The calcareous sponge Sycon raphanus (Schmidt, 1862).

(A) The species S. raphanus had been grouped by Haeckel [28] to the taxon Sycandra. Here a scheme of the morphology and the skeletal structure of Sycandra hystrix is given [28]. (B) S. raphanus specimens, growing on the mussel (m) Mytilus galloprovincialis. The oscule of the specimens is surrounded by a pronounced corona (co), formed of spicules. On the basis of the specimens stolons/buds (st) are seen. They develop after release from the parent sponge asexually to a descendent. (C and D) Cross section through S. raphanus specimens, displaying the external and internal surface layer. In the center the atrium (a) is shown into which the water canals flow in. Radial aquiferous canals traverse the body that originate at the surface of the animal, via the inhalant openings (io), and end at the internal surface via exhalant pores (eo). Between the canals the mesenchyme (m) compartments is radially arranged. The slices were stained with ASTRIN. (E) Non-stained section through the outer part of the sponge showing the location of the two major types of spicules; (i) the diactines spicules (ds), protruding from the distal cones of the outer surface of the specimens, and (ii) the triactines (ts) that are localized within the mesohyl. The mesohyl compartment is filled with eggs/embryos (e).