Two new genera and species of Dernospongiae are described from a northwestern Mediterranean littoral cave characterized by cold homotherrny, which shelters deep-sea invertebrates. The two new sponges have neither mineral nor fibrous skeleton. Their cytology is described using transmission electron microscopy. Thymosiopsis cuticulatus n.g. n.sp. (Chondrillidae) shares some characters with Thymosia Topsent, but lacks the diagnostic spongin fibres. Myceliospongia araneosa n.g. n.sp. has unusual anatomy, cytology,
and mode of growth. No clear relationship with any order of the
Demospongiae is indicated and the sponge is classified as incertae sedis within the Demospongiae.