Age-determination by winter rings

According to the remarks by E. FRÖMMING in the preceding article age determination be means of winter rings is not possible, or — to say the least — highly subjective. He there critisizes the views which I had brought forward in my paper “Groei en ouderdom bij Viviparus contectus (Millet, 1813) en Viviparus viviparus (Linne, 1758)” (VAN DER SPOEL, 1958), and, although being very much obliged for this criticism, I am afraid I cannot agree with him in all points. Firstly, the “Wachstumsabsatze” are indeed identical with my winter rings and with my intermediate rings; both are stagnations in growth. Secondly the cessation of the taking in of food cannot be the only cause of a ring, because Viviparus at times enlarges its shell very rapidly by extending only the outer layer of its shell, building up the lower layers at a later stage. The ring appears only, and this happened also in my breeding experiments, where the outer layer ends, independently of the moment when the ingestion ceased. It is, moreover, very difficult to ascertain just when the feeding stops, as this can take place also through the gills (P. M. COOK, 1949).