The authors who have studied the Rhizocephala have a quite different opinion as to the limits of the species, especially those of the genus Sacculina. On the one side KOSSMANN (1874) and GIARD (1887, 1888) are convinced that every specimen of Sacculina found on a certain crab has to be separated specifically from those living on another host. On the other hand SMITH (1906) unites all the described species of Sacculina into one widely distributed species which has to keep the name Sacculina carcini. A better way in separating the forms of Sacculina into different species is without doubt the one avoiding both the extremes of the above-cited authors, as explained by GUÉRINGANIVET (1911). Only in the cases where morphological structures of specific value are found the forms are to be regarded as different species. A more detailed account of the questions dealing with the difficulties of the species-problem in Sacculina will be published in the monograph on the Rhizocephala of the Siboga Expedition, now in preparation by Professor VAN KAMPEN and the present author. Here only may be pointed to some particularities found in the material dealt with in this paper. The material collected by Dr. VAN DER HORST at Curaçao contains four specimens of Rhizocephalid parasites. Three of these belong to the genus Sacculina, the fourth is a representative of the genus Lernaeodiscus. All of these differ from the hitherto known species of the two genera and are therefore described as new species in this paper. Though the material is comparatively small it is of great interest since the three species of Sacculina have a quite different structure. In this paper the structure of the external cuticle is used as a base in the diagnosis of the species. Many species of Sacculina, especially those from tropical regions, possess appendages on this chitinous cuticle, which have in a certain specimen a well-defined form. The size of the appendages may be different in the various regions of the mantle (e. g. those in the neighbourhood of the stalk may be of smaller size than those next to the mantle-opening), but within certain limits the form of these appendages is constant. Already KOSSMANN (1874) made use of the appendages of the external cuticle in the diagnosis of his species, but most authors have not employed this method. Some years ago VAN KAMPEN (1919), however, pointed out that the principles of KOSSMANN’s method could give good results in the taxonomy of this group.