Calcarea

Calcarea, or Calcispongiae, are a group of sponges including those in which the skeletal structures are formed of carbonate of lime. There are two main divisions, the "Homocoela" and the "Heterocoela." In the former there is a large central digestive or gastric cavity, the whole of which is lined with the "collared cells" which are so characteristic of the sponges (q.v.); while in the latter these cells occur only as the lining of certain special cavities or "ampullae." The Ascones are the most typical sponges of the former class, while Homoderma and its allies form a transition to the Heterocoela, as, in addition to the central gastric cavity, there are series of radial tubes. Among the Heterocoela the Sycones and Leucones are the most typical groups. They also include the Teichoneae, in which the sponges are flat and leaf-like, and the small pores all open on the one side and the larger oscula all open on the other. The spicules of the calcareous sponges are very rarely found fossil.