Page:Scientific results HMS Challenger vol 18 part 1.djvu/134

cxiv which are always quite simple in form and usually irregularly distributed; this main division includes only the one order, with six genera, among which is Actinelius, the common stem-form of all the. The more recent group, Icosacantha, includes all the other (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 with twenty radial spines, disposed according to Müller's law, may be subdivided into two large groups or orders:—the  (p. 740, Pls. 130-132) and the  (p. 791, Pls. 133-140). The latter possess a complete extracapsular lattice-shell, which the former have not. The more recent may be derived phylogenetically from the more primitive  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.

174. Differentiation of the Acanthonida.—The order, 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 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 the most important is the primitive group Astrolonchida, for from this the various stem-forms of the  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