{cxiv}172. _Adelacantha and Icosacantha._--The numerous forms of ACANTHARIA, here disposed in twelve families and sixty-five genera, may be divided phylogenetically into two main groups of very different extent--_Adelacantha_ and _Icosacantha_. The more primitive group, _Adelacantha_, have an indefinite and variable number of radial spines, which are always quite simple in form and usually irregularly distributed; this main division includes only the one order #Actinelida#, with six genera, among which is _Actinelius_, the common stem-form of all the ACANTHARIA. The more recent group, Icosacantha, includes all the other ACANTHARIA (fifty-nine genera), and is very markedly distinguished from the Adelacantha by the fact that the radial spines are always twenty in number, and arranged according to Müller's law (compare pp. 717-725, and § 110). Since this regular disposition (in five alternating zones each of four spines) has been retained by inheritance in the whole of the Icosacantha, it is probable that this large group has been developed monophyletically from a twig of the Adelacantha; _Actinastrum_ (p. 732) and _Chiastolus_ (p. 738) still present connecting links between the former and the latter, between _Actinelius_ and _Acanthometron_. 173. _Acanthonida and Acanthophracta._--The extensive main division Icosacantha (§ 110), which embraces all ACANTHARIA with twenty radial spines, disposed according to Müller's law, may be subdivided into two large groups or orders:--the #Acanthonida# (p. 740, Pls. 130-132) and the #Acanthophracta# (p. 791, Pls. 133-140). The latter possess a complete extracapsular lattice-shell, which the former have not. The more recent #Acanthophracta# may be derived phylogenetically from the more primitive #Acanthonida# simply by the development of this lattice-shell, with which process are usually (perhaps always) connected certain alterations in the malacoma, _e.g._, degeneration of the myophriscs (§ 96). The most primitive form of all Icosacantha is the genus _Acanthometron_ (p. 324), in which all the twenty acanthin spines are of the simplest constitution and of equal dimensions.