XXViii THE VOYAGE OF H.M.S. CHALLENGEB. 



Spyroidea, on the contrary, and the majority of the Cyrtoidea, the 

 central capsule forms at its basis rounded 'lobes, which protrude and hang down 

 from the meshes of the cortinar plate ; and since this latter has usually three 

 or four large pores, the capsule similarly develops three or four processes 

 (PI. 53, fig. 19; PL 55, figs. 4-11 ; PL 59, figs. 4-13; PL 60, figs. 3-7; 

 PL 65, fig. 1). 



56. The Membrane of the Central Capsule. The capsule-membrane or envelope of 

 the central capsule is both morphologically and physiologically one of the most 

 important parts of the Radiolarian body, for it separates its two main constituents, the 

 capsule with its nucleus and endoplasm and the extracapsulum with the calymma and 

 exoplasm. The capsule-membrane is invariably present at some time or other during 

 the life of the organism, even though in a few species it may persist only for a short 

 time. It is characterised in general by its power of resistance to chemical and physical 

 reagents, and appears to be related to the elastic tissues or perhaps even more to the 

 chitinous substances. Its thickness is usually less than O'OOOl, though in certain groups 

 it ranges between O'OOl and 0'002, and in many of the larger Eadiolaria (such as Collida 

 and PH^ODARIA) it may attain a thickness of 0'003 to O'OOG or more. In the three 

 legions SPUMELLARIA, ACANTHARIA, and NASSELLARIA the capsule-membrane is single, 

 while in the PH^ODARIA it is always double, being composed of a firm outer and a 

 delicate inner membrane, which are in contact at only few points. Usually it is quite 

 structureless, except for its apertures ; the thicker membrane showing occasionally a fine 

 concentric lamination. In certain large Colloidea (e.g., Thalassicolla, PL 1, fig. 5&) 

 the membrane is covered on the inner surface by a network of polygonal ridges, and 

 in some large PH^ODARIA with remarkable small curved rods (PL 114, fig. 13). 

 In all Radiolaria the membrane is perforated by definite openings or pores, 

 through which the intracapsular and extracapsular protoplasm are in direct communi- 

 cation. These openings (or " pylae ") show very characteristic and constant differences 

 in the four legions, which have given rise to the names PERIPYLEA, ACTIPYLEA, 

 MONOPYLEA, CANNOPYLEA. 



The capsule-membrane was first indicated as the most important and absolutely constant 

 component of all Eadiolaria, and as the differential character of the class, in my Monograph (1862, 

 pp. 69-71). The careful investigations of E. Hertwig have confirmed this view and at the same 

 time have yielded the most important conclusions regarding the nature and systematic significance 

 of the openings in the capsule (op. cit., 1879, pp. 105-107). On the contrary, Karl Brandt has 

 recently propounded the theory that the capsule-membrane is by no means a constant part of the 

 Eadiolarian organism, but is lacking in certain species of Collozoum and Sphcerozoum (1881, p. 392). 

 This contradiction is explained by the fact that in some Collodaria and Acanthometra 

 the formation of the central capsule takes place much later than in the other Eadiolaria, in some 



