Chapter II: Front Matter (2)

_Definition._--#Plagonida# with three radial spines.

Genus 384. _Triplagia_,[3] Haeckel, 1881, Prodromus, p. 423.

_Definition._--#Plagonida# with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus _Triplagia_ and the following closely allied _Plagiacantha_ may be regarded as the simplest and most primitive forms of the #Plectoidea#, perhaps as the common ancestral stock of this suborder. The skeleton is composed of three simple or branched radial spines, arising from one common central point. These three spines in _Triplagia_ lie in one and the same plane, whilst in _Plagiacantha_ they lie in different planes. Therefore the former exhibits the simplest type of the triradial structure, common to the majority of NASSELLARIA.

{909}1. _Triplagia primordialis,_ n. sp. (Pl. 91, fig. 2).

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, each with two pairs of opposite lateral branches, which are correspondingly parallel to the two other spines; the proximal branches twice as long as the distal branches.

_Dimensions._--Length of each spine 0.2, of the basal branches 0.07.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

2. _Triplagia triradialis,_ n. sp.

Spines straight, of equal size and similar form, equidistant, three-sided prismatic, regularly pinnate, with six pairs of opposite pinnulæ, tapering gradually towards the distal ends; the basal pinnulæ are again branched, with straight ramules.

_Dimensions._--Length of each spine 0.27, of the basal branches 0.08.

_Habitat._--North Pacific, Station 256, surface.

3. _Triplagia horizontalis,_ n. sp.

Spines curved, cylindrical, irregularly branched, with three primary and nine to twelve secondary branches, which are also slightly curved. The angles between the bases of the three spines are equal, but the size and form of the branches different.

_Dimensions._--Length of the three spines 0.15 to 0.25, of the basal branches 0.05 to 0.08.

_Habitat._--West Tropical Pacific, Station 226, depth 4475 fathoms.

Genus 385. _Plagiacantha_,[4] Claparède, 1856, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, November 13.

_Definition._--#Plagonida# with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus _Plagiacantha_ agrees with the preceding _Triplagia_ in the simple structure of the triradial skeleton, composed of three diverging radial spines, which are united in a common centre at the oral pole of the central capsule. But whilst the three radial rods of _Triplagia_ lie in one horizontal plane, here they lie in different planes and correspond to the three edges of a flat pyramid. _Plagiacantha arachnoides_, described in 1856 by Claparède, was the earliest known form of all #Plectoidea#.

{910}1. _Plagiacantha arachnoides_, Claparède.

_Plagiacantha arachnoides_, Claparède, 1856, Monatsber. d. k. preuss.
Akad. d. Wiss. Berlin, November 13.

_Plagiacantha arachnoides_, Claparède, 1858, Études sur les Infusoires et
les Rhizopodes, p. 462 (pl. xxii. fig. 8).

_Acanthometra arachnoides_, Claparède, 1855, Monatsber. d. k. preuss.
Akad. d. Wiss. Berlin, p. 675.

Spines straight, cylindrical, divided into three divergent straight branches of equal size; each branch two to three times as long as the simple thicker basal part.

_Dimensions._--Length of the spines 0.12 to 0.2 of the branches 0.08 to 0.14.

_Habitat._--North Atlantic, coast of Norway, Claparède, surface.

2. _Plagiacantha furcata_, n. sp.

Spines straight, cylindrical, divided into two divergent straight branches of equal size, of about the same length as the simple basal part. The spines and their branches are smooth.

_Dimensions._--Length of the spines 0.24, of their fork-branches 0.12.

_Habitat._--North Pacific, Station 254, surface.

3. _Plagiacantha dodecantha_, n. sp.

Spines straight, three-sided prismatic, in the middle with one verticil composed of three divergent straight branches of equal size, of about the same length as the simple basal part. The edges of the spines and their branches are thorny.

_Dimensions._--Length of the spines 0.24, of the branches 0.1.

_Habitat._--Central Pacific, Station 274, surface.

4. _Plagiacantha abietina_, Richard Hertwig.

_Plagiacantha abietina_, Richard Hertwig, 1879, Organismus d. Radiol., p.
72, Taf. vii. figs. 6-6_b_.

Spines straight, three-sided prismatic, with three verticils of three divergent straight lateral branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the second, and four times as long as those of the third verticil. All nine branches of each spine simple, straight, three-sided pyramidal.

_Dimensions._--Length of the spines 0.2 to 0.3, of the basal branches 0.06 to 0.08.

_Habitat._--Mediterranean, Messina (R. Hertwig), surface.

5. _Plagiacantha verticillata_, n. sp.

Spines curved, three-sided prismatic, with six to nine verticils of three divergent branches, decreasing in size towards the distal end; the branches of the first verticil about twice as long as those of the fourth verticil. All branches simple, slightly curved, bristle-shaped.

_Dimensions._--Length of the spines 0.27, of the basal branches 0.12.

_Habitat._--South Pacific, Station 296, surface.

{911}6. _Plagiacantha elatine_, n. sp.

Spines straight, three-sided prismatic, with prominent wing-like edges and ten to twelve verticils of three divergent branches, decreasing in size towards the distal end; the branches of the three or four basal verticils are again ramified and much larger than the simple branches of the distal verticils.

_Dimensions._--Length of the spines 0.36, of the basal branches 0.14.

_Habitat._--Central Pacific, Station 268, depth 2900 fathoms.

Subfamily 2. TETRAPLAGIDA, Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with four radial spines.

Genus 386. _Tetraplagia_,[5] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with four equal radial spines, arising from one common central point, and corresponding to the four axes of a tetrahedron.

The genus _Tetraplagia_ is one of the most important #Plectoidea#, and possibly the common ancestral form of many NASSELLARIA (compare above, p. 901). The skeleton is composed of four radial rods, diverging from one common centre in different directions, and corresponding to the four axes, which extend from the central point of a tetrahedron to the central points of its four faces. The whole skeleton of _Tetraplagia_ exhibits therefore the same form, which is observed in the single quadriradiate spicula of some #Beloidea# (in some species of _Lampoxanthium_, _Sphærozoum_, &c.).

1. _Tetraplagia geometrica_, n. sp.

Spines straight, cylindrical, perfectly equal, corresponding in regular disposition exactly to the four geometrical axes of a regular tetrahedron; in the middle arise from each spine two opposite equal lateral branches of half the length of the spine.

_Dimensions._--Length of the spines 0.12, of the branches 0.06.

_Habitat._--Central Pacific, Station 266, surface.

2. _Tetraplagia phænaxonia_, n. sp. (Pl. 91, fig. 3).

Spines straight or slightly curved, three-sided prismatic, with irregular short branches arising from the three edges; the branches are thorny, tapering towards the apex.

_Dimensions._--Length of the spines 0.15, of the basal branches 0.03.

_Habitat._--Tropical Atlantic, Station 332, surface.

{912}3. _Tetraplagia abietina_, Haeckel.

_Plagiacantha abietina_, var. _quadrispina_, Richard Hertwig, 1879,
Organismus d. Radiol., p. 73.

Spines straight, three-sided prismatic, verticillate, with six to eight verticils of three simple straight branches; the branches of each edge are parallel, tapering towards the distal end. R. Hertwig regards this species only as a four-spined variety of his three-spined _Plagiacantha abietina_; but a specimen, observed by me in Corfu, exhibited all the characters of _Tetraplagia_.

_Dimensions._--Length of the spines 0.2, of the basal branches 0.07.

_Habitat._--Mediterranean (Messina, Corfu), surface.

Genus 387. _Plagoniscus_,[6] n. gen.

_Definition._--#Plagonida# with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.

The genus _Plagoniscus_ agrees with the preceding _Tetraplagia_ (its probable ancestral form) in the possession of four radial spines, diverging from one common central point. But whilst in this latter all four spines are equal, corresponding exactly to the four axes of a tetrahedron, here in _Plagiocarpa_ an important difference exists between one vertical or apical spine and three other divergent basal spines; these latter corresponding probably to the three "feet," the former to the single "apical horn" of the majority of NASSELLARIA. Perhaps we find here one of the oldest and simplest types of their "triradial or cortinar structure" (compare above, p. 902).

1. _Plagoniscus tripodiscus_, n. sp. (Pl. 91, fig. 4).

Spines three-sided prismatic, thorny. Apical spine nearly straight, verticillate, with four to five verticils of three thorny branches, tapering towards the apex. Three basal spines somewhat shorter, curved, with three thorny edges.

_Dimensions._--Length of the apical spine 0.2, of the basal spines 0.15.

_Habitat._--Central Pacific, Station 263, surface.

2. _Plagoniscus euscenium_, n. sp.

Spines three-sided prismatic, thorny, with dentate edges, and three to six verticils of three short branches. Apical spine straight, with six verticils, nearly twice as long as the three curved basal spines, each of which bears three verticils; the basal verticils larger and ramified. Resembles somewhat Euscenium eucolpium, Pl. 53, fig. 12, but has no latticed shell.

_Dimensions._--Length of the apical spine 0.3, of the basal spines 0.16.

_Habitat._--North Pacific, Station 247, surface,

{913}3. _Plagoniscus cortinaris_, n. sp.

Spines three-sided prismatic, straight, verticillate. Apical spine with twelve verticils, one and a half times as long as the three basal spines, each of which bears eight verticils of three branches. The branches are also prismatic, straight, on each edge parallel, tapering towards the apex, in the three basal spines forked, in the apical spine more branched. Similar to the cortinar skeleton of _Clathrocorys_ (Pl. 64, figs. 8-10).

_Dimensions._--Length of the apical spine 0.24, of the basal spines 0.16.

_Habitat._--South Atlantic, Station 338, surface.

4. _Plagoniscus nassellaris_, n. sp.

Spines cylindrical, curved, irregularly branched. Apical spine half as long as, and less branched than the three basal spines, which are sigmoidal, nearly horizontally expanded in the proximal half, descending in the distal half.

_Dimensions._--Length of the apical spine 0.1, of the three basal spines 0.18.

_Habitat._--West Tropical Pacific, Station 224, surface.

Genus 388. _Plagonidium_,[7] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with four equal radial spines, arising in pairs from the two poles of a common central rod.

The genus _Plagonidium_, and the following _Plagiocarpa_ possess four radial spines, like the two preceding genera. But whilst the four rods in these latter arise from a common centre, here they arise in two pairs from the two poles of a common middle rod; they have therefore exactly the same form which we find in the single spicula of some #Beloidea# (e.g., _Thalassoxanthium furcatum_, _Sphærozoum furcatum_, &c.). Probably the middle rod is horizontal and serves as supporting base for the central capsule, whilst two opposite spines are directed upwards, two other downwards.

1. _Plagonidium bigeminum_, n. sp.

Spines straight, three-sided prismatic, four to six times as long as the common central rod, pinnate, with four to five pairs of opposite pinnulæ, the distal of which are simple, the proximal again branched.

_Dimensions._--Length of the spines 0.16, of the middle rod 0.032.

_Habitat._--Indian Ocean, Sunda Strait (Rabbe), surface.

{914}2. _Plagonidium quadrigeminum_, n. sp.

Spines cylindrical, curved, eight to ten times as long as the common central rod, in the distal half forked; the fork-branches curved, somewhat longer than the basal part.

_Dimensions._--Length of the spines 0.18, of the middle rod 0.02.

_Habitat._--Antarctic Ocean, Kerguelen Island, Station 159, surface.

Genus 389. _Plagiocarpa_,[8] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus _Plagiocarpa_ agrees with the preceding _Plagonidium_ in the possession of a common horizontal middle rod, the two poles of which bear two pairs of divergent spines; but whilst in the preceding all four spines are equal, here they are differentiated in the same manner as in _Plagoniscus_, which differs only in the absence of the middle rod. The two observed and closely allied species of this genus are of peculiar interest, since they belong possibly to the common ancestral forms of the NASSELLARIA; the basal middle rod corresponds perhaps to the basal part of a sagittal ring, the apical spine to its dorsal part, the three other spines to the basal feet (compare above, p. 902).

1. _Plagiocarpa procortina_, n. sp. (Pl. 91, fig. 5).

Spines cylindrical, curved, thorny, three to four times as long as the common middle rod. Apical spine and the meeting caudal spine (or the posterior basal spine) somewhat longer and with more numerous thorns than the two paired pectoral (or anterior) spines. From the common base of the latter arises an anterior prolongation of the horizontal middle rod, which in the sagittal plane is curved upwards and corresponds to the sternal foot of many #Cyrtellaria#. An ascending branch of this spine is opposed to a descending branch of the apical spine, both together forming an incomplete sagittal ring.

_Dimensions._--Length of the two larger spines 0.15, of the two smaller 0.12.

_Habitat._--Mediterranean (Portofino near Genoa), surface.

2. _Plagiocarpa procyrtella_, n. sp.

Spines of form and arrangement similar to those of the preceding species, but longer and more branched, six to eight times as long as the shorter common middle rod. The two characteristic {915}opposed branches (the ascending branch of the basal rod and the descending branch of the apical spine), which in the preceding species nearly compose a sagittal ring, are here absent.

_Dimensions._--Length of the two larger 0.27, of the two smaller 0.21.

_Habitat._--North Atlantic, Iceland (Steenstrup), surface.

Subfamily 3. HEXAPLAGIDA, Haeckel.

_Definition._--#Plagonida# with six radial spines.

Genus 390. _Hexaplagia_,[9] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with six radial spines, arising from one common central point.

The genus _Hexaplagia_ differs from the preceding genera of Plagonida in the possession of six radial spines, diverging from one common central point. Commonly, these six spines seem to lie opposite in pairs in three different meridian planes, and in this case _Hexaplagia_ may be regarded as a _Plagiacantha_, the three radial spines of which are prolonged over the basal pole of the central capsule. But in other species the six spines seem to lie in different planes. Further observations are required.

1. _Hexaplagia arctica_, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, straight, three-sided prismatic, with dentate edges. The three pairs are equal, but the upper spine of each pair only half as long as the lower spine.

_Dimensions._--Length of the upper spine 0.13, of the lower 0.27.

_Habitat._--Arctic Ocean (Greenland), in the stomach of a Medusa (Olrik).

2. _Hexaplagia antarctica_, n. sp.

Spines opposite in pairs in three diameters, crossed in the common centre, slightly curved, three-sided prismatic, with verticillate branches; the three pairs are equal, but the lower spine of each pair bears three to four verticils, each of three branches, and is two to three times as long as the upper spine, which bears two verticils only.

_Dimensions._--Length of the upper spine 0.12, of the lower 0.3.

_Habitat._--Antarctic Ocean, Station 157, depth 1950 fathoms.

{916}3. _Hexaplagia collaris_, n. sp.

Spines in opposite pairs in three diameters, crossed in the common centre, straight, cylindrical, pinnate, with three to four pairs of opposite lateral branches or regular pinnulæ. All six spines are equidistant, of equal size and similar form, and lie with their branches nearly in one horizontal plane. Therefore they are similar to the six radial rods in the collar septum of many #Spyroidea# and #Cyrtoidea# (_e.g._, _Calpophæna_, Pl. 53, fig. 18).

_Dimensions._--Length of all six spines 0.18, of their basal pinnulæ 0.04.

_Habitat._--East of New Zealand, Station 169, surface.

4. _Hexaplagia australis_, n. sp.

Spines unequal, at unequal intervals, not opposite in pairs, cylindrical, irregularly branched and curved. In this species a definite arrangement of the six different spines could not be detected.

_Dimensions._--Length of the spines 0.2 to 0.3, of their largest branches 0.1.

_Habitat._--South of Australia, Station 160, surface.

Genus 391. _Plagonium_,[10] Haeckel, 1881, Prodromus, p. 423.

_Definition._--#Plagonida# with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus _Plagonium_ differs from the preceding closely allied _Hexaplagia_ in the remarkable peculiarity, that the six radial spines do not arise from one common central point, but from the two poles of a common horizontal middle rod; three divergent spines on each pole. The skeleton of _Plagonium_ exhibits therefore the same remarkable form which is found in the isolated spicula of numerous #Beloidea# (_e.g._, _Lampoxanthium punctatum_, _Sphærozoum punctatum_), and bears the same relation to _Hexaplagia_ that _Plagonidium_ does to _Tetraplagia_.

1. _Plagonium sphærozoum_, n. sp. (Pl. 91, fig. 6).

Spines straight and stout, three-sided prismatic, about twice as long as the common middle rod, irregularly branched or nearly verticillate, with short thorny branches. Similar to the single spicula of some species of _Sphærozoum_ and _Lampoxanthium_.

_Dimensions._--Length of the spines 0.12, of the middle rod 0.06.

_Habitat._--Equatorial Atlantic, Station 347, surface.

{917}2. _Plagonium lampoxanthium_, n. sp.

Spines irregularly curved, slender, cylindrical, six to eight times as long as the common middle rod, in the proximal half smooth, in the distal half covered with short thorns. (Similar to an isolated spiculum of _Lampoxanthium punctatum_ or of _Sphærozoum variabile_, Pl. 4, fig. 5.)

_Dimensions._--Length of the spines 0.18, of the middle rod 0.03.

_Habitat._--North Pacific, Station 240, surface.

3. _Plagonium arborescens_, n. sp.

Spines irregularly curved and branched, slender, cylindrical, thorny, twelve to sixteen times as long as the middle rod; the branches are large, arborescent, their ramules again ramified and very thorny.

_Dimensions._--Length of the spines 0.2 to 0.25, of the middle rod 0.02.

_Habitat._--Indian Ocean, Madagascar (Rabbe), surface.

4. _Plagonium trigeminum_, n. sp.

Spines straight, slender, cylindrical, four to six times as long as the middle rod, in the basal half smooth, in the distal half arborescent, with three to four irregular verticils of ramified branches. (Similar to a single spiculum of _Sphærozoum verticillatum_, Pl. 4, fig. 7.)

_Dimensions._--Length of the spines 0.15 to 0.2, of the middle rod 0.05.

_Habitat._--Central Pacific, Station 265, depth 2900 fathoms.

5. _Plagonium distriactis_, n. sp.

Spines straight and stout, three-sided prismatic, smooth, ten to twelve times as long as the middle rod; each on the distal end cleft into three divergent straight branches, which are again trichotomous.

_Dimensions._--Length of the spines 0.15 to 0.18, of the middle rod 0.015.

_Habitat._--South Pacific, Station 288, surface.

Subfamily 4. POLYPLAGIDA, Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plagonida# with numerous (seven to nine or more) radial spines.

Genus 392. _Polyplagia_,[11] n. gen.

_Definition._--#Plagonida# with numerous (seven to nine or more) radial spines, arising from a common centre and lying in different planes.

{918}The genus _Polyplagia_ alone represents the small subfamily of Polyplagida, distinguished from the other Plagonida by the multiplication of the radial spines, the number of which amounts to seven to nine or more. This increased number is commonly the result of an intercalation of new spines between the three or four primary spines; it is sometimes also effected by stronger development of branches of the latter, which become independent. The following five species of this genus are very different, require further investigation, and perhaps represent different genera:--

1. _Polyplagia septenaria_, n. sp.

Seven radial spines, straight, three-sided prismatic, verticillate, of different sizes; four larger spines correspond to the four axes of a tetrahedron (running from the centre to the four corners), each with five to six verticils of three simple slender branches; one of these four main spines seems to be the apical, the three others the basal spines of _Plagiocarpa_; in the three meridian planes between the latter and the former lie the three smaller spines, diverging upwards, each with two to three verticils. (Similar to _Polyplecta heptacantha_, Pl. 91, fig. 12, but without connection between the branches.)

_Dimensions._--Length of the four major spines 0.26, of the three minor 0.11.

_Habitat._--Central Pacific, Station 274, surface.

2. _Polyplagia octonaria_, n. sp.

Eight radial spines, straight, three-sided prismatic, of equal size, arising in two opposite groups from the two poles of a short common middle rod (as in _Sphærozoum arborescens_, Pl. 4, fig. 8, and in other #Beloidea#). The four spines of each group are divergent, six to eight times as long as the middle rod, each spine armed with three to four verticils of thorny branches.

_Dimensions._--Length of the spines 0.15, of the middle rod 0.022.

_Habitat._--Indian Ocean, Cocos Islands (Rabbe), surface.

3. _Polyplagia novenaria_, n. sp.

Nine radial spines of equal size, straight, cylindrical, lying nearly in a horizontal plane, or forming a very flat pyramid. Near the common central point every three spines arise from a short common rod, so that the centre of the skeleton is triradial. Each spine bears towards the apex two divergent straight lateral branches. This species may have been derived from _Plagiacantha arachnoides_ by shortening and reduction of the basal parts of the three original branched primary spines.

_Dimensions._--Length of the nine spines 0.24, of the three basal rods 0.02.

_Habitat._--North Atlantic, Færöe Channel (Gulf Stream), John Murray, surface.

4. _Polyplagia duodenaria_, n. sp.

Twelve radial spines of equal size, arising from a common central point, and diverging in different directions. The twelve spines are very large, opposite in six pairs, cylindrical, longitudinally {919}striped (the expression of concentric lamellæ), and with spinulate surface, covered with innumerable small thorns. The basal quarter of each spine is straight and simple, the second quarter twice forked, and these four fork-branches are again in the outer half of the spine richly forked or ramified, with diverging, slightly curved thin branches; each of the twelve spines with about sixty to eighty terminal branches, the ends of which seem to fall into a spherical face. The position of this remarkable species in this family is doubtful.

_Dimensions._--Length of the spines 0.12 to 0.16, of the simple basal part 0.04.

_Habitat._--South Pacific (off Juan Fernandez), Station 299, surface.

5. _Polyplagia viminaria_, n. sp.

Numerous (sixteen to twenty or more) radial spines of about equal size, arising from a common central point and diverging in different directions, richly and more or less irregularly branched. The ends of the numerous small branches seem to fall into a spherical face. The large spines of this species have the same form and structure as in the preceding, nearly allied species, but are more numerous and more irregularly branched and disposed.

_Dimensions._--Length of the spines 0.2 to 0.25, of the simple basal part 0.05.

_Habitat._--North Pacific, Station 241, surface.

Family XLVII. #Plectanida#, Haeckel.

_Plectanida_, Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectoidea# with a wattled skeleton, composed of the meeting and united branches of radial spines, which arise from a common central point or central rod, and protect the partly enclosed central capsule.

The family #Plectanida# comprises those NASSELLARIA in which the skeleton is composed of radial spines, arising from a common centre, and of a loose wickerwork, produced by concrescence of the meeting branches of those spines. This rudimentary wattled skeleton is either quite irregular or only slightly regular, but it never assumes the form of a complete lattice-shell, as in the #Cyrtellaria# (the #Spyroidea#, #Botryodea#, and #Cyrtoidea#), nor does it exhibit a ring (as in the #Stephoidea#). The central capsule is partly or wholly protected, and often entirely enclosed by the wattled skeleton.

Three species only of Plectanida have been hitherto known. The first described form is _Plectophora arachnoides_, which its discoverer Claparède observed in a living state in 1855 on the western coast of Norway, and considered as a mere variety of his _Plagiacantha arachnoides_. Two other species were afterwards observed in the Mediterranean, _Polyplecta dumetum_, 1856, by Johannes Müller (united by him with _Acanthodesmia_) and _Polyplecta polybrocha_ by myself in 1864. Many new forms are found in the {920}collection of the Challenger, so that we may here distinguish eight genera and twenty-seven species.

The family Plectanida may be divided into four different subfamilies, according to the number of the radial spines which compose the skeleton. Those four subfamilies, as also the genera contained in them, correspond exactly to the four subfamilies of Plagonida, from which they have originated: Triplectida with three, Tetraplectida with four, Hexaplectida with six, and Polyplectida with numerous (seven to nine or more) radial spines. These spines are usually united (as in the corresponding ancestral Plagonida) in one common central point, upon which rests the basal pole of the central capsule, with the porochora. More rarely (in the genera _Periplecta_ and _Plectanium_) the spines arise in two opposite groups (each of two or three spines) from the two poles of a common horizontal central rod, which supports the overlying porochora of the central capsule.

The different genera of Plectanida, which are enumerated in the following synopsis, correspond so exactly to the different genera of Plagonida described above, that probably each of the former has arisen from a corresponding genus of the latter. The only difference between the two is, that in all Plagonida the branches and ramules of the radial spines end freely, without growing together, whilst in all Plectanida the meeting ends of the neighbouring spines grow together and so produce a loose and irregular wickerwork. Therefore the latter constantly possess meshes, which are missing in the former. As soon as any form of Plagonida begins to produce meshes by concrescence of meeting branches of the spines, it passes over into a corresponding form of Plectanida. The loose wickerwork or wattlework so produced is sometimes very irregular, at other times more or less regular; but it never assumes the regular form of a complete lattice-shell, as in the #Cyrtellaria# (#Spyroidea#, #Botryodea#, and #Cyrtoidea#). The general differences which this wickerwork exhibits in the different genera of Plectanida have been already described above (compare pp. 900-904).

_The Central Capsule_ of the Plectanida agrees perfectly with that of the ancestral Plagonida, as well in general form and structure as in the peculiar topographical relation to the radial spines (compare above, p. 905). A slight difference between the two families is effected by the higher development of the skeleton in the Plectanida. Since the branches of the radial spines in this family become united and form a loose wickerwork, the central capsule becomes more or less enclosed by the latter, and more perfectly protected, than in the Plagonida, where the branches remain free. In this respect the Plectanida approach more nearly to the #Cyrtoidea# with which they are connected by such transitional forms as _Plectaniscus_ and _Periplecta_ on the one hand, _Cladoscenium_ and _Pteroscenium_ on the other.

{921}_Synopsis of the Genera of Plectanida._

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I. Subfamily Triplectida. Three radial spines.
-------------------------------------------------------------------------
Three spines lying in one horizontal plane, 393. _Triplecta_.

Three spines corresponding to the edges of a flat
pyramid, 394. _Plectophora_.
-------------------------------------------------------------------------
II. Subfamily Tetraplectida Four radial spines.
-------------------------------------------------------------------------
{ All four spines equal, 395. _Tetraplecta_.
Four spines arising from {
one common central { One apical spine opposed
point. { to three basal spines, 396. _Plectaniscus_.

Four spines arising in two pairs from the poles of
a common central rod; one apical spine different
common spines, 397. _Periplecta_.
-----------------------------------------------------------------
III. Subfamily Hexaplectida. Six radial spines.
-----------------------------------------------------------------
Six spines arising from one common central point, 398. _Hexaplecta_.

Six spines arising in two opposite groups from the
poles of a common control rod, 399. _Plectanium_.
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IV. Subfamily Polyplectida. Numerous radial spines.
-----------------------------------------------------------------
Numerous (seven to nine or more) radial spines
arising from a common centre (either a central
point or a branched rod), 400. _Polyplecta_.

Subfamily 1. TRIPLECTIDA, Haeckel, 1881, Prodromus, p. 424.

_Definition_.--#Plectanida# with three radial spines.

Genus 393. _Triplecta_,[12] Haeckel, 1881, Prodromus, p. 424.

_Definition_.--#Plectanida# with three radial spines, arising from one common central point and lying in one horizontal plane.

The genus _Triplecta_ is the simplest and most primitive of the Plectanida, and may therefore be regarded as the prototype of this family. The skeleton represents a triangular lattice-plate with three radial beams. On the central union of the latter rests the oral pole of the central capsule. Since the axis of the latter is vertical, the lattice-plate must be horizontal, serving for the expansion of the pseudopodia. _Triplecta_ has arisen from _Triplagia_ by union of the branches of its three radial spines. In the simplest case only three large meshes are formed, corresponding probably to the three cortinar meshes in the collar septum of many #Cyrtellaria#.

1. _Triplecta triangulum_, n. sp.

Spines straight, equal, smooth, cylindrical, each in the basal half with one pair of divergent straight lateral branches. The opposed branches of every two neighbouring spines are united by a {922}thin convex bow. In this way arises a very simple, equilateral triangular skeleton, with three equal large meshes.

_Dimensions._--Length of the spines 0.14, sides of the triangle 0.16.

_Habitat._--Tropical Atlantic, Station 348, depth 2450 fathoms.

2. _Triplecta triactis_, n. sp. (Pl. 91, fig. 7).

Spines straight, equal, three-sided prismatic, pinnate, each with three to four pairs of opposite slender pinnulæ or lateral branches; the pinnulæ of each spine are correspondingly parallel to the main rods of the other two spines. All pinnulæ branched and connected by a few slender bows, marking the sides of a regular triangle.

_Dimensions_.--Length of the spines 0.15, sides of the triangle 0.17.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

Genus 394. _Plectophora_,[13] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with three radial spines, arising from one common central point and corresponding to the edges of a three-sided pyramid.

The genus _Plectophora_ has been derived from _Plagiacantha_ by union of the meeting neighbouring branches of the three radial spines, and exhibits therefore to it the same relation that _Triplecta_ bears to _Triplagia_. The loose framework so produced corresponds to the three sides of a flat pyramid and supports the central capsule.

1. _Plectophora triomma_, n. sp.

Spines equal, straight, three-sided prismatic, thorny, connected by three convex bows, the meeting branches of lateral spines arising in one pair from the middle part of each spine. Therefore the skeleton exhibits three large ovate meshes only.

_Dimensions._--Length of the spines 0.12, of the meshes 0.08.

_Habitat._--North Pacific, Station 238, surface.

2. _Plectophora arachnoides_, Haeckel.

_Plagiacantha arachnoides_, var., Claparède, 1858, Études sur les
Infusoires et les Rhizopodes, p. 462, Taf. xxii. fig. 9.

Spines straight, cylindrical, trifid, with one pair of lateral branches. The nine diverging, straight, and smooth branches are connected in the distal part by slender concave bows, so that the whole skeleton exhibits nine wide meshes, three larger pentagonal and six smaller triangular.

_Dimensions._--Length of the spines 0.14, of their branches 0.07.

_Habitat._--North Atlantic, west coast of Norway (Claparède), surface.

{923}3. _Plectophora novena_, n. sp.

Spines slightly curved, three-sided prismatic, thorny, with two pairs of lateral branches. The six smaller distal branches end freely, whilst the six larger basal branches are connected by slender convex bows. There are therefore nine wide meshes, as in the preceding closely allied species.

_Dimensions._--Length of the spines 0.18, of the basal branches 0.08.

_Habitat._--North Atlantic, Færöe Channel (Gulf Stream) (John Murray), surface.

4. _Plectophora pyramidalis_, n. sp.

Spines straight, three-sided prismatic, with three to four verticils of short lateral branches. The branches of the basal verticils are again ramified, and form by connecting bows a delicate loose framework, covering the three sides of a flat pyramid, the three edges of which are the three radial spines.

_Dimensions._--Length of the spines 0.2, base of the pyramid 0.16.

_Habitat._--Central Pacific, Station 267, surface.

Subfamily 2. TETRAPLECTIDA, Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with four radial spines.

Genus 395. _Tetraplecta_,[14] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with four equal radial spines, arising from one common central point and corresponding to the four axes of a tetrahedron.

The genus _Tetraplecta_ has been derived from _Tetraplagia_ by union of the neighbouring branches of the four radial spines, diverging from a common point in different directions. In some forms of this genus the four rods seem to correspond exactly to the four axes, which are directed from the centre of a tetrahedron towards its four corners; whilst in other forms the four rods and the angles between them are perhaps not perfectly equal.

1. _Tetraplecta tetrahedra_, n. sp.

Spines straight, equal, three-sided prismatic, pinnate, each with three or four pairs of opposite straight slender pinnulæ or lateral branches; the pinnulæ of each side are correspondingly parallel. {924}All pinnulæ connected by a few slender bridges, thus producing a delicate network with irregular rhomboidal meshes.

_Dimensions._--Length of the spines 0.15, of the basal branches 0.05.

_Habitat._--Central Pacific, Station 272, depth 2600 fathoms.

2. _Tetraplecta quadricornis_, n. sp.

Spines in the basal half straight, in the distal half slightly curved and irregularly branched, with two to three pairs of unequal alternate lateral branches, the distal ends of which are connected by a few slender bows, marking the six edges of an irregular tetrahedron.

_Dimensions._--Length of the spines 0.22, of the basal branches 0.14.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

3. _Tetraplecta pinigera_, n. sp. (Pl. 91, fig. 8).

Spines three-sided prismatic, straight, or in the basal half slightly curved, verticillate; each spine with six to eight three-branched regular verticils, tapering gradually towards the distal end. All branches of each spine lie parallel in three equidistant meridian planes, and are connected by delicate parallel threads, perpendicular to the branches. Therefore the skeleton consists of four pine-shaped trees and twelve delicate triangular wings with rectangular meshes.

_Dimensions._--Length of the spines 0.25 to 0.3, of the basal branches 0.1 to 0.15.

_Habitat._--Central Pacific, Station 271, surface.

Genus 396. _Plectaniscus_,[15] nov. gen.

_Definition._--#Plectanida# with four unequal radial spines, arising from one common central point; one vertical or apical spine opposed to three divergent or basal spines.

The genus _Plectaniscus_ has probably been derived from _Plagoniscus_ by reticular union of the neighbouring branches of the four radial spines, and exhibits therefore to it the same relation that _Tetraplecta_ bears to _Tetraplagia_. Whilst in these two latter genera the four spines are equal, in the two former genera there is an important difference between a vertical spine (or apical horn) and three divergent (commonly larger) spines, corresponding to the three basal feet of the triradiate NASSELLARIA. Perhaps, therefore, _Plectaniscus_ is one of the remotest ancestors of the latter; but differs in the absence of a complete lattice-shell.

{925}1. _Plectaniscus cortiniscus_, n. sp. (Pl. 91, fig. 9).

Spines straight, three-sided prismatic, with three to four verticils of short perpendicular branches; the branches of the distal verticils are simple and free, those of the proximal verticils again ramified and connected by an arachnoidal network of delicate threads. The vertical apical spine (or horn) is scarcely one-third or half as long as the three divergent basal spines (or feet), and the angle between it and the latter is smaller.

_Dimensions._--Length of the apical spine 0.05 to 0.1, of the three basal spines 0.18 to 0.2.

_Habitat._--North Pacific, Station 244, surface.

2. _Plectaniscus tripodiscus_, n. sp.

Spines curved, cylindrical, with a variable number of irregular branches, which in the distal half of the spines are free, in the basal half again ramified and connected by a loose spongy framework. The apical spine is shorter and simpler, nearly straight, less ramified.

_Dimensions._--Length of the apical spine 0.1, of the three basal spines 0.15 to 0.18.

_Habitat._--North Pacific, Station 238, surface.

3. _Plectaniscus archiscenium_, n. sp.

Spines slightly curved, three-sided prismatic, with two to three verticils of short curved branches. The vertical apical spine is about twice as long as the three basal spines, and connected with them by an irregular loose framework. The three basal spines are connected together only by a simple ring, so that between them remain three large collar holes, like those of the similar and closely allied _Archiscenium quadrispinum_ (Pl. 53, fig. 11).

_Dimensions._--Length of the apical spine 0.32, of the three basal spines 0.18.

_Habitat._--South Pacific, Station 295, depth 1500 fathoms.

4. _Plectaniscus cladoscenium_, n. sp.

Spines curved, three-sided prismatic. The three basal spines pinnate, with four to five pairs of opposite curved pinnulæ; the distal pairs are simple and free, the basal pairs branched and connected by an irregular delicate framework, which together with the apical spine forms a spongy cap-shaped shell, similar to _Cladoscenium_. Apical spine about half as long, simple.

_Dimensions_.--Length of the apical spine 0.1, of the three basal spines 0.18.

_Habitat._--Central Pacific, Station 274, surface.

5. _Plectaniscus clathrocorys_, n. sp.

Spines straight, three-sided prismatic, with seven or eight verticils of ramified branches, which in the basal half are connected by an irregular loose framework. In the distal half each of the three divergent basal spines (or feet) is connected with the longer apical spine (or horn) by a {926}fenestrated triangular lattice wing. Therefore the skeleton becomes very similar to _Clathrocorys_ (Pl. 64, figs. 8-10), but wants the regular central cephalis.

_Dimensions._--Length of the apical spine 0.2, of the basal spines 0.14.

_Habitat._--Central Pacific, Station 271, depth 2425 fathoms.

Genus 397. _Periplecta_,[16] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with four unequal radial spines, arising in pairs from the two poles of a common central rod; one ascending apical spine opposed to three descending basal spines.

The genus _Periplecta_ has probably been derived from _Plagiocarpa_ (by concrescence of the meeting spine branches), and has the same important relation to a part of the triradial #Stephoidea# (_Cortina_, &c.) and #Cyrtoidea# (_Pteroscenium_, &c.). It differs from the latter only in the fact that the loose irregular framework connecting the bases of the four cortinar spines is not a regular lattice-shell.

1. _Periplecta cortina_, n. sp. (Pl. 91, fig. 10).

Spines of very different size and shape, three-sided prismatic. The apical spine (or horn) is larger, nearly straight, and bears a verticil of three large divergent branches, which are again ramified. The three basal spines (or feet) are curved, pinnate, each with three to five pairs of opposite simple curved branches. A small common central rod separates the two united pectoral feet from the two other spines (the caudal foot and the apical horn). The three basal feet are connected by three convex bows composing a horizontal collar ring, and since the central rod is prolonged between the pectoral feet to the ring, a very remarkable collar septum is formed with four cortinar meshes. Above this septum an irregular spongy shell, including the central capsule, is formed by a delicate framework, interwoven between the apical horn and the three basal feet.

_Dimensions._--Length of the apical spine 0.25 to 0.3, of the three basal spines 0.15 to 0.2.

_Habitat._--Central Pacific, Station 271, surface.

2. _Periplecta pteroscenium_, n. sp.

Spines of very different size and shape, three-sided prismatic, verticillate. The apical spine (or horn) with ten to twelve verticils, is one and a half times as long as the three basal spines (or feet), which bear six to eight verticils only; each verticil with three branches which by communicating ramules form a loose irregular wickerwork. Similar to _Pteroscenium pinnatum_, Pl. 53, figs. 14-16, but without a regular lattice-shell. A short basal central rod separates two equal (pectoral) spines from two unequal spines, the larger of the latter is the apical, the shorter the caudal spine.

_Dimensions._--Length of the apical spine 0.26, of the three basal spines 0.17.

_Habitat._--Central Pacific, Station 274, surface.

{927}3. _Periplecta monocyrtis_, n. sp.

Spines of nearly equal size and shape, cylindrical, irregularly branched. The apical spine is straight, more branched and nearly twice as long as the three basal spines. All four spines in the basal third connected by a loose spongy framework, approaching the form of some Monocyrtida.

_Dimensions._--Length of the apical spine 0.24, of the basal spines 0.13.

_Habitat._--Central Pacific, Station 274, surface.

Subfamily 3. HEXAPLECTICA, Haeckel.

_Definition._--#Plectanida# with six radial spines.

Genus 398. _Hexaplecta_,[17] Haeckel, 1881, Prodromus, p. 425.

_Definition._--#Plectanida# with six radial spines, arising from one common central point.

The genus _Hexaplecta_ has probably been derived from _Hexaplagia_ by concrescence of the meeting branches of the six spines, and therefore has to it the same relation that _Plectophora_ bears to _Plagiacantha_. The two species here described represent perhaps two different genera, since in the first all six spines are equal, opposed in pairs, in the second different in pairs (three upper smaller and three lower larger).

1. _Hexaplecta triaxonia_, n. sp.

Spines equal, opposite in three crossed pairs, straight, three-sided prismatic, thorny, in the distal half with three leaf-shaped, dentate edges, in the basal half with three verticils of slender branches, which are connected by parallel threads, and so form an arachnoidal network with rectangular or rhomboidal meshes.

_Dimensions._--Length of the spines 0.22, of their basal branches 0.06.

_Habitat._--South Pacific, Station 285, depth 2375 fathoms.

2. _Hexaplecta tricladonia_, n. sp.

Spines unequal, three larger spines (in the lower half of the body) being opposite to three smaller spines (in the upper half). The latter are nearly horizontally expanded, each bearing three straight divergent branches. The larger are stronger, twice as long and armed with three verticils, each of three branches. All six spines are slender, three-sided prismatic. Their branches are very thin, thread-shaped, and form by their union a loose network with irregular polygonal meshes.

_Dimensions._--Length of the larger spines 0.25, of the smaller 0.12.

_Habitat._--South Pacific, Station 291, surface.

{928}Genus 399. _Plectanium_,[18] Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with six radial spines, arising in two opposite divergent groups from the two poles of a common central rod.

The genus _Plectanium_ has been derived from _Plagonium_ by concrescence of the meeting branches, and bears therefore to it the same relation that the preceding _Hexaplecta_ exhibits to _Hexaplagia_. Whilst in these two latter genera the six spines arise from a common central point, they arise here in two divergent groups from the two poles of a horizontal common middle rod, similar to the spicula of many #Beloidea#.

1. _Plectanium trigeminum_, n. sp. (Pl. 91, fig. 11).

Spines straight and stout, six to eight times as long as the common middle rod, three-sided prismatic; in the distal half thickened, with three divergent terminal thorns; in the basal half with three to four verticils of thin lateral branches, which are again ramified, and by their united threads produce an irregular loose framework.

_Dimensions._--Length of the spines 0.25, basal breadth 0.005, terminal breadth 0.02; length of the middle rod 0.03.

_Habitat._--North Pacific, Station 244, surface.

2. _Plectanium ovodimare_, n. sp.

Spines straight and stout, three-sided prismatic, about four times as long as the common middle rod, gradually thinned towards the distal end; in the basal half with two to three verticils of forked branches, which are dichotomously ramified, and by their united threads form a loose ovate framework.

_Dimensions._--Length of the spines 0.17, of the middle rod 0.04.

_Habitat._--North Pacific, Station 236, surface.

3. _Plectanium sphærozoum_, n. sp.

Spines straight, cylindrical, ten to twelve times as long as the common middle rod, with six to eight verticils of thorny branches, tapering towards the distal end; the branches of the verticils are simple in the distal half, again ramified in the basal half, and here connected together by irregularly branched threads forming a loose framework. (Resembles one spiculum of _Sphærozoum verticillatum_, Pl. 4, fig. 7, but is more richly branched, with united ramules.)

_Dimensions._--Length of the spines 0.2, of the middle rod 0.01.

_Habitat._--Central Pacific, Station 266, surface.

{929}Subfamily 4. POLYPLECTIDA, Haeckel, 1881, Prodromus, p. 424.

_Definition._--#Plectanida# with numerous (seven to nine or more) radial spines.

Genus 400. _Polyplecta_,[19] Haeckel.

_Definition._--#Plectanida# with numerous (seven to nine or more) radial spines, arising from a common centre and lying in different planes.

The genus _Polyplecta_ comprises provisionally all Plectanida possessing seven or more radial spines, united in the centre of the framework. It may be derived from _Polyplagia_ by meeting and concrescence of the free branches. But as in the latter genus, here also the few observed species are very different, perhaps of different origin, and may be afterwards better separated as representatives of diverse genera.

1. _Polyplecta heptacantha_, n. sp. (Pl. 91, fig. 12).

_Heptaplegma heptacantha_, Haeckel, 1882, MS.

Seven unequal spines divergent from one common central point, slender, slightly curved, three-sided prismatic. Four spines are much larger, twice to three times as long and as thick as the three smaller spines. One of the four larger spines is directed upwards (as "apical horn"), whilst the three others diverge downwards (as "basal feet"). Each of the four larger spines bears five to seven equidistant verticils of three divergent branches, the proximal of which are branched, the distal simple. The three smaller spines lie nearly horizontally, midway between the odd apical spine and the three basal spines, and in the same meridian planes with them; each bears one verticil of three divergent branches. All the branches are united by arachnoidal threads, composing a loose wickerwork with irregular, generally quadrangular meshes.

_Dimensions._--Length of the four larger spines 0.27 to 0.33, of the four smaller 0.1 to 0.14.

_Habitat._--Central Pacific, Station 271, surface.

2. _Polyplecta enneacantha_, n. sp.

_Enneaplegma enneacantha_, Haeckel, 1881, Prodromus, p. 425.

Nine equal and equidistant, straight, cylindrical radial spines, lying nearly in one plane, arise from an irregular spongy central framework; six of them seem to be secondary, intercalated between three equidistant primary spines, which are united in the centre. (This species resembles in the nine-radial structure the remarkable _Enneaphormis rotula_, Pl. 57, fig. 9, and may perhaps be derived from a similar species; but it has no regular latticed shell.)

_Dimensions._--Length of the spines 0.22, diameter of the framework 0.17.

_Habitat._--Central Pacific, Station 266, depth 2750 fathoms.

{930}3. _Polyplecta decacantha_, n. sp.

_Pentaplegma decacantha_, Haeckel, Prodromus, p. 425.

Ten radial spines, curved, cylindrical, irregularly branched, diverge in different directions and seem to arise in pairs from an irregular central framework, in the centre of which five primary spines are united; the latter correspond probably to the five spines of _Pentaspyris_, &c. The density of the spongy central framework did not allow of an accurate investigation, and makes it doubtful whether this species is not a _Spongiomma_.

_Dimensions._--Length of the spines 0.3 to 0.4, diameter of the framework 0.18.

_Habitat._--Tropical Atlantic, Station 338, depth 1990 fathoms.

4. _Polyplecta polybrocha_, Haeckel.

? _Acanthodesmia polybrocha_, Haeckel, 1865, Zeitschr. f. wiss. Zool.,
Bd. xv. p. 368, Taf. xxvi. fig. 3.

_Plegmosphæra polybrocha_, Haeckel, 1881, Prodromus, p. 455.

Numerous (twenty to thirty or more) radial spines, thin, cylindrical, curved and irregularly branched, arising from an irregular central spongy framework, are connected by numerous slender arches. The specimen observed by me in 1880 in Portofino was a true _Polyplecta_, with three primary spines centrally united, between which numerous other spines were intercalated. The similar specimen, however, observed in 1864 in Villafranca, and figured, _loc. cit._, was perhaps a _Plegmosphæra_.

_Dimensions._--Length of the radial spines 0.05 to 0.1, diameter of the framework 0.16.

_Habitat._--Mediterranean (Villafranca, Portofino), surface.

5. _Polyplecta dumetum_, Haeckel.

_Acanthodesmia dumetum_, J. Müller, 1868, Abhandl. d. k. Akad. d. Wiss.
Berlin, p. 30, Taf. i. Fig. 3.

Numerous (ten to twelve or more) radial spines, thin and straight, with a few straight lateral branches, diverge in different directions and are connected by a few slender curved arches. Some similar forms, but more developed, with numerous branches and curved rods, are found in the Pacific Radiolarian ooze, and represent probably different species.

_Dimensions._--Length of the radial spines 0.05 to 0.08, of the branches 0.02 to 0.03.

_Habitat._--Mediterranean, French shore (Cette and Saint Tropez), Johannes Müller, surface.

{931}----

Suborder III. STEPHOIDEA, Haeckel.

_Stephoidea_ vel _Stephida_, Haeckel, 1881, Prodromus, p. 444.
_Acanthodesmida_ (_sensu ampliori_), Bütschli, 1882, Zeitschr. f. wiss.
Zool., vol. xxxvi. p. 495.

_Definition._--NASSELLARIA without complete lattice-shell, with a skeleton composed of one or more simple rings, which may be united by a loose framework and are separated by large openings or gates. One primary or sagittal ring, determining the sagittal or median plane of the bilateral body, encloses the monaxonian central capsule.

The suborder #Stephoidea#, hitherto known by a few species only of "Acanthodesmida," comprises a large number of interesting NASSELLARIA (now more than two hundred species), which possess peculiar interest for the morphology and phylogeny of this legion. The monaxonian central capsule of the #Stephoidea# is surrounded either by one simple ring or by a complex system of several loosely connected rings; these may be united by a loose framework of connected branches, but never produce a complete lattice-shell, as is constantly the case in the #Spyroidea#, #Botryodea#, and #Cyrtoidea#. Therefore there remain between the parts of the connected rings a few large openings which we call "gates," separating them from the numerous small "pores" of the complete lattice-shells. In the most simple case, if only one ring be formed, there is also present only one "gate," the aperture of this simple ring.

The first known species of #Stephoidea# were observed in the Mediterranean by Johannes Müller in 1856, and described and figured in his last treatise (1858) under the names _Lithocircus annularis_ (_loc. cit._, Taf. i. fig. 1) and _Acanthodesmia vinculata_ (_loc. cit._, Taf. i. figs. 4-7). In the following year I myself observed two other living species in the Mediterranean, and described them in my Monograph (1862, pp. 268, 270) as _Zygostephanus mülleri_ (Taf. xii. fig. 2) and _Prismatium tripleurum_ (Taf. iv. fig. 6). For these four longest known #Stephoidea# I founded the new family of Acanthodesmida (_loc. cit._, p. 265), but united with them two other similar genera which I afterwards separated:--_Plagiacantha_ (belonging to the #Plectoidea#) and _Dictyocha_ (belonging to the PHÆODARIA).

When, in 1876, I received the rich material of the Challenger collection, I was astonished to find in it an enormous number of new, similar, and partly very interesting "Acanthodesmida," which were afterwards arranged in my Prodromus (1881, p. 444) in thirty-eight different genera and four "subfamilies," all united in one single large family, "Stephida or Monopylaria cricoidea." I retain here this natural group in the same sense, but give to it the rank of a "suborder," separating at the same time its four subfamilies as substantial "families." Since the names of the {932}latter, proposed in the Prodromus, were not quite suitable, I replace them here by the following more convenient names:--

1. Family Stephanida (= Monostephida, 1881, p. 447). Skeleton entirely
formed by one simple ring (the primary vertical sagittal ring). The only
"gate" is the simple aperture of the ring.

2. Family Semantida (= Dyostephida, 1881, p. 446). Skeleton composed of
two rings, perpendicular to one another; the primary vertical sagittal
ring bears at the base a horizontal basal ring; between the two rings two
or more "basal gates remain."

3. Family Coronida (= Triostephida, 1881, p. 445). Skeleton composed of
two crossed vertical or meridional rings, perpendicular to one
another--the primary sagittal ring and the secondary frontal ring.
Commonly the two vertical rings are united at the base by a horizontal
basal ring, and between them remain two or more "basal gates."

4. Family Tympanida (= Parastephida, 1881, p. 446). Skeleton composed of
two parallel horizontal rings, and upper mitral ring, and a lower basal
ring, both connected by vertical or divergent columellæ, which are parts
of vertical rings (primary sagittal, and secondary frontal ring).

The peculiar structure of the central capsule of the #Stephoidea#, and their character as true MONOPYLEA, were first recognised by Richard Hertwig, who in 1879, in his Organismus der Radiolarien, gave an excellent detailed description of it (_loc. cit._, pp. 68-72, Taf. vii. figs. 4, 5). He also pointed out the near affinity of these "Acanthodesmida" with the #Spyroidea# or Zygocyrtida, uniting the latter with the former family.

A fuller explanation of this affinity, and of the morphological and phylogenetic importance of the "Acanthodesmida," as ancestral forms of the #Spyroidea# and #Cyrtoidea#, was given in 1882 by Bütschli (Zeitschr. f. wiss. Zool., vol. xxxvi. pp. 495-501). He described in detail some important fossil forms of Acanthodesmida as different species of "_Stephanolithis_," a name which Ehrenberg had employed for various ring-like fragments of Radiolarian shells, sponges, and other fossil bodies. The four fossil species which Bütschli described represent four different genera of #Stephoidea#, viz., _Semantis_ (_spinescens_), _Semantrum_ (_mülleri_), _Semantidium_ (_haeckelii_), and _Tristephanium_ (_hertwigii_). On the phylogenetic conclusions, derived from these accurate observations, compare above, p. 893, &c. The topographical signification of the parts, employed by Bütschli, is contrary to mine; he calls my dorsal side the "anterior," and my ventral side the "posterior."

The geometrical fundamental form of the body in nearly all #Stephoidea# (with few exceptions) is distinctly bilateral or "dipleuric," so that we can easily distinguish the {933}three different dimensive axes: the principal axis with different apical and basal poles, the sagittal axis with different dorsal and ventral poles, and the lateral axis with equivalent right and left poles. In only a few genera this bilateral symmetry is not expressed, and a simpler, more regular fundamental form appears. The latter may be either primary (in the monaxonian _Archicircus_ and _Lithocircus_) or secondary, afterwards acquired (in the octahedral _Trissocircus_ and _Trissocyclus_, the cubical _Lithocubus_, the prismatic _Eutympanium_, and some other forms).

The most important element of the skeleton, with which the formation begins, in all #Stephoidea# is the simple primary or sagittal ring, lying vertically in the sagittal or median plane of the body and surrounding the monaxonian central capsule. This sagittal ring is the only essential element of the skeleton in all Stephanida, and is completely preserved in all Semantida, also in the greater part of the Coronida and Tympanida. It is partially reduced in the small groups of the true Acanthodesmida (subfamily of Coronida) and the Dystympanida and Eutympanida (subfamilies of Tympanida). Here only the vertical parts of it are preserved (dorsal and ventral rod), whilst the horizontal parts are lost (mitral and basal rod).

The sagittal ring lies constantly in the vertical median plane of the body, and therefore divides the enclosed central capsule into a right and a left half. It is rarely regular or subregular, commonly dipleuric or distinctly bilateral, so that we may easily distinguish its dorsal and ventral, apical and basal parts. The most important of these four parts or "rods" is the "basal rod" or the inferior part, because here the ring is in closer connection with the central capsule and its "porous area"; here peculiar spines or branches are commonly developed, which even on the isolated ring immediately determine the basal pole. The opposite upper part, or the "mitral rod," is also often distinguished by the peculiar appendages. The posterior part, or the "dorsal rod" (the anterior rod, _a_, in the description of Bütschli), is commonly more straight, often quite vertical. The opposite anterior part, or the "ventral rod" (the posterior rod, _b_, of Bütschli), is usually more convex, and often strongly curved or semicircular.