148. _The Ontogeny of the Nassellaria._--The individual development of the NASSELLARIA 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 _Tripospyris_ the basal tripod were to appear first in the ontogeny, and the sagittal ring were developed from this, then the #Plectoidea# 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; (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 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 {xcviii}usually different), more rarely the frontal axis (with equal right and left poles). In the zygothalamous #Spyroidea# (as in the #Stephoidea#) the formation of the shell proceeds from the sagittal ring, whilst in the polythalamous #Cyrtoidea# the latticed cephalis is always the starting point, from which a series of joints (thorax, abdomen, and in the Stichocyrtida, the numerous post-abdominal joints) successively arise