#Phæoconchia#. Bipedal #Spyroidea# and #Stephoidea#.
III. CENTROPLANA. The geometrical centre of the body is a plane (the sagittal plane). Constant transverse axes (perpendicular to the main axis) are wanting in the Monaxonia (which have circular transverse sections); on the contrary they are differentiated in the Stauraxonia (which have polygonal transverse sections). | +-V. BILATERALIA (or ZEUGITA). Bilateral forms in the general sense, with right and left halves. | +-14. _Amphipleura_ | (Bilaterally radial ground-form.) | | Many #Cyrtoidea# and #Spyroidea# multiradiata. | +-15. _Zygopleura._ (Bilaterally symmetrical ground-form.)
Most NASSELLARIA (primitively at least), many Challengerida.
IV. ACENTRA. There is no geometrical centre. | +-VI. ANAXONIA. No definite axes can be determined. | +-16. _Irregularia._ (Absolutely irregular ground-forms.)
_Collodastrum_, _Collosphæra_, Phorticida, Soreumida.
40. _Mechanical Causes of the Geometrical Ground-Forms._--The great variety of ground-forms exhibited by the Radiolaria is of special interest, since in most instances their causes admit of recognition, and since they are so intimately related to each other that even in the remaining cases the assumption that they have arisen by purely mechanical _causæ efficientes_ seems justified. In this respect the first rank is taken by statical conditions, especially the indifferent or stable equilibrium of the whole organism, which floats freely in the water. With regard to these fundamental statical relations, three principal groups of ground-forms may be distinguished, pantostatic, polystatic, and monostatic.
41. _Pantostatic Ground-Forms._--By pantostatic or indifferently stable ground-forms are meant those in which the centre of gravity coincides with the centre of the body, so that they are in equilibrium in any given position. Strictly speaking, the only form which possesses perfectly indifferent equilibrium is the sphere, that being the only truly homaxon and perfectly regular form. Nevertheless, in a somewhat wider sense many Polyaxonia, especially the endospherical polyhedra with very numerous sides, may be { xx}included in this category. Such indifferently stable bodies are found among the SPUMELLARIA in many #Collodaria# and #Sphæroidea#, as well as in the Astrolophida among the ACANTHARIA. On the contrary they are entirely wanting among the NASSELLARIA and PHÆODARIA, since their central capsule constantly presents a main axis with a differentiated basal pole, and determines the position of stable equilibrium.