Die Ektoplasmahülle von Heterostegina depressa (Foraminifera:Nummulitidae)

The structure of the rigid ectoplasm sheath was studied in living individuals of the large foraminifer Heterostegina depressa d'Orbigny 1826 (Nummulitidae). H. depressa is able to form pseudopods, which emerge from the marginal cord within a few minutes. Normally, however, the living test is entirely covered by a rigid hyalin elastic sheath, which is fixed to the substrate by terminally branched processes. In older individuals, the sheath is often incomplete and restricted to the marginal cord only. Probably, the sheath is secreted by the ectoplasm which fills the canal system of the chamber walls and the marginal cord, or it consists of transformed ectoplasm; we call it “Ektoplasmahülle”, and the processes radiating to all sides, “Ruhepseudopodien”. H. depressa is able to move while inside the elastic sheath by means of pseudopods which penetrate the sheath and attach themselves to the substrate out-side. In the same manner, H. depressa leaves a complete sheath by disrupting it through the tractive powers of the pseudopods. The procedure of breaking the sheath is described. Locomotion when leaving the sheath is performed in a zigzag manner (alternating action of several pseudopods). The sheath counteracts transportation by water movement and protects chamber formation (which takes place inside the sheath). Due to its symbiotic unicellular algae, H. depressa does not need to collect food particles; hence locomotion seems unneccssary. The formation of a rigid ectoplasm sheath may be connected with H. depressa's sessile mode of life.