REPORT ON THE RADIOLARIA. xcvii 



this form passes over into au ellipsoid by prolongation of one axis ; on the contrary, in 

 the Hexalaspida (as in the D i s c o i d e a) the discoidal or lenticular form arises by 

 shortening of an axis. Finally, in the Diploconida, and in some Hexalaspida in which the 

 growth is different in all three dimensive axes (as in the L a r c o i d e a), both the central 

 capsule and the shell assume the lentelliptical form. The lattice-shell of the A c a n t h o- 

 phracta is usually successive in its development, since from each of the twenty radial 

 spines two or four tangential apophyses proceed, whose branches subsequently unite and 

 combine to form the lattice-shell. Only in the peculiar Sphaerocapsida can the pavement- 

 like shell arise simultaneously or in a moment of lorication. 



148. The Ontogeny of the Nassellaria. — The individual development of the Nassel- 

 LARiA in the simplest instance remains stationary at the skeletonless Nasselid stage 

 [Cystidium, Nassella), which can be immediately derived from the foregoing Actissa- 

 stage by the disappearance of the pores in the upper (apical) hemisphere of the central 

 capsule, whilst in the lower (basal) portion they are modified to form a porochora ; the 

 podoconus is developed within the endoplasm upon this latter. Usually the spherical 

 form of the central capsule passes over into an ovoid or ellipsoidal one, the vertical axis 

 which passes through the centre of the porochora being elongated. From the skeletonless 

 Nassellida the other Nassellaria may be derived both ontogenetically and phylogenetically 

 by the excretion of an extracapsular siliceous skeleton. Unfortunately, the earliest stages 

 in the formation of this skeleton are unknown, and hence no answer can at present be 

 given to the important question, in what order the three primary skeletal elements of the 

 Nassellaria (the basal tripod, sagittal ring, and latticed cephalis) appear (compare 

 §§111 and 182). If, for example, in Cortina and Tripo&pyris the basal tripod were to 

 appear first in the ontogeny, and the sagittal ring were developed from this, then the 

 P 1 e c t o i d e a would be rightly considered to be the oldest forms in the phylogeny of 

 the skeleton-forming Nassellaria ; and in the contrary case the Stephoidea would 

 be so regarded. The relations of growth in the three dimensive axes are very variable 

 in the Nassellaria ; the three most important factors in this respect (partly separately 

 and partly in combination) are ; (l) the development of the basal tripod to a triradial staur- 

 axon form (the ground-form being a three-sided pyramid) ; (2) the development of the 

 sagittal ring in the median plane of the body (the vertical axis having the poles different) ; 

 (3) the development of the latticed cephalis outside the central capsule (the poles of the 

 vertical axis being again different). Since the development both of the skeleton and of 

 the malacoma is characterised in most Nassellaria by the stronger growth of the vertical 

 axis and the differentiation of the two poles, the allopolar monaxon ground-form acquires 

 a predominant significance in this legion (§ 32) ; the starting point of most of the further 

 modifications is the basal pole of the vertical main axis. Next to this the sagittal axis 

 is usually the most important determining factor (its dorsal and ventral poles being 



(ZOOL. CHALL. EXP. — PART XL. — 1886.) ^"^ " 



