Page:Scientific results HMS Challenger vol 18 part 1.djvu/39

Principal Groups of Ground-Forms.		Subsidiary Groups of Ground-Forms.		Geometrical Type.		Examples.
II. Centraxonia. The geometrical centre of the body is a straight line (the vertical main axis).   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).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	III. Monaxonia. Uniaxial ground-forms or centraxonia without transverse axes. The transverse planes (perpendicular to the main axis) are circles.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	8. Monaxonia isopola. (Spheroids and ellipsoids; both poles of the main axis similar.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Central capsule and lattice-shell of of many Discoidea (lenses) and Prunoidea (ellipsoids), Belonaspida, &c.
				9. Monaxonia allopola. (Cone, ovoid and hemisphere; the two poles of the axis dissimilar.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Central capsule and lattice-shell of many Nassellaria, especially the Cyrtoidea eradiata (Cyrtocalpida, &c.).
		IV. Stauraxonia. Pyramidal ground-forms or centraxonia with transverse axes. The transverse planes (perpendicular to the main axis) are either regular or amphithect polygons.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	10. Dipyramides regulares. (Quadratic octahedron, or quadrilonchial forms and regular double pyramids.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Acantharia with twenty radial spines, the four equatorial being equal. Multiradial Discoidea and Staurosphærida.
				11. Dipyramides amphithectæ. (Rhombic octahedron, lentellipsoid, and amphithect double pyramids.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Acantharia with twenty radial spines, whose four equatorial spines are unequal but paired. Many Larcoidea.
				12. Pyramides regulares. (Regular pyramids.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	Many Nassellaria (triradial and multiradial). Medusettida and Tuscarorida.
				13. Pyramides amphithectæ. (Rhombic pyramids.)	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	Phæoconchia. Bipedal Spyroidea and Stephoidea.
III. Centroplana. The geometrical centre of the body is a plane (the sagittal plane).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	V. Bilateralia (or Zeugita). Bilateral forms in the general sense, with right and left halves.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right.}$	14. Amphipleura (Bilaterally radial ground-form.)	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Many Cyrtoidea and Spyroidea multiradiata.
				15. Zygopleura. (Bilaterally symmetrical ground-form.)	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	Most Nassellaria (primitively at least), many Challengerida.
IV. Acentra. There is no geometrical centre.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	VI. Anaxonia. No definite axes can be determined.	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	16. Irregularia. (Absolutely irregular ground-forms.)	$\scriptstyle {\left.{\begin{matrix}\ \\\ \end{matrix}}\right\}\,}$	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