Whilst in all Plagonida the branches of the spine remain perfectly free, in all Plectanida, again, the meeting ends of the branches become united and grow together, and by this concrescence a loose network arises, like wickerwork, which partly encloses the central capsule and the central parts of the spines, on which it rests. The meshes of this loose wickerwork are large, either quite irregular, of very different size and form, or more or less regular, with a certain form and arrangement of the meshes, effected by the peculiar kind of ramification. Commonly the siliceous threads of the arachnoidal wickerwork are very thin, often extremely delicate, representing "pseudopodia metamorphosed into silex." Sometimes the wickerwork is spongy. Its surface is constantly rough and bristly, with free ends of the spine-branches, never covered with a regular lattice-plate, as in the Cyrtellaria (Spyroidea, Botryodea, and Cyrtoidea).
The entire form of the central wickerwork is in the minority of Plectanida quite irregular and indefinite; in the majority, however, a certain more or less regular entire form is recognisable, effected by a certain, more or less regular origin and mode of the connection of the meeting branches. So in some species of Triplecta (Pl. [91], fig. 7) the network represents a triangular plate, of Plectophora and Plectaniscus a three-sided pyramid, of Tetraplecta (Pl. [91], fig. 3) a tetrahedron, and in many other species a polyhedron of more or less regular form. Some species of Plectanida become very similar to certain species of Stephoidea, Spyroidea, and Cyrtoidea; so Plectaniscus and Periplecta approach to Cortina and Cortiniscus, Pteroscenium and Clathrocorys, &c. (compare Pls. [92], [93], [53], [64], &c.). They may represent a true phylogenetic connection between both groups. But in these cases also the distinction is determined by the fact that the true Plectoidea never possess a complete sagittal ring (like the Stephoidea) nor a regular lattice-shell (like the Spyroidea, Botryodea, and Cyrtoidea).
The Central Capsule of the Plectoidea constantly exhibits the peculiar characters of the Monopylea or Nassellaria. It is commonly ovate, more rarely ellipsoidal or even spherical, sometimes conical or lentelliptical. The lower or basal pole of its vertical main axis constantly exhibits the characteristic "porochora" (or the area porosa) of the Monopylea, and upon this rests the peculiar "podoconus" (or the pseudopodial-cone) of this legion. On this porochora the central capsule is in immediate connection with the central point of the skeleton, or the horizontal common central rod, from which the radial spines arise. The endoplasm, or the protoplasm of the central capsule (besides the podoconus), contains commonly one large alveole or several small vacuoles, and often pigment-granules. The nucleus is large, spherical or ovate, and exhibits the same character as in all the other Monopylea; it encloses usually a single nucleolus.
The position of the central capsule and its topographical relation to the skeleton offers in the different Plectoidea some important and as yet unsolved problems, which can be answered only by fresh and accurate observations on living specimens. In Triplagia and Triplecta, where the triangular skeleton lies in a horizontal plane, the vertical main axis of the central capsule is perpendicular to the central point of that supporting triangle. In Plagiacantha and Plectophora, where the three radial spines correspond to the edges of a flat pyramid, the capsule is enclosed in the pyramidal space of the latter, its basal pole touching the apex; therefore in the normal position of the body the three divergent rays are directed upwards. In Tetraplagia and Tetraplecta probably the same position is retained, and therefore the fourth free spine, here developed, is probably directed vertically downwards. In Plagoniscus and Plectaniscus, however, and moreover, in the closely allied Plagiocarpa and the corresponding Periplecta (Pl. [91], figs. 5, 10) the position of the central capsule, relative to the skeleton, seems to be inverse, and to agree with that of the Stephoidea (Cortina, Cortiniscus, &c.) and the Cyrtoidea (Pteroscenium, Clathrocorys, &c.); the three divergent spines are here directed downwards (as basal feet), whilst the opposite fourth spine is vertically directed upwards (as an apical horn); the capsule rests here upon the tripod, which lies below it, and is inclined with its dorsal face to the apical spine. In the majority of the other Plectoidea the position of the central capsule and its relation to the skeleton are not yet sufficiently observed, and require further accurate researches. Its position seems to be very different in the several genera. The capsule is never perforated by parts of the skeleton; this latter is constantly extra-capsular.
The physiological value of the skeleton, with regard to the central capsule, is different in the Plagonida and Plectanida; in the former it supports, in the latter it encloses the capsule like a shell. In the Nassellida, where no skeleton is developed, the central capsule is quite free and naked, enveloped only by the calymma.
The calymma or the extracapsular jelly-veil in all Plectoidea is voluminous, and encloses not only the central capsule completely, but also the skeleton wholly or partially. Its form is of the greatest value for the development and configuration of the skeleton. Sometimes the calymma is alveolate and foamy, as in Nassella and the common Thalassicolla. In several other Plectoidea the calymma seems to include numerous small vacuoles, sometimes also pigment-granules. Xanthellæ are commonly scattered in it in great numbers. The pseudopodia, arising in a large bunch from the porochora of the capsule, and running along the branches of the radial spines, seem to be always numerous, richly branched, and with a strong tendency to form anastomoses. The peculiar form of their network is often exactly preserved in the conformation of the skeleton, produced by them. The peculiarities of this network require further accurate observations, as does the whole organisation of the Plectoidea.
Synopsis of the Families of Plectoidea.
| I. Skeleton (originally tripodal) composed of radial spines united in a common centre and supporting the central capsule, without wicker-work, | 1. Plagonida. |
| II. Skeleton (originally tripodal) wattled, with irregular wicker-work, composed of the united branches of radial spines and enveloping the central capsule, | 2. Plectanida. |
Family XLVI. Plagonida, Haeckel.
Plagonida, Haeckel, 1881, Prodromus, p. 423.