The Sponges described hereafter were all preserved in spirit. Those which were treated with chromic acid or some similar reagent were in a state that did not allow any investigation whatever. On the other hand some of the Sponges preserved in spirit were also in a bad condition, so that I could only make out something about the skeleton or the spicules. Some on the contrary were very well preserved so that even a few histological details could be observed. This is one of the reasons why I have treated the Sponges in the unequal manner that the reader will notice. Another reason is that I began my task when I was lecturer in the Hague and had much more time at my disposal than is now the case. I feel obliged to apologise. In the middle of 1882 I had the honour to be called as assistant at Prof. Dohrn’s Station in Naples and since then I could, of course, not devote very much time to my Polar Sponges. Still I had accepted the task and was thus obliged to finish it. I am convinced that many points more are to be found out with the material I had, but I thought it my duty not to spend more time. I hope the Commitee for publishing the results of the Barents-Expedition will excuse me for having made them wait so long a time and giving them so little.