RADIOLARIA. 875 



boiling caustic alkalies ; and in two families of the same sub-group (Circo- 

 porida, Tuscarorida) it has a porcellanous aspect by reflected light, and 

 consists of fine siliceous needles imbedded in a granular matrix. Its 

 elements are solid, except in most Phaeodaria, where they are hollow 

 siliceous tubes filled with a gelatinous substance. The shapes taken by 

 the skeleton are numerous. It may be spicular (beloid), as in the Beloidea 

 among Spumellaria, and the Phaeocystina among Phaeodaria, the spicules 

 being of various shapes and disposed more or less tangentially, radially only 

 in the Phaeodarian family A ulocanthida. In all other instances it forms 

 a connected whole ; a simple ring or several rings united in various planes, 

 the Nassellarian Stephoidea ; radial spines united at a centre which lies ex- 

 centrically near the oral pole, the Nassellarian Plectoidea ; a lattice-shell, 

 the bars of which lie either in one plane, or in different planes, and in this 

 case spongy in texture, as is not uncommon among Spumellaria, in 

 shape spherical, ellipsoidal (= prunoid), discoidal, lent-elliptical (=larcoid) 

 i.e. with three principal axes of unequal length, cyrtoid i.e. with two dis- 

 similar poles, an apical and basal, or finally conchoid. The last-named 

 consists of two valves, and is confined to a few Phaeodaria. The cyrtoid 

 skeleton may be monothalamous, and then ovate or cap-shaped, as in some 

 Nassellaria, where it is termed cephalis, a few Phaeodaria, e.g. Challengerida, 

 and very rarely in Spumellaria ; or it may be in some Nassellaria poly- 

 thalamous, the cephalis being simple, bilobate, or multilobate, with, 

 especially when simple, new joints added at the basal pole ; incomplete 

 internal septa correspond to the external constrictions. The shell is rarely 

 multiple, except in the Spumellarian Sphaeroidea, where there may be two 

 spheres, three, four, five, or even ten, rarely more, one within the other, all 

 united by radial bars 1 . 



The division of the body into a peripheral or extra-capsular, and a 

 central or intra-capsular portion is constant. The central capsule is de- 

 limited by a resistent chitinoid (?) membrane, a skeletal structure which 

 is double in Phaeodaria^ and is absent in the young stages of some forms, 

 but appears just before sporulation. Collozoum inerme is said to develope 

 it only when forming isospores, and in Sphaerozoum neapolitanum it is per- 

 manently absent (Brandt). When it is thus absent the contour of the 

 body is variable. The capsule is normally spherical, but it becomes 

 adapted to the general shape of the skeleton as well as to its peculiarities. 

 Its surface is lobed in many Spumellarian Sphaeroidea ; and in the Nassel- 

 larian Spyroidea and Cyrtoidea it has 3-4 lobes projecting through the 

 basal or cortinar plate of the shell. When the shell is double or multiple 

 the capsule generally incloses the inner or two or three of the inner shells 

 during its growth. Its membrane is always perforated either by innumerable 



1 The innermost or two innermost shells, termed medullary, differ sometimes in the character 

 of their lattice-work from the outer or cortical shells. See Haeckel, op. cit. supra, pp. Ixxxv-vi. 



