage particle, normally filled with nucleic acid to form the 'head.'"
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