Diazona is represented in European waters only by Diazona violacea Savigny. It is a compound ascidian forming massive colonies of spectacular size and appearance. In many ways it is the most interesting of all ascidians, for in its adult structure it straddles two commonly accepted orders and in itself is a strong argument against such a division; it is the only oviparous and small egged compound ascidian, two features undoubtedly primitive; and its manner of budding is the simplest and probably is the basic type for the group as a whole. Only fragmentary descriptions of the morphology and reproduction exist, and a more or less complete account of the various stages of the life cycle may be of some value. The family Diazonidae includes, in addition to Diazona itself, the genera Tylobranchion of subantarctic regions, and Rhopalea of Mediterranean and northern waters. In its entirety Diazona appears to link with such divergent forms as Ciona on the one hand and Archidistoma on the other. The fact that Diazona is obtained by dredging in relatively swift offshore waters and lives poorly in an aquarium probably accounts for the existing unsatisfactory state of knowledge of most of its phases. Most of what is known concerns asexual reproduction; and attention has been given, at various times, primarily to the process of regeneration, rather than bud formation, for example by Della Valle (1884), Caullery (1914), Oka (1906) under the name Aphanobranchion, and by Salfi (1926).

The material of the present account was collected at various times in the Plymouth area from the Mewstone and Eddystone grounds.