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



latticed shell." In the majority of them I observed that the skeleton did not 

 consist of silex, but of a very peculiar organic substance, which I called " acanthin." 

 At that time I divided the family Acanthometrida into four subfamilies : (l) Acantho- 

 staurida, (2) Astrolithida, (3) Litholophida, (4) Acanthochiasmida. The two former 

 now represent the suborder Acanthonida, the two latter the suborder A c t i n e- 

 1 i d a.. The number of genera which I distinguished in my Monograph amounted to nine, 

 the number of species to fifty. By the rich collections of the Challenger this number is 

 so much increased that we can here describe twenty-seven genera and one hundred and 

 sixty species. 



Richard Hertwig in his work on the Organismus der Radiolarien (1879, pp. 625) 

 adopted my family Acanthometrida, and gave a very accurate description of its 

 anatomical structure. He confirmed my observations that the radial spines of this 

 family are never hollow, but solid, and that their chemical substance is not silex, but the 

 organic matter " acanthin." He found that the simple nucleus of the Acanthometrida 

 is commonly very early cleft, and that the peculiar brushes of filaments on the 

 calymma, described by Johannes Muller and by me as " Gallert-cilien," are peculiar 

 " contractile filaments," comparable to the " muscle-fibrillaB " of some Infusoria, or the 

 " My ophan -filaments " (Myophrisca). 



The order Acanthometra is here divided into two different suborders of very 

 unequal extent and value, the Actinelida and Acanthonida. The first may be 

 regarded as the common ancestral stock, not only of the second, but of all ACANTHABIA. 

 In the small group of Actinelida the number of radial spines is variable and commonly 

 indefinite, often very large (more than a hundred) ; they are therefore Adelacantha. 

 The second suborder, the Acanthonida, comprise by far the greatest part of the order, 

 and possess constantly twenty radial spines, regularly disposed after the Miillerian law ; 

 they are therefore (like all Acanthophracta) Icosacantha (compare above, p. 717). 



The Actinelida possess constantly simple radial spines, without any apophyses ; 

 their form is commonly very simple and primitive. This suborder comprises three small 

 but very different families, the Astrolophida, Litholophida, and Chiastolida. The first 

 family, the Astrolophida, is the original ancestral group. A large and variable, 

 commonly indefinite number of radial spines is here united in the centre of the spherical 

 central capsule and radiating within a spherical space. In the second family, the 

 Litholophida, a small and variable number of radial spines (between ten and twenty) is 

 united in the apex of a conical central capsule and radiating within the quadrant or 

 octant of a spherical space. In the third family, the Chiastolida, a variable number 

 of radial spines is grown together by pairs, in such a manner that every two opposite 

 spines (placed originally in one axis of the spherical central capsule) forms a single 

 " diametral spine " ; all these diametral spines are not united in the centre of the 

 central capsule but only crossed loosely near the centre. 



