Definition.—Phæodaria with a bivalved lattice-shell, composed of two hemispherical valves, a dorsal and a ventral. A conical cupola or a pyramidal galea arises from the apical pole of both valves, therefore at the opposite poles of the sagittal axis. Rhinocanna and frenula wanting. Three or more 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 Cœlodendrida differs from the preceding Concharida (its probable ancestral group) in the development of a conical galea or pyramidal cupola on the apical pole of each valve, and of three or more hollow radial tubes arising from each galea. They do not possess, however, the peculiar sagittal nasal tube or rhinocanna, which is constantly developed from the base of each cupola (and connected with its apex by a frenulum) in the following family, the Cœlographida. These latter differ also from the former in the constant possession of prominent verticillate styles.
The family Cœlodendrida was founded in 1862 in my Monograph (p. 360) and represented hitherto only by two species of the genus Cœlodendrum, there described (p. 361, Taf. xiii. figs. 1-3, and Taf. xxxii. fig. 1). This first description, however, contained some errors, which were afterwards (in 1879) corrected by Richard Hertwig; this author also gave the first accurate description of the central capsule and its three openings. In the rich collection of the Challenger, the Cœlodendrida are represented by four genera, but only seventeen species, some of which, however, are cosmopolitan and very common, particularly Cœlodendrum.
The two valves of the lattice-shell, dorsal and ventral, are either hemispherical, or somewhat more flatly vaulted or cap-shaped. They are never connected in the equatorial zone of the body, as I supposed in my Monograph (1862, loc. cit.); but they are separated by the girdle-fissure, a free circular equatorial interval, in which lie the three openings of the enclosed central capsule. Though the two valves, therefore, have no direct connection, they are, however, always opposed so accurately, that their equal free circular edges correspond exactly one to the other, so that the apex of each valve lies in one pole of the sagittal axis. From this apex there arises on each valve an irregular conical or three-sided pyramidal cupola, the galea (Pl. [121], figs. 3, 4, 8). The Cœlodendrida differ in the possession of this galea from the Concharida, and agree with the Cœlographida; but they never exhibit the peculiar rhinocanna or nasal tube, which arises from each galea in the latter family.
The siliceous lattice-plate of the two valves, and of the galea arising from them, is very thin and fragile, and its irregular roundish pores are extremely variable in size, number, and disposition. Sometimes the pores are so small and so scarce, that the plate appears nearly solid. At other times the siliceous plate seems to be really solid, and covered by a network of thin crests, the small dimples between which give to it the appearance of being fenestrated. Often the pores or the dimples are wanting in the central part of each valve, while they are very numerous and dense in the peripheral part. The same may be said of the lattice-plate of the galea, which is sometimes nearly solid, at other times richly fenestrated. The Cœlodendrida agree in this structure with the following family, the Cœlographida, and differ from the preceding family, the Concharida, in which the siliceous wall of the two valves is much thicker, and perforated by regular circular or roundish pores.
The galea or conical cupola in the apex of the two valves ("der kegelförmige Aufsatz" of the German authors) has in all Cœlodendrida a triangular base and an irregularly conical or nearly three-sided pyramidal form. Its cavity is about one-third or one-fourth as large in diameter as the cavity of the hemispherical valve upon which it rests. The galea is relatively smaller and more irregularly formed than in the Cœlographida, and differs essentially from that of the latter in the constant absence of a rhinocanna; there are also wanting, therefore, the characteristic frenula, which connect the nasal tube with the apex of the galea. The cavity of the galea probably always communicates with that of the valves by pores in the separating siliceous plate, and is besides pierced by irregular pores in its outer wall, very variable in form, size, and number, but it does not communicate with the cavity of the hollow radial tubes, from which it is separated by a thin, solid, siliceous plate.
The hollow radial tubes which arise from the galea in the Cœlodendrida do not seem to possess that constant regularity in number, origin, and disposition, which is found in the following family, and there serves for distinction of genera. In my first description of the Cœlodendrida (1862, loc. cit., p. 362), I pointed out this irregularity, and mentioned that the number of radial tubes arising from each galea varies from three to eight; the total number therefore amounts to from six to sixteen, the same minimum and maximum numbers which we shall encounter also in the radial styles of the following family. But whilst it is easy to determine the position and relation of these hollow tubes in the Cœlographida, owing to the constant sagittal position of their rhinocanna, this task is very difficult in the Cœlodendrida, where the rhinocanna is wanting. In the most frequent cases there arise from each galea three or four tubes, more rarely five or six, and very rarely seven or eight. The simplest and probably the original case is the development of three tubes, two of which are paired (divergent on the right and left), while the third is odd, lying in the sagittal plane. Perhaps these three primary tubes may be compared to the three cortinar feet of the Nassellaria, so that we may regard the two paired anterior as pectoral, and the odd posterior as a caudal tube. Usually the two paired or pectoral tubes arise from two corners of the triangular base of the galea, whilst the third odd or caudal tube does not arise from the third corner of the base, but more or less above it, and often even from the highest point or the apex of the galea. In the majority of species observed, this odd sagittal tube is forked even at its origin, so that two divergent tubes (an anterior and a posterior) arise from the apex of the galea (Pl. [121], figs. 3, 8). More rarely the two paired or pectoral tubes are also forked at the base, so that three pairs of tubes arise from each galea, and the total number of tubes amounts to twelve. Very rarely four separate tubes or four pairs of tubes arise from each galea, viz., two from the two anterior corners of the basal triangle, one from the posterior corner, and one from the apex of the galea. It is possible that this difference in the origin, furcation, and number of the hollow radial tubes may be employed for the distinction of genera of Cœlodendrida, in the same manner as it is employed in the next following family, the Cœlographida. But I have not been able, in spite of numerous and accurate examinations, to demonstrate in the former the same regularity in number and arrangement of the tubes as in the latter. It seems that these relations here are very variable, even in one and the same Species, and not yet fixed.
It is, however, probable, on the other hand, that the primary tubes (all or partly) are identical to the Cœlodendrida and Cœlographida. This is most probably the case with the posterior odd or caudal tube, which seems to be never wanting, and in both families is developed in the form of a dichotomous brush (never in the form of a verticillate style). Possibly also the two paired pectoral tubes are homologous in both families.
The hollow tubes are perfectly simple and unbranched only in one genus, Cœlodoras, which is probably the common ancestral form of both families, and which may have been derived from Concharium by development of a galea and tubes on the sagittal apex of the valves. All the other Cœlodendrida have branched spines, and the ramification is constantly dichotomous, or repeatedly forked. There never occur in this family those characteristic "styles," or verticillate prolonged tubes, which we find in all Cœlographida. Usually the cylindrical tubes are slightly curved and forked even near their base. The furcation is repeated a variable number of times in the different species. In the largest species each tube becomes a brush with more than one hundred terminal bristles.
We divide the Cœlodendrida into two subfamilies, according to the different development of the distal branches of the hollow tubes. In the Cœlodorida all the branches of the tubes remain free and are never connected by anastomoses, so that the surface of the bivalved skeleton is protected by the free radial distal branches of the tubes. In the larger species of Cœlodendrum (e.g., Cœlodendrum furcatissimum, Pl. [121], fig. 1), the numerous branches of the dichotomous tubes form a dense thicket, similar to that in the Cœlotholida.