In the second subfamily, Cœlodrymida, the distal branches of the tubes are connected by numerous anastomoses, and compose either a simple lattice-plate on the surface of the skeleton (Cœlodrymus), or a thicker envelope of spongy framework (Cœlodasea). The lattice-mantle so produced is always bivalved, and its two outer hemispherical valves (dorsal and ventral) correspond exactly to the two inner valves, from which arise the hollow tubes. The free margins of the two external mantle-valves come externally into contact in the equatorial plane of the body, in which the girdle-fissure lies internally between the two central shell-valves. The free edges of the two external mantle-valves, opposed to one another in the circle of the equator, seem usually to catch one into another in the same way as the corresponding mantle-valves of the Cœloplegmida are loosely connected (Pl. [128], figs. 1, 7). A true concrescence between the two valves seems never to take place.
The two subfamilies of Cœlodendrida therefore exactly correspond to the two subfamilies of the following family, the Cœlographida. The Cœlodorida and Cœlotholida form in a similar way a thicket, by dichotomous ramification of the hollow tubes, all the branches of which remain free. The Cœlodrymida and Cœloplegmida, on the other hand, form an outer lattice-mantle by anastomosing branches. The latter two subfamilies, of course, have been derived correspondingly from the two former, and the common ancestral form of all four is probably Cœlodoras, derived from the Concharida.
Though the two corresponding subfamilies in both groups are very similar, they are, however, separated by important hereditary characters. All Cœlodendrida (the Cœlodorida without a mantle as well as the Cœlodrymida with a mantle) possess no rhinocanna and no frenula on the galea, and they never develop prominent verticillate styles; the surface of their calymma is probably always spherical or subspherical. All Cœlographida, however (the Cœlotholida without a mantle as well as the Cœloplegmida with a mantle), possess a rhinocanna and frenula on the galea, and always develop prominent verticillate styles; the surface of their calymma is probably always symmetrically polyhedral.
The superficial armature of the skeleton in the Cœlodendrida is rather simple, and by no means so manifold and differentiated as in the more highly developed Cœlographida. The thin terminal branches of the hollow tubes are in the Cœlodorida closed at the distal end, and armed with a variable number of short teeth (Pl. [121], fig. 2), or with a spinulate terminal knob, or a corona of recurved hooks (ibid., figs. 5-7). In the Cœlodrymida, however, where the distal ends of the branches by anastomosing form the lattice-mantle, the spherical surface of this latter is armed with numerous thin spathillæ or radial bristles (often zig-zag or spinulate), and each bristle usually bears at the distal end a small anchor with two, three, or four recurved teeth; the outer convex edge of these teeth is usually smooth, the inner concave edge denticulate. All these ramules and branches of the tubes (also the thinnest terminal threads) are hollow, and filled up by jelly.
The central capsule of the Cœlodendrida does not lie outside the two central valves (as I supposed in my first description, in 1862, being deceived by the dark enveloping phæodium, Monogr. d. Radiol., Taf. xxxii. fig. 1), but it is enclosed between the two valves, as in the preceding and the following family. The first accurate description of it was given by Richard Hertwig in 1879 (loc. cit., p. 95, Taf. x. fig. 3). Its constant position between the two lattice-valves (dorsal and ventral) is such, that its three openings lie in the frontal plane, in the open fissure between the valves. The astropyle or the main-opening, with the radiate operculum and the tubular proboscis arising from it, lies on the anterior (or oral) pole of the main axis, whilst the two lateral accessory openings, or parapylæ, lie on both sides of the posterior (or aboral) pole, to the right and left. The position of the capsule is therefore the same as in the preceding Concharida (Pls. [123]-[125]), and the following Cœlographida (Pls. [126]-[128]). The large nucleus, enclosed in the central capsule, is usually half as broad, and contains numerous nucleoli.
The calymma, or the extracapsular jelly-veil, is in the Cœlodendrida usually spherical, very voluminous, and includes the entire skeleton, the thicket of the Cœlodorida as well as the lattice-mantle of the Cœlodrymida. Only the outermost terminal branches of the tubes in the former, and the radial bristles and spathillæ on the surface of the latter, remain free and project beyond the surface of the calymma. The phæodium is usually very large, three to four times as broad as the central capsule, and envelops it often completely. Usually it envelops only the anterior half of it, and the proboscis (Pl. [121], figs. 1, 9). Often numerous green, brown, or blackish phæodellæ are scattered through the whole calymma, and sometimes accumulate in a superficial layer on its surface. The galea of both valves is usually also filled up by the phæodium.
Synopsis of the Genera of Cœlodendrida.
I. Subfamily Cœlodorida. Hollow tubes, arising from the galea of both valves, simple or dichotomously branched; the branches always free, not anastomosing. No outer lattice-mantle. |  | Tubes simple, not branched, | 727. Cœlodoras. |
| Tubes forked or dichotomously branched, | 728. Cœlodendrum. | ||
II. Subfamily Cœlodrymida. Hollow tubes, arising from the galea of both valves, dichotomously branched; the branches anastomose and form an outer bivalved lattice-mantle. |  | Lattice-mantle simple; its meshes lying in a spherical surface, | 729. Cœlodrymus. |
| Lattice-mantle spongy; its meshes lying in different planes, | 730. Cœlodasea. |
I. Subfamily Cœlodorida. Hollow tubes, arising from the galea of both valves, simple or dichotomously branched; the branches always free, not anastomosing. No outer lattice-mantle. | ||||
| Tubes simple, not branched, | ||||
| 727. Cœlodoras. | ||||
| Tubes forked or dichotomously branched, | ||||
| 728. Cœlodendrum. | ||||
II. Subfamily Cœlodrymida. Hollow tubes, arising from the galea of both valves, dichotomously branched; the branches anastomose and form an outer bivalved lattice-mantle. | ||||
| Lattice-mantle simple; its meshes lying in a spherical surface, | ||||
| 729. Cœlodrymus. | ||||
| Lattice-mantle spongy; its meshes lying in different planes, | ||||
| 730. Cœlodasea. | ||||