The importance of research on the ecological distribution for the taxanomy of calcareous spongcs. The particular difficulties of the taxonomy of calcarcous sponges are due to their great plasticity and their capacity for adaptation which obscure the natural limits
of specics. An attempt was made to define the natural populations of the Mediterranean species of the genera Clathrina and Ascandra through a study of their ecological distributions.
The populations not connccted by the transirory forms were defined by their ecological preferences, and, as they are found in a close cohabitation, were treated as havîng the specific values. The variabilîty of different morphological features is discussed, the features significant for taxonomy are pointed out and the morphologically close specîes arc defined.