Definition and classification probably rank second only to marriage as a source of controversy, but it is not in that spirit in which I want to write; I only want to show that there may be more than one viewpoint about Mollusca. Problems of definition and classification are different. Workers may agree on the defintions of phyla and classes, yet disagree on their arrangement. Definition and classification each have two separate and distinct phases: the work itself on developing a concept; and the general acceptance of this concept. New ideas by their very nature go against prevailing opinion; it is not a new idea itself which is the critical feature as is its acceptance. Acceptance of definition and/or classification tends to be a passive process and most teachers are content to quote uncritically that which is in the latest textbook.