_Calcaromma calcarea_ was described by Wyville Thomson as having a shell of apposed calcareous discs, and _Myxobrachia_, by Haeckel, as having collections of the calcareous Coccoliths and Coccospheres. In both cases we have to do with a Radiolarian not possessing a skeleton, but retaining the undigested shells of its food, in the former case (_Actissa_) in a continuous layer, in the latter (_Thalassicolla_) in accumulations that, by their weight, droop and pull out the lower hemisphere into distinct arms. The (siliceous) skeleton of the Nassellaria is absent only in the Nassoidea, and is never represented by distinct spicules. Its simplest form is a "tripod" with the legs downward, and the central capsule resting on its apex. The addition of a fourth limb converts the tripod into a "calthrop," the central capsule in this case resting between the upturned leg and two of the lower three regarded as the "anterolateral"; the odd lower leg, like the upturned one, being "posterior." Again, the skeleton may present a "sagittal ring," often branched and spiny (Fig. 26, A), or combined with the tripod or calthrop, or complicated by the addition of one or more horizontal rings. Another type is presented by the "latticed chamber" surrounding the central capsule, with a wide mouth ("pylome") below. This is termed the "cephalis"; it may be combined in various ways with the sagittal ring and the tripod or calthrop; and, again, it may be prolonged by the addition of one, two, or three chambers below, {84}the last one opening by a pylome (Fig. 25, B). These are termed "thorax," "abdomen," and "post-abdomen" respectively. In the Phaeodaria the skeleton may be absent, spicular (of loose or connected spicules) or latticed, continuous or bivalve. It is composed of silica combined with organic matter, so that it chars when heated, is more readily dissolved, and is not preserved in fossilisation. The spicules or lattice-work are hollow, often with a central filament running in the centre of the gelatinous contents. The latticed structure of the shell of the Challengeridae (Fig. 28) is so fine as to recall that of the Diatomaceae. In the Phaeoconchida the shell is in two halves, parted along the "frontal" plane of the three apertures of the capsule. [Illustration: FIG. 27.—Scheme of various possible skeletal forms deposited in the meshes of an alveolar system, most of which are realised in the Radiolaria. (From Verworn, after Dreyer.)] The central capsule (rarely inconspicuous and difficult, if not impossible to demonstrate) is of a substance which resembles chitin, though its chemical reactions have not been fully studied hitherto, and indeed vary from species to species. It is composed of a single layer, except in _Phaeodaria_, where it is double. The operculum in this group, _i.e._ the area around the aperture, is composed of an outer layer, which is radially thickened, and a thin inner layer; the former is produced into the projecting tube ("proboscis").