1. Nassella thalassicolla, n. sp.

Central capsule spherical. Podoconus with simple circular porochora, half as long as the capsule. Nucleus spherical. Numerous oil-globules in the endoplasm. Calymma spherical, without pigment, with numerous xanthellæ and large alveoles.

Dimensions.--Diameter of the central capsule 0.12, nucleus 0.04, calymma 0.6.

Habitat.--South Pacific, Station 300 (off Juan Fernandez), surface.

2. Nassella nassiterna, n. sp.

Central capsule ovate. Podoconus two-thirds as long as the capsule, trifid, with three equal circular lobes (as in Cystidium inerme). Nucleus ovate. Three equal large oil-globules in the endoplasm, corresponding to the three lobes of the porochora. Calymma ovate, in the upper half much more voluminous than in the lower, including numerous large alveoli and xanthellæ, and around the mouth masses of black pigment.

Dimensions.--Diameter of the central capsule 0.1, nucleus 0.03, calymma 0.8.

Habitat.--Indian Ocean, Madagascar (Rabbe), surface.

----

Suborder II. PLECTOIDEA, Haeckel.

Plectoidea vel Plectida, Haeckel, 1881, Prodromus, p. 423. Plagiacanthida (sensu ampliori), R. Hertwig, 1879, Organismus d. Radiol., p. 72.

Definition.--NASSELLARIA with a rudimentary, originally tripodal skeleton, composed of radial spines, arising from one common central point or central rod; the spines are simple or branched, and the branches may form by concrescence of their meeting ends a loose wickerwork, but never a complete lattice-shell. Never a ring in the skeleton.

{899}The suborder #Plectoidea#, hitherto known by few species only of "Plagiacanthida," comprises a large number of interesting NASSELARIA, which belong partly to the simplest and most primitive forms of this legion. It may be divided into two different families, Plagonida and Plectanida. In the first family, Plagonida, the monopylean central capsule is supported by a simple or rudimentary skeleton, composed only of a variable number of radial spines united in a common centre. In the second family, Plectanida, the branches of these radial spines become united and form a loose irregular framework with wide meshes, partly enclosing the central capsule, but never a perfect lattice-shell.

The #Plectoidea# differ from the following suborder, the #Stephoidea#, in the absence of the ring, characteristic of the latter. Some slight traces, however, indicate a near affinity between the ringless #Plectoidea# and the ring-bearing #Stephoidea#. Both these suborders of #Plectellaria# differ from the closely allied #Cyrtellaria# (#Spyroidea#, #Botryodea#, and #Cyrtoidea#) in the absence of a complete lattice-shell. The morphological relation and phylogenetic affinity between the former and the latter have already been discussed in the preceding description of the legion NASSELLARIA (compare pp. 891-894).

The first known species of #Plectoidea# was observed in the North Atlantic (on the Norwegian shore) in 1855 by my late friend Edouard Claparède, and described and figured in his Études, &c. (1858), under the name Plagiacantha arachnoides. He considered it as a new genus of Acanthometrina. Another species, from the Mediterranean, was described in the same year by Johannes Müller as Acanthodesmia dumetum (1858, loc. cit., Taf. i. fig. 3). A third species, also Mediterranean, was figured by me in 1865 under the name Acanthodesmia polybrocha. Finally, Richard Hertwig, 1879, in his Organismus der Radiolarien, gave a very accurate description of another Mediterranean form, Plagiacantha abietina (loc. cit., Taf. vii. fig. 6). He first recognised the true character of MONOPYLEA in their monaxonian central capsule, and observed at the same time the first NASSELLARIUM without skeleton, called by him Cyrtidium inerme (loc. cit., Taf. vii. fig. 1). To these four known species, representing three different genera, the rich collection of the Challenger has added so many new forms that we may distinguish here not less than seventeen genera and sixty-one species. In my Prodromus (1881, p. 423) I arranged these in two subfamilies, the Plagonida and Plectanida, constituting together the family Plectida (identical with the "Plagiacanthida" of Hertwig and Bütschli). But at present, regarding the important relations of these Plectida to the other NASSELLARIA, it seems more convenient to give to them the rank of an independent suborder of Radiolaria, under the name #Plectoidea#.

The peculiar structure of the central capsule of the #Plectoidea#, first recognised by Richard Hertwig, allows no doubt of their being true MONOPYLEA or NASSELLARIA; and also their siliceous, originally triradiate skeleton indicates the nearest affinity to {900}the other families of this legion. But a very difficult and as yet unsolved problem is the important question, in what manner these different groups of NASSELARIA are phylogenetically connected. Either the #Plectoidea#--as the simplest of all--are the original common ancestral group of this whole legion (as I assumed in my Prodromus, 1881), or they are derived from the #Stephoidea# (by reduction of the sagittal ring), or they have originated independently from them (if we suppose a polyphyletic origin of the MONOPYLEA. Compare above, p. 893, &c.). In any case the typical "triradial structure" of the #Plectoidea#, prevalent also in the other groups of this legion, is a very important and interesting fact.

The triradial skeleton of the #Plectoidea# exhibits in the two families of Plagonida and Plectanida a complete homology of development, so that each genus of the latter may be derived from a corresponding genus of the former, simply arisen by concrescence or union of the branches of the radial spines. Therefore the only difference between the two closely allied families is, that the branches of the radial spines in the Plagonida remain free, whilst in the Plectanida they produce a loose framework or wickerwork by union of their meeting ends. We express this complete homology in the nomenclature of the #Plectoidea#, in each genus of Plagonida retaining the syllable "Plag-"; in each genus of Plectanida, correspondingly, the syllable "Plect-."

The number of radial spines composing the skeleton is originally three, and in all not triradial genera is probably derived from three. For better survey we may divide each family, according to the different number of rays, into four different subfamilies: A, with three radial spines (Triplagida and Triplectida); B, with four radial spines (Tetraplagida and Tetraplectida); C, with six radial spines (Hexaplagida and Hexaplectida); and D, with numerous (seven to nine or more) radial spines (Polyplagida and Polyplectida). The last three subfamilies have arisen probably from the first triradial subfamily, by a secondary increase in the number of rays.

The important signification of the triradial structure, recurring in the most different groups of NASSELLARIA, has been already pointed out sufficiently by myself and by R. Hertwig. But the triradial #Plectoidea# offer also another interesting relation of this characteristic structure, some simple forms of this order appearing nearly identical with the isolated triradial spicula of certain #Beloidea# (Thalassosphærida and Sphærozoida). Even some more complex quadriradial and sexradial forms of the latter reappear in exactly the same shape also in the former. This identity may be perhaps an important indication of true affinity (compare below).

The simplest and probably the most original kind of triradial structure is exhibited by the genera Triplagia and Triplecta (Pl. 91, figs. 2, 7). Here three equal radial spines lie in one horizontal plane and are united in a common central point at equal angles, so that three lines connecting their distal ends form a regular equilateral triangle. Simple triradial spicula of the same regular form are also found in many {901}#Beloidea# (Lampoxanthium, Sphærozoum, &c., Pls. 2 and 4). The central capsule of these simplest #Plectoidea# (with vertical main axis) rests perpendicularly on the horizontal triangle, formed by the triradial skeleton; the porochora of the former (or the "area porosa") rests upon the central point of the latter.

Another kind of triradial structure characterises the genera Plagiacantha and Plectophora. The three radial spines united in the central point lie here not in one plane, but diverge in different planes, so that they correspond to the three lateral edges of a three-sided pyramid. Commonly the three spines are of equal size, and also the angles between them equal, so that the pyramid is regular, sometimes very flat, at other times more elevated. Spicula of exactly the same form are also found in some #Beloidea#. Probably the three divergent spines are homologous to the three basal feet of numerous #Spyroidea# and #Cyrtoidea#. The central capsule, according to Hertwig, is placed in the apical part of the pyramid, the axes of both being identical, and the porochora resting in the apex itself. This fact seems to contradict the above-mentioned affinity; but since in Triplecta and Triplagia the three spines lie horizontally, they may have changed this original position in different direction, in Plagiacantha and Plectophora becoming divergent upwards, whereas in Plagoniscus and Plectaniscus (as in the #Spyroidea# and #Cyrtoidea#) directed downwards.