The size of the shell varies between 0.2 and 0.8 mm., and is usually between 0.3 and 0.5 mm. Its form is in the majority of species a geometrical sphere; rarely it is somewhat irregular, slightly ellipsoidal (prolonged in the axis of the mouth and proboscis), or polyhedral (by conical protuberances from the bases of the radial spines). The ideal fundamental form of the shell, however, is constantly monaxonian, since the place of the shell-mouth and of the proboscis of the capsule (both lying in one axis of the sphere), causes a principal axis to be different from all others. The longer radial spines, quite variable in number and disposition, are never arranged regularly or symmetrically, and therefore determine no constant axes.
The lattice-work of the spherical shell offers also but slight differences. It is usually more or less irregular, with circular or subcircular pores of unequal sizes. More rarely the network is perfectly regular, with equidistant circular pores of equal size, and sometimes these are surrounded by hexagonal frames (fig. 6), as in many Sphæroidea. More rarely the pores are perfectly irregular, roundish or sometimes polygonal, of unequal sizes and dissimilar forms (fig. 2). The size of the pores is very variable between 0.01 and 0.05, usually between 0.015 and 0.025. They are commonly somewhat broader than the bars between them, more rarely smaller. The substance of the shell-wall is in the the majority of Castanellida hyaline, transparent (as in the similar Sphæroidea); in some larger forms, however, a network of fine axial filaments is visible in the lattice (fig. 2), and sometimes this seems to be hollow, with an axial canal (as in the large Orosphærida). Such an axial filament or fine axial canal is usually visible in the larger radial spines. The shell-wall of the Castanellida, however, never exhibits that characteristic structure (with tangential needles in a cement-like fundamental substance), which we find in all Circoporida. The latter differ also in the constant presence of circles of large pores around the bases of the radial spines, which are never found in the Castanellida. The shell becomes usually stained more or less purple by carmine, and when burned by fire, becomes of a brown colour.
The inner surface of the shell is always quite smooth, the outer surface armed with numerous short and thin radial bristles. Sometimes these bristles or "by-spines" are very short, but never perfectly wanting. Usually their length is about one-third or one-fourth of the radius of the shell, often less, rarely more. They arise from the nodal points between every three or four neighbouring spines, and are commonly very thin and straight, pointed, with slightly thickened conical basis. Rarely they are directed not quite radially, but more or less obliquely.
The larger radial spines, which arise between the thin bristles or "by-spines" and may be opposed to the latter as "main-spines," are wanting in two genera only, in Castanarium and Castanella (Pl. [113], fig. 6). They are simple in Castanidium (figs. 2, 5, 7) and Castanissa (fig. 1), irregularly branched in Castanopsis and Castanura (figs. 3, 4). Their number and disposition is never regular nor symmetrical (as in the majority of Sphæroidea) and varies even in each single species. In general the number of main-spines varies between ten and forty (rarely less or more) usually between twenty and thirty. Their length is commonly about equal to the diameter of the shell, often somewhat shorter, rarely considerably longer. Their thickness is in the majority of species about equal to the diameter of one or two pores of the lattice. Sometimes they are much stronger, rarely thinner.
The form of the radial main-spines is not very variable, usually it is cylindrical or cylindro-conical, rarely angular, prismatic or pyramidal. Sometimes they are straight (perfectly radial), at other times more or less irregularly curved. Usually they are simple, rarely branched; the branches are always very irregular and exhibit a remarkable tendency to unite by concrescence (figs. 3, 4). Some thicker forms of spines exhibit a remarkable structure, the surface being covered with small dimples and spinules between them (fig. 1); sometimes the dimples are hexagonal (fig. 5a). In the axis of the thicker spines there is often visible the same axial filament or funicle which we find in many other Phæodaria, more rarely a hollow axial canal.
The mouth of the shell is usually circular, more rarely polygonal or irregularly roundish (fig. 2). It is either quite smooth (fig. 3, 5, 7) or armed with a corona of strong teeth (figs. 1, 4, 6). These teeth are different from the other spines of the shell; they are much thicker than the radial bristles, and shorter than the main-spines. Their number varies between three and twenty, usually between five and seven. They are usually parallel to the main axis, which is determined by the excentric position of the mouth and the centre of the sphere (figs. 1, 6). The teeth are usually conical, more or less irregular in size and form, more rarely of equal size. The corona of teeth is usually sharply separated from the surrounding lattice-work of the shell, but has no influence on its regular spherical form.
The central capsule of the Castanellida immediately determines beyond doubt their true Phæodarian nature; it lies excentrically in the cavity of the enclosing shell, and is surrounded at the oral pole by the large and dark phæodium. The latter covers the radiate operculum of the capsule membrane and the tubular proboscis, which arises from it and is directed towards the shell mouth. Usually the diameter of the spherical central capsule is about one-third of the shell diameter, rarely one-quarter only, and in a few cases it attains nearly its half. The calymma, therefore, is larger than the capsule, and fills up the whole cavity of the shell which is not occupied by the latter. The greater part of the calymma again is filled up by the dark green or brown, sometimes black phæodella, the roundish granules which compose the voluminous non-transparent phæodium. Sometimes the latter proceeds through the mouth and is partially placed outside the shell (fig. 5).
The astropyle, or the main-opening of the central capsule, is placed on the oral pole of its main axis, while its radiate operculum and the tubular proboscis arising from it possess the same structure as in the other Phæodaria. But the two lateral parapylæ or accessory openings which are present in the majority of the latter seem to be absent in the Castanellida as well as in the Medusettida and the Challengerida. I could never observe a trace of them. The proboscis is a small cylindrical tube and is directed towards the mouth of the shell, therefore placed in its main axis; it is usually quite hidden in the dark phæodium, which occupies the oral half of the shell-cavity, whilst the capsule lies in the aboral half. The large nucleus of the central capsule and its double membrane present the same characters as in the other Phæodaria.
Synopsis of the Genera of Castanellida.
| No long radial main-spines (in addition to the constant short bristles of the shell-surface). |  | Mouth smooth, | 705. Castanarium. | ||
| Mouth dentate, | 706. Castanella. | ||||
| Long radial main-spines scattered between the short constant bristles of the shell-surface. |  | Main spines simple. | | Mouth smooth, | 707. Castanidium. |
| Mouth dentate, | 708. Castanissa. | ||||
| Main spines branched. | | Mouth smooth, | 709. Castanopsis. | ||
| Mouth dentate, | 710. Castanura. | ||||