43. _Monostatic Ground-Forms._--Those ground-forms are classed as monostatic or unistable in which the body is in equilibrium only in one position, since the centre of gravity of the body lies in a constant vertical axis below its centre. This fixed position is only rarely and exceptionally found among the SPUMELLARIA (_e.g._, in _Xiphostylus_, _Sphærostylus_, _Lithomespilus_, _Lithapium_) and among the ACANTHARIA (_e.g._, in _Zygostaurus_ and _Amphibelone_). On the contrary it is quite usual among the NASSELLARIA and PHÆODARIA (with but few exceptions); for here a vertical main axis, with a differentiated basal pole, is determined even by the formation of the central capsule, and usually also by the corresponding structure of the skeleton. Among the NASSELLARIA this basal pole, with the porochora of the central capsule, appears always to be the lower; as also in most #Phæogromia# among the PHÆODARIA. In the peculiar bivalved #Phæoconchia#, on the other hand, the basal pole with the cannopyle is directed upwards; as also in the Challengerida and Tuscarorida. The #Phæosphæria# and #Phæocystina# are probably to a large extent polystatic. In general unistable equilibrium may be assumed in the following categories of ground-forms:--(1) Allopolar monaxon (conical and ovoid); (2) pyramidal (regular and amphithect); (3) Centroplana (amphipleura and zygopleura); (4) Anaxonia. {xxi}44. _Principal Axes._--From the foregoing consideration of the statical conditions and their direct causal connection with the geometrical ground-forms of the Radiolaria, the great mechanical significance of the differentiation of definite axes in these unicellular free-swimming organisms will be manifest. The most important of these is the primary main axis (axis principalis, or protaxon), which in all cases has a vertical direction. It is wanting in the Centrostigma (spheres and endospherical polyhedra), and in the Anaxonia (acentra). It is isopolar in the phacotypic forms (Monaxonia isopola), and in the double pyramids (Stauraxonia isopola). It is allopolar in all monastatic ground-forms, in the conotypic forms (Monaxonia allopola), pyramids (Stauraxonia allopola), and the Centroplana (or bilateral forms).