REPORT ON THE RADIOLARIA. XXV 



soft body. The fact that it has recently been reported as absent by various authors is due to their 

 having observed young or unripe specimens, before the formation of the spores. In some species of 

 Polycyttaria and Acantharia the membrane persists only a very short time. 



52. The Primary Form of the Central Capsule. — The form of the central capsule is 

 originally a geometrical sphere ; and if in accordance with our monophyletic hypothesis 

 all Radiolaria are to be derived from one common stem-form {Actissa, see p. 12), then the 

 central capsule of this common stem-form must be regarded as perfectly spherical 

 (Procyttanum, p. 13, PI. 1, fig. 1). Since, further, the enclosed nucleus and the 

 surrounding calymma of this primitive archaic form must also be spheres, and since the 

 nucleus lies in the centre of the body, and the protoj^lasm is evenly distributed between 

 it and the membrane, it follows that no axes or excentrically differentiated parts are to 

 be distinguished in this most primitive Radiolarian. Rather in the primary central 

 capsule all parts are concentrically and evenly arranged round its centre. This j^rimary 

 spherical form becomes modified in most Radiolaria into various secondary ground-forms, 

 which are correlated partly with the structure of the capsule itself, and partly also with 

 the development of oj)enings in its membrane. In general the ground-form of the 

 central capsule is polyaxon in the Porulosa (Spumellaria and Acanthafja) ; but in the 

 Osculosa centraxon forms are more frequently observed ; in the Nassellaria the ovoid 

 (allopolar monaxon) form is predominant, and in the Ph^odaria the rhomboid or 

 amphithect pyramid. In these latter, the astropyle indicates the basal pole of the 

 vertical main axis, whilst the two parapylse (right and left) mark the poles of the 

 frontal transverse axis. In the Nassellaria the centre of the porochora corresponds with 

 the basal pole of the main axis, whilst no transverse axes are originally present. 



53. The Secondary Forms of the Central Capsule. — The original purely spherical form 

 of the central capsule persists only in the minority of the Radiolaria, namely, the greater 

 part of the Spumellaria and Acantharia ; it passes over into various other secondary 

 forms in the majority of the class, in the whole of the Nassellaria and Ph/EODAria, 

 and in a considerable portion of the Spumellaria and Acantharia. These secondary or 

 derived forms may be divided into two quite distinct groups, which may be designated 

 endometamorphic and exometamorphic ; in the former the cause of the divergence of 

 the secondary form from the sphere lies in the internal structure of the central capsule ; 

 in the latter it lies in the external influence exerted by the growth of the skeleton. 

 Obviously the former series of modifications is more significant than the latter. 



54. T/ie Endometamorphic Forms of the Central Capsule. — The secondary forms of 

 the central capsule, which are due to internal causes connected with its growth, are as 

 follows : — 



(ZOOL. CHALL. EXP. — PART XL.— 1886.) Rr il 



