This paper reports on the design and development of SPONGIA, an expert system to help in the identification of marine sponges. In this field, taxonomic difficulty is noticeable and systematists are scarce. SPONGIA is directed towards identification at any single taxonomic level starting from the class level. With this feature, the system is not restricted to identify only the species rank and the identification stops at a given taxonomic rank (order, family...) depending on the available data about the sample. The design of SPONGIA takes advantage of an architecture for the construction of expert systems in taxonomic domains. This architecture is based on the MILORD II language for expert systems construction. In this paper, we mainly focus on the conceptualisation and implementation of the sponges domain model which constitutes the main effort in the expert system development. We also refer to the particular classification task and problem-solving method for identification that we have adopted from those provided by the architecture to implement SPONGIA.