REPORT ON THE RADIOLARIA. 



725 



exoplasm on the surface of the calymma. These and other differentiations seem to 

 indicate that the pseudopodia in the ACANTHARIA are more highly developed than in 

 the SPUMELLARIA, and justify the denomination of the former as " Actipylea." 



Synopsis of the Orders and Suborders of ACANTHARIA. 



I. ACANTHOMETRA. 1 Radial spines in variable and indefinite number, 

 Skeleton composed only of disposed irregularly, .... 

 acanthinic radial spines not 1- 

 forming a complete lattice- Radial spines constantly twenty, disposed regularly 



1. Actinelida. 



shell 



II. ACANTHOPHRACTA. 



Skeleton composed of twenty 

 aeanthinic radial spines (dis- 

 posed after the Mullerian law) 

 and of a spherical or variously 

 shaped complete lattice-shell. 



J after the Miillerian law of Icosacantha, 



2. Acanthonida. 



Radial spines all twenty of equal size ; shell and 



central capsule spherical, . . . 3. Sphsarophracta. 



Radial spines of different sizes ; shell and central 



capsule ellipsoidal, discoidal, or heteromorphous, 4. Prunophracta. 



Order III. ACANTHOMETKA, Johannes Muller, 1855. 



Acanthometra, J. Muller, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin. 

 Acanthometrida, Haeckel, 1862, Monogr. d. Radiol., p. 371. 

 Acanthometrea, R. Hertwig, 1879, Organismus d. Eadiol., p. 133. 

 Acanthonida et Litholophida, Haeckel, 1881, Prodromus, pp. 465, 469. 



Definition. ACANTHARIA without complete latticed shell. 



The order Acanthometra, the third order of Eadiolaria, comprises all those 

 ACANTHARIA in which the acanthinic skeleton is only composed of radial spines arising 

 from one common central point, but never forms a complete latticed shell. By the 

 absence of such a latticed or fenestrated shell the Acanthometra differ principally 

 from the nearly allied Acanthophracta, the second order of ACANTHARIA, which 

 constantly possess such a complete shell. 



Johannes Muller, who first detected and described the Acanthometra (in 

 1855, loe. cit.~), defined them as follows: " Radiolaria without shell, with siliceous 

 radial spines" (1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 46). He described and 

 figured eighteen species of them, disposed in four genera (Acanthometra with fifteen 

 species, and Zygacantha, Lithophyllium, Lithoptera, each with a single species). 

 Among those eighteen species, however, were two " Acanthometrse cataphractse," apper- 

 taining to the following order, the Acanthophracta. 



In my Monograph (1862, p. 371) all true Acanthometra were united into a 

 single family, Acanthometrida, with the following definition : " Skeleton composed of a 

 number of radial spines, piercing the central capsule and united in its centre, without 



