A group of sponges that first appeared in the Cambrian. They are of normal sponge shape and have skeletal spicules consisting of opaline silica. Large spicules (megascleres) and small spicules (microscleres) are connected in various patterns, and this forms the basis for the classification of the group. They were common in the Palaeozoic but many forms disappeared in the Permian. They were again important in the Jurassic and Cretaceous, but are now confined to much deeper marine waters.
Subjects: Earth Sciences and Geography.