Orders: Testacea, Tunicata.
The shell-bearing forms belonging to this group which were known to Linnaeus were placed by him (in 1748) in the third order of his class Vermes under the name " Testacea," whilst the Echinoderms, Hydroids and Annelids, with the naked Mollusca, formed his second order termed " Zoophyta."
Ten years later he replaced the name " Zoophyta " by " Mollusca," which was thus in the first instance applied, not to the Mollusca at present so termed, but to a group consisting chiefly of other organisms. Gradually, however, the term Mollusca became used to include those Mollusca formerly placed among the " Testacea," as well as the naked Mollusca.
Poli, Testacea utriusque Siciliae, eorumque historic et anatome, tabulis aeneis 49 illustrate, vols.