Page:Scientific results HMS Challenger vol 18 part 1.djvu/92

72 lattice-shell, whilst the others (Aphracta) are without it. The, then, or Radiolaria without a complete skeleton, are the (p. 9), the  (p. 725), the  (p. 895), and the  (p. 1543). On the other hand, the, or Radiolaria with a complete skeleton, are the (p. 49), the  (p. 791), the  (p. 1015), and the  (p. 1590).

"Upon this basis the first subdivision of the Radiolaria was made by Johannes Müller, who recognised three groups:—"I. Thalassicolla, without receptacle, naked or with spicules; II. Polycystina, with a siliceous receptacle; III. Acanthometra, without receptacle, but with siliceous radial spines" (L. N. 12, p. 16)."

106. The Ectolithia and Entolithia (Extracapsular and Intracapsular Skeletons).—The relation of the skeleton to the central capsule in the Radiolaria is very various in many respects; in the first instance two great groups, Ectolithia and Entolithia (see note A), may be distinguished topographically by mere external observation; in the former the skeleton lies entirely outside the central capsule; in the latter, partially at all events, within it. The Ectolithia, with a completely extracapsular skeleton, include all and, as well as a great part of the  (all  and the most archaic forms of ); the Entolithia, on the other hand, in which the skeleton lies partly within, partly without the central capsule, include all  and the majority of the  (most , see note B).

"A. The difference between Ectolithia and Entolithia was applied in my Monograph in 1862 (p. 222) to separate the Monocyttaria into two main groups. The arrangement was, however, quite artificial, being contrary to the natural relations of the larger groups, as was shown seventeen years later by the discovery of the different structural relations of the central capsule.

B. Among the, which all possess primitively an intracapsular and centrogenous skeleton, the remarkable Cenocapsa (Pl. 133, fig. 11), seems to furnish the single exception; in it the skeleton consists of a simple spherical shell which encloses the concentric central capsule. The exception is, however, only apparent; the twenty perspinal pores of the shell show that they were originally in connection with twenty centrogenous acanthin spines, and that those have disappeared by retrograde metamorphosis."

107. Perigenous and Centrogenous Skeletons.—Much more important than the topographical relation of the skeleton to the central capsule, according to which the Ectolithia and Entolithia are separated from each other (§ 106), is the original development of the skeleton within or without the central capsule, which gives rise to the distinction between perigenous and centrogenous skeletons. Centrogenous skeletons are found only in the, which are further distinguished from all other Radiolaria by their skeleton being formed of acanthin; in all the formation of the skeleton begins in the middle of the central capsule, from which twenty (the number is inconstant only in the