CX1V THE VOYAGE OF H.M.S. CHALLENGER. 



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 Mliller'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, be- 

 tween Actinelius and Acanthometron. 



173. Acanthonida and Acanthophracta. The extensive main division Icosacantha 

 ( 110), which embraces all ACANTHARIA with twenty radial spines, disposed according to 

 Mliller's law, may be subdivided into two large groups or orders : the Acanthonida 

 (p. 740, Pis. 130-132) and the Acan thophrac ta (p. 791, Pis. 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. 



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, Pis. 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 Acanthoinetron ( 173). All the Astrolonchida (p. 740, 

 PI. 130) have twenty radial spines of equal size and similar form. On the other hand, in 

 the Quadrilonchida (p. 766, PI. 131) the four equatorial spines differ from the others in 

 size and sometimes also in form. In the Amphilonchida (p. 781, PI. 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 im- 

 portant 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 



