Page:Scientific results HMS Challenger vol 18 part 1.djvu/117

Rh this form passes over into an ellipsoid by prolongation of one axis; on the contrary, in the Hexalaspida (as in the ) 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 ), both the central capsule and the shell assume the lentelliptical form. The lattice-shell of the 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 Sphærocapsida can the pavement-like shell arise simultaneously or in a moment of lorication.

148. The Ontogeny of the Nassellaria.—The individual development of the 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 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 (the basal tripod, sagittal ring, and latticed cephalis) appear (compare §§ 111 and 182). If, for example, in Cortina and Tripospyris the basal tripod were to appear first in the ontogeny, and the sagittal ring were developed from this, then the would be rightly considered to be the oldest forms in the phylogeny of the skeleton-forming ; and in the contrary case the  would be so regarded. The relations of growth in the three dimensive axes are very variable in the ; the three most important factors in this respect (partly separately and partly in combination) are; (1) the development of the basal tripod to a triradial stauraxon 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 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