155. _Systematic Categories._--The categories or different orders of divisions have in the Radiolaria, as in all other organisms, no _absolute_ significance, but only a _relative_ value. In itself it is quite unimportant whether the whole group be regarded, as at first, as a _family_ (Ehrenberg, 1847), or as an _order_ (J. Müller, 1858), or as a _class_ (Haeckel, {cii}1881). These different views are regulated, on the one hand, by the known extent of the group and by the amount of our acquaintance with it, and on the other, by comparison with related groups and by reference to their conventional disposition. When, therefore, the whole class, Radiolaria, is here divided into two subclasses, four legions, eight orders, eighty-five families, &c., these artificial categories are drawn up only in the conviction that by this means the easiest survey and most thorough insight into the system as a whole may be attained; this latter will indeed approach as far as possible the ideal of a natural system, but must on numerous practical grounds always remain more or less artificial. Since it is to be expected that with the progress of our systematic knowledge the rank of the various categories will rise, it is possible that in the future the arrangement of the group may be somewhat as follows:--_Phylum_, RADIOLARIA; _Four Classes_, SPUMELLARIA, NASSELLARIA, ACANTHARIA, PHÆODARIA; _Eight Legions_ (Nos. I.-VIII. in the following Table); _Twenty Orders_ (Nos. 1-20 in the Table), &c. Four Legions. Eight Sublegions. Twenty Orders. Typical Families. { {1. Colloidea, { 1a. Thalassicollida. {I. COLLODARIA { { 1b. Collozoida. { (Spumellaria { { palliata) {2. Beloidea, { 2a. Thalassosphærida. { { { 2b. Sphærozoida. I. Legion { (or Subclass){ { { 3a. Ethmosphærida. SPUMELLARIA { {3. Sphæroidea, { 3b. Collosphærida. (PERIPYLEA) { { { {4. Prunoidea, { 4a. Ellipsida. [Porulosa {II. SPHÆRELLARIA { { 4b. Zygartida. peripylea.] { (Spumellaria { { loricata) {5. Discoidea, { 5a. Phacodiscida. { { { 5b. Porodiscida. { { { {6. Larcoidea, { 6a. Larnacida. { { { 6b. Pylonida.