174. _Differentiation of the Acanthonida._--The order #Acanthonida#, which embraces all Icosacantha which have no complete lattice-shell, divides early into three main branches, the three families Astrolonchida, Quadrilonchida, and Amphilonchida (p. 727, Pls. 130-132). The first of these constitutes the common stem-group from which the other two as well as the whole group #Acanthophracta# have been developed; the common stem-form of all is _Acanthometron_ (§ 173). All the Astrolonchida (p. 740, Pl. 130) have twenty radial spines of equal size and similar form. On the other hand, in the Quadrilonchida (p. 766, Pl. 131) the four equatorial spines differ from the others in size and sometimes also in form. In the Amphilonchida (p. 781, Pl. 132) two opposite equatorial spines (lying in the hydrotomical axis) are much larger than the other eighteen and of a different shape. Of the three families of the #Acanthonida# the most important is the primitive group Astrolonchida, for from this the various stem-forms of the #Acanthophracta# arise. They are subdivided according to the formation of the spines into three subfamilies: the Zygacanthida, with simple spines without apophyses (or transverse processes); the Phractacanthida, with two opposite apophyses on each radial {cxv}spine, and the Stauracanthida, with four crossed apophyses on each radial spine. The three genera of the Zygacanthida represent the stem-forms of the three families, since the radial spines in _Acanthometron_ (the most primitive form of #Acanthonida#) are cylindrical, in _Zygacantha_ two-edged, and in _Acanthonia_ four-edged (p. 741). 175. _Capsophracta and Cladophracta._--The extensive order #Acanthophracta#, which embraces all ACANTHARIA with a complete lattice-shell, is polyphyletic, its main subdivisions have been developed independently from different branches of the #Acanthonida#. The whole order may be divided directly into two main groups, the #Capsophracta# and #Cladophracta# (p. 793), which differ in the structure and the origin of their lattice-shell. The group (or suborder) #Capsophracta# includes only the single family Sphærocapsida (p. 795, Pl. 133, figs. 7-11; Pl. 135, figs. 6-10); the lattice-shell arises independently of the twenty radial spines, being made up like a pavement of innumerable small acanthin plates, united by a kind of cement; each plate being perforated by a fine pore. In addition twenty larger main pores (or groups of four pores each) are present, corresponding to the twenty radial spines; these are always equal, quadrangular prismatic, without transverse processes as in _Acanthonia_. In the #Cladophracta#, which include the five remaining families of the #Acanthophracta#, the structure and origin of the lattice-shell are quite different; the lattice-shell is here made up of the branches of the transverse processes, which radiate tangentially from the twenty radial spines and are only united secondarily.