Page:Scientific results HMS Challenger vol 18 part 1.djvu/933

Rh exoplasm on the surface of the calymma. These and other differentiations seem to indicate that the pseudopodia in the are more highly developed than in the, and justify the denomination of the former as "Actipylea."

Acanthometra, J. Müller, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin. Acanthometrida, Haeckel, 1862, Monogr. d. Radiol., p. 371. Acanthometrea, R. Hertwig, 1879, Organismus d. Radiol., p. 133. Acanthonida et Litholophida, Haeckel, 1881, Prodromus, pp. 465, 469.

Definition.— without complete latticed shell.

The order, the third order of Radiolaria, comprises all those 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 differ principally from the nearly allied, the second order of , which constantly possess such a complete shell.

Johannes Müller, who first detected and described the (in 1855, loc. 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 "Acanthometræ cataphractæ," appertaining to the following order, the.

In my Monograph (1862, p. 371) all true 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