Next to the study of shells the interest in the animals of the Mollusca and their interior parts is increasing rapidly. Not only are the beauty of the unexpected colours and the surprising variety of structure a never-ending source of intense enjoyment, but these studies are furnishing reliable characters for the more exact determination of the species. In the seventeenth and eighteenth centuries it was already known that Mollusks possessed a radula as well as a mandibula, but owing to the lack of suitable magnifying instruments it was not possible to observe even the gross structure accurately, let alone to depict their detailed structure.