21. _The Centroplana or Zygotypic Ground-Forms._--The third principal group of ground-forms includes those which are bilaterally symmetrical in the ordinary sense, or zeugitic or zygotypic; the natural centre of their body is a plane. These forms are the only ones in which the distinction between right and left is possible, since their body is divided by the median plane (planum sagittale) into two symmetrical halves (right and left). In all these zeugites the position of every part is determined by three axes perpendicular to each other, and of these three dimensive axes two are allopolar, one is isopolar. The two unlike poles of the principal (or longitudinal) axis are the oral and aboral, the two unlike poles of the sagittal (or vertical) axis are the dorsal and ventral; the two similar poles of the frontal (or transverse) axis, however, are the right and left. This important group of zeugitic or bilateral forms may also be divided into two clearly distinct lesser groups, the _Amphipleura_ and the _Zygopleura_. In the Amphipleura (or bilaterally radial ground-forms) the "radial two-sided" body is produced by modification of a regular pyramid (as _Spatangus_ from _Echinus_), and hence is composed of several (not less than three) antimeres. In the Zygopleura (or bilaterally symmetrical ground-forms) on the other hand, the bodies consist of two antimeres (as in all the higher animals, Vertebrata, Arthropoda, &c.). (See Gener. Morphol., Bd. i. pp. 495-527.) 22. _The Acentrica or Atypic Ground-Forms._--Among the acentrica or anaxonia are included all those ground-forms which are absolutely irregular, and in which neither a definite centre nor constant axes can be distinguished (_e.g._, most Sponges). These quite irregular ground-forms are very rare among the Radiolaria, but nevertheless there may be referred to them the amoeboid central capsule of some #Colloidea# (_Collodastrum_, p. 27, Pl. 3, figs. 4, 5) among the SPUMELLARIA, the irregular shells of many Collosphærida {xii}(Pl. 8, fig. 2), and the absolutely irregular shells of the Phorticida and Soreumida among the #Larcoidea#. (See Gener. Morphol., Bd. i. p. 400.)