Plagiacanthida (sensu strictiori), Richard Hertwig, 1879, Organismus d. Radiol., p. 72.
Definition.—Plectoidea with a spiny skeleton, composed of radial spines, which arise from a common central point or central rod, and support the free central capsule.
The family Plagonida comprises those Nassellaria in which the skeleton is only composed of united radial spines, arising from a common centre, without any connection of meeting branches of the spines; the rudimentary skeleton exhibits therefore neither a loose wickerwork (as in the closely allied Plectanida), nor a ring (as in the Stephoidea), nor a complete lattice-shell (as in the Cyrtellaria, the Spyroidea, Botryodea, and Cyrtoidea). The central capsule, which possesses all the characters of the Monopylea, is therefore free, not enclosed, and only on one side supported or partly protected by the radial spines or their branches.
Two species only of Plagonida have been hitherto known. The first form described is the Plagiacantha arachnoides, discovered thirty years ago (1855) by Claparède on the western shore of Norway. Another species of the same genus, from the Mediterranean, was very accurately described by Richard Hertwig in 1879 in his Organismus der Radiolarien (Plagiacantha abietina). Upon these two species the latter founded his family Plagiacanthida, a term which was afterwards employed by Bütschli and others, for the whole group of Plectoidea. Many new forms are contained in the collection of the Challenger, so that we may describe here nine genera and thirty-four species.
The family Plagonida may be divided into four different subfamilies, according to the numbers of the radial spines which compose the skeleton: Triplagida with three, Tetraplagida with four, Hexaplagida with six, and Polyplagida with numerous (seven to nine or more) radial spines. These are united commonly in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera Plagonidium, Plagiocarpa, and Plagonium) the spines arise in two opposite groups (each with two or three spines) from the two poles of a common central rod; in this case the basal pole of the central capsule with the porochora rests upon the horizontal common rod, which corresponds probably to the basal part of the sagittal ring of the Stephoidea and Cyrtellaria.
The different forms which the skeleton of the Plagonida assumes in the different genera of this family, and the important relations which these exhibit on the one hand to the spicula of the Beloidea, and on the other hand to the shell of some Stephoidea (Cortina, Cortiniscus, &c.) and Cyrtoidea (Pteroscenium, Clathrocorys, &c.), have been already pointed out in the preceding description of the suborder Plectoidea. There it is also demonstrated, that all these different forms may be derived from the simplest triradial forms, Triplagia and Plagiacantha (compare above, pp. [900]-[904]).
Whilst the genera of the Plagonida are characterised by the number of the radial spines and the peculiar mode of junction in a common central point or at the two poles of a common central rod, the different species of this family may be defined by the peculiar form of the spines and their branches. These morphological characters have also been already described above. We repeat here only that the radial spines in the majority of species are three-sided prismatic and verticillate, each verticil commonly with three branches. The distal ends of these branches remain constantly free, and are never united, as is always the case in the following family.
The Central Capsule of the Plagonida exhibits the general characters of all Monopylea (compare above, p. [890]). It is commonly ovate or ellipsoidal, with vertical main axis; on the lower pole of the latter is the porochora (or the "area porosa," from which all pseudopodia radiate). This is in immediate connection with the central point or central rod of the skeleton, in which its radial spines are united. The topographical relation of the supporting skeleton to the central capsule seems to exhibit in the different genera of the Plagonida remarkable differences, as already demonstrated above (p. [905]).
Synopsis of the Genera of Plagonida.
I. Subfamily Triplagida. Three radial spines. |  | Three spines lying in one horizontal plane, | 384. Triplagia. | ||
| Three spines corresponding to the edges of a flat pyramid, | 385. Plagiacantha. | ||||
II. Subfamily Tetraplagida. Four radial spines. |  | Four spines arising from one common central point. |  | All four spines equal, | 386. Tetraplagia. |
| One apical spine opposed to three basal spines, | 387. Plagoniscus. | ||||
| Four spines arising in two pairs from the poles of a common central rod. | | All four spines equal, | 388. Plagonidium. | ||
| One apical spine opposed to three basal spines, | 389. Plagiocarpa. | ||||
III. Subfamily Hexaplagida. Six radial spines. | | Six spines arising from one common central point, | 390. Hexaplagia. | ||
| Six spines arising in two opposite groups from the poles of a common central rod, | 391. Plagonium. | ||||
IV. Subfamily Polyplagida. Numerous radial spines. |  | Numerous (seven to nine or more) radial spines arising from a common centre (either a central point or a branched rod), | 392. Polyplagia. | ||