147. _The Ontogeny of the Acantharia._--The individual development of the ACANTHARIA in the simplest case (_Actinelius_) stops at a point which differs from the _Actissa_-stage only in the change of radial axial threads into acanthin spines. In the small group #Actinelida#, their number remains variable and usually indeterminate (Adelacantha), whilst in the great majority of the legion (#Acanthonida# and #Acanthophracta#) the number is constantly twenty, and those spines are regularly arranged according to the Müllerian law in five parallel circles, each containing four crossed spines (Icosacantha). The simplest form among these latter is _Acanthometron_, which may be regarded both ontogenetically and phylogenetically as the common starting-point of all the Icosacantha. Within this extensive group variations in the length of the dimensive axes appear, similar to those observed in the SPUMELLARIA. In the Astrolonchida and #Sphærophracta# the central capsule remains spherical, extending equally in all directions; and correspondingly the lattice-shell, which is excreted on the surface of the spherical calymma, remains spherical. In the Belonaspida (just as in the #Prunoidea#) {xcvii}this form passes over into an ellipsoid by prolongation of one axis; on the contrary, in the Hexalaspida (as in the #Discoidea#) the discoidal or lenticular form arises by shortening of an axis. Finally, in the Diploconida, and in some Hexalaspida in which the growth is different in all three dimensive axes (as in the #Larcoidea#), both the central capsule and the shell assume the lentelliptical form. The lattice-shell of the #Acanthophracta# is usually successive in its development, since from each of the twenty radial spines two or four tangential apophyses proceed, whose branches subsequently unite and combine to form the lattice-shell. Only in the peculiar Sphærocapsida can the pavement-like shell arise simultaneously or in a moment of lorication.