This paper deals with the slugs Alderia modesta (Lovén) and Limapontia depressa Alder & Hancock, both very common in the intertidal zone of the Dutch salt-marshes. Van Benthem Jutting & Engel did not mention them as native species in „Fauna van Nederland” 8, a monograph on Dutch Opisthobranchia, published in 1936. Alderia and Limapontia do not belong to the order of Nudibranchia, but to a separate order, Saccoglossa (Ascoglossa). They possess an uniseriate radula, the worn teeth of which fall in a pouch (saccus, ascus). Their way of life is different, the Nudibranchs feed mainly on hydroids, sea anemones, bryozoa, sponges, etc.; only a few species e.g. Polycera quadrilineata O. F. Müller feed on algae, while all west-european Saccoglossa feed exclusively on algae.