Habitat.—Equatorial Atlantic, Station 347, depth 2250 fathoms.
Family LXXXV. Cœlographida, n. fam. (Pls. [122], [126]-[128]).
Definition.—Phæodaria with a bivalved lattice-shell, composed of two hemispherical valves, a dorsal and a ventral. A conical cupola or a helmet-shaped galea arises on the apical pole of each valve, therefore on the opposite poles of the sagittal axis. The cavity of the galea communicates with the sagittal rhinocanna, a peculiar nasal tube, which rests upon the valve, and is connected with the galea by a simple or double frenulum; its opening being directed towards the proboscis. Three or more branched hollow radial tubes arise from each valve, and are symmetrically disposed. Sometimes their branches form an outer bivalved mantle. The central capsule is so enclosed between the two inner valves, that its three openings lie in the open frontal fissure between them.
The family Cœlographida, the last family of the Phæodaria, exhibits the highest degree of morphological development, not only in this group, but among all Radiolaria. They attain also the greatest size of all members of the class, since the diameter of their body is sometimes more than 20 mm., and in a few species even more than 30 mm. The complexity of their structure attains at the same time such a high degree, that they may be regarded as the most complicated, and (in a morphological sense) as the most highly developed of all Protozoa. Nevertheless their body always remains a single cell, and is closely allied to the preceding Cœlodendrida; they differ from the latter mainly in the development of a peculiar new organ, the "rhinocanna," or "nasal tube." This is a hollow tube placed in the sagittal plane, arising from the base of each galea, and is connected with its apex by a simple or double frenulum. Between the oral openings of the two opposed rhinocannæ (one dorsal and one ventral) lies the proboscis of the central capsule.
The first observed species of Cœlographida was Cœlographis gracillima, some parts of which (but not the entire skeleton) were figured in my Monograph (1862, Taf. xxxii. figs. 2, 3). But I confounded these with Cœlodendrum gracillimum, in the branched hollow trees of which the fragments of the former were entangled. I detected this error afterwards, when I had the opportunity of the observing some complete specimens. The first description of a complete skeleton was given in 1882 by O. Bütschli, who examined a large specimen of Cœlothamnus davidoffii, captured by Davidoff in the Mediterranean (Zeitschr. f. wiss. Zool., Bd. xxxvi. p. 486, Taf. xxxi.). In the rich collection of the Challenger I was able to distinguish not less than nine genera and twenty-six species of Cœlographida, but the majority of their large and most fragile skeletons were more or less injured, or quite broken. It was, therefore, of the highest importance for the minute study of this difficult group, that Dr. John Murray, during his expedition to the Færöe Channel (in 1882, in H.M.S. "Triton"), discovered in the Gulf Stream the beautiful Cœloplegma murrayanum, and brought up home hundreds of well-preserved specimens (Pl. [127]). Only by the complete examination of this excellent material it was possible to answer many difficult questions as to their morphology, and to correct the errors in my description and in that of Bütschli.
We divide the family Cœlographida into two rather different subfamilies, which may afterwards be separated as two divergent families, the Cœlotholida (Pl. [122]) and Cœloplegmida (Pls. [126]-[128]). Both groups may be easily distinguished at a glance, since the numerous branches, arising from the hollow radial tubes, remain constantly free and independent in the former, and represent a spiny thicket, whilst in the latter they constantly become united, and by anastomosing form a peculiar "mantle," or outer envelope of delicate network. But besides, there are other and more important differences between the two groups. The peculiar hollow tube, arising from the base of the galea on each valve, which is filled with phæodella, and which we call the rhinocanna, develops in the Cœlotholida on its open mouth two paired lateral frenula (right and left), which connect it (like two lateral bridges) with the base of two paired hollow main tubes (the "frontal tubes"). In the Cœlospathida, however, the mouth of the rhinocanna develops a single odd frenulum only (in the sagittal plane), and is connected by it with the base of an odd, single, hollow main tube, directed forwards, the "nasal style."
The central bivalve lattice-shell, from which the galea and the tubes arise, exhibits in the Cœlographida essentially the same form and structure as in the preceding Cœlodendrida. The only (but important) difference between them is indicated by the constant presence of the peculiar rhinocanna in the former, whilst this is always absent in the latter. The two valves of the shell, dorsal and ventral, are either hemispherical or somewhat flatter, sometimes nearly cap-shaped, and formed of an extremely delicate and irregularly fenestrated plate of silica, as in the Cœlodendrida. As in the latter, so also in the Cœlographida both valves are of similar form and usually of equal size, but sometimes the dorsal is a little smaller than the ventral valve. The remarkable difference which Bütschli describes in his Cœlothamnus davidoffii, and the inverse origin of the three tubes in both valves (loc. cit., Taf. xxxi; figs. 2, 4), depends upon an error of observation, produced by the artificial inversion of one valve, and the dislocation of their natural arrangement. The valves are never in direct contact, but separated by the equatorial fissure or girdle-cleft, in which the girdle zone of the enclosed central capsule and its three openings lie freely (Pl. [127], figs. 4, 5; Pl. [128], fig. 2). The free margins of both valves, which are opposite to one another, and bound the girdle-cleft, are always equidistant, so that the cleft in the whole equatorial circumference is of equal breadth. The margins are usually irregularly denticulate, sometimes armed with longer bristles (Pl. [127], fig. 8), more rarely smooth (fig. 5). The delicate lattice-work of the valves is always irregular and very variable, usually with numerous small and unequal pores, sometimes rudimentary, so that the valves appear partly solid and hyaline. The size of the valves is usually between 0.2 and 0.5 (in diameter).
The galea (g) or the apical cupola, which arises from the vaulted apex of each valve (or its sagittal pole) is more developed in the Cœlographida than in the preceding Cœlodendrida, and differs from the latter in the peculiar rhinocanna arising from its base, and in the single or double frenulum, connecting the open mouth of the rhinocanna with the odd or paired main tube arising from the galea. The two opposite galeæ lie therefore on the poles of the sagittal axis of the bivalve shell, and are so symmetrically disposed in the sagittal plane, that the open mouths of their rhinocannæ are directed towards the oral pole of the main axis, and nearly come in contact with the proboscis arising from the radiate operculum of the central capsule (Pl. [127], figs. 4, 5).
The size and form of the galea are very variable, even in one and the same species. The volume of its cavity is generally about as great as that of the hemispherical valve from which it arises, sometimes larger, at other times smaller. Its fundamental form is constantly dipleuric or bilateral, since the radial hollow tubes arise symmetrically on both its sides, and the rhinocanna proceeding from its base determines the sagittal plane. Usually the galea has the form of a vaulted helmet, the convex crest of which is inclined towards the mouth (Pl. [127], figs. 4, 5, 8, 9). Its anterior or apical part is broad and truncated in the Cœlotholida, more or less conical in the Cœlospathida; sometimes it assumes nearly the form of a bilateral three-sided pyramid, at other times it is more pear-shaped (Pls. [126]-[128]). The thin siliceous wall of the galea has the same irregular and delicate network as the valve from which it arises, and sometimes the small irregular pores are also here reduced, so that the wall becomes partly solid. In some cases the thin, solid, siliceous plate of the galea and of the valve is covered by an irregular delicate network of crests; the dimples between these crests may be easily confounded with true pores.
The cavity of the galea is filled with phæodella and does not communicate with the cavity of the shell-valves, nor with the cavity of the radial tubes filled by jelly; it is closed towards the latter and the former by a thin solid plate of silex. Bütschli (1882, loc. cit., p. 488) describes in Cœlothamnus a large circular opening (Taf. xxxi. figs. 2a, 4a), and states that this is a direct communication between the cavities of the valves and of their galeas which are called by him "der dreiseitige kastenförmige Aufsatz" (ε). This error was caused by the fact that he observed the valves from the apical face only. The apparent opening of communication does not exist, and is the optical section of the rhinocanna, the shortened walls of which he describes as "trapezförmige Kiesellamelle" (γ); the two lateral edges of the latter ("die seitlichen Zipfel," δ) are the paired frenula, which connect the open mouth of the rhinocanna with the base of the two frontal tubes.