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



lattice-shell, whilst the others (Aphracta) are without it. The RADIOLARIA APHRACTA, 

 then, or Radiolaria without a complete skeleton, are the C o 1 1 o d a r i a (p. 9), the 

 Acanthometra (p. 725), the Plectellaria (p. 895), and the Phseocystina 

 (p. 1543). On the other hand, the RADIOLARIA CATAPHRACTA, or Radiolaria with a 

 complete skeleton, are the Sphserellaria (p. 49), the Acanthophracta 

 (p. 791), the Cyrtellaria(p. 1015), and the Phaeocoscina (p. 1590). 



Upon this basis the first subdivision of the Kadiolaria was made by Johannes Miiller, who re- 

 cognised three groups : " I. Thalassicolla, without receptacle, naked or with spicules ; II. Poly- 

 cystina, 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 NASSEL- 

 LARIA and PH^EODARIA, as well as a great part of the SPUMELLARIA (all Collodaria 

 and the most archaic forms of Sphserellaria); the Entolithia, on the other hand, 

 in which the skeleton lies partly within, partly without the central capsule, include all 

 ACANTHARIA and the majority of the SPUMELLARIA (most Sphserellaria, 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 ACANTHARIA, which all possess primitively an intracapsular and centrogenous 

 skeleton, the remarkable Cenocapsa (PI. 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 these have disappeared 

 by retrograde metamorphosis. 



107. Perigenous and Centrogenous Skeletons. Much more important than the topo- 

 graphical 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 

 ACANTHARIA, which are further distinguished from all other Radiolaria by their skeleton 

 being formed of acanthin ; in all ACANTHARIA the formation of the skeleton begins in the 

 middle of the central capsule, from which twenty (the number is inconstant only in the 



