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Transcriber's note:

Text enclosed by underscores is in italics (_Actissa_).

The underscore also denotes a subscript, so _c__1 is an
italic c with a subscript (note that there are two
underscoreds between the "c" and the "1").

Text enclosed by hash marks is in "gesperrt"
(wide-spaced) type (#Larcoidea#).

Page numbers enclosed by curly braces (example: {25}) have
been incorporated to facilitate the use of the Index.

Some typographical errors in the printed work have been
corrected. They are listed at the end of the text.

The Addenda & Errata (Second Part, pp. 1763-4) have been
applied without further comment.

REPORT ON THE SCIENTIFIC RESULTS OF THE VOYAGE OF H.M.S. CHALLENGER DURING THE YEARS 1873-76

Under the Command of Captain George S. Nares, R.N., F.R.S. and the Late Captain Frank Tourle Thomson, R.N.

Prepared Under the Superintendence of the Late Sir C. Wyville Thomson, KNT., F.R.S., &c. Regius Professor of Natural History in the University of Edinburgh Director of the Civilian Scientific Staff on Board and Now of John Murray One of the Naturalists of the Expedition

ZOOLOGY--VOL. XVIII.

SECOND PART

Published by Order of Her Majesty's Government

Printed for Her Majesty's Stationary Office and Sold by London:--Eyre & Spottiswoode, East Harding Street, Fetter Lane Edinburgh:--Adam & Charles Black Dublin:--Hodges, Figgis, & Co. 1887

Price (in Two Parts, with a Volume of Plates) £5, 10s.

CONTENTS.

REPORT on the RADIOLARIA collected by H.M.S. CHALLENGER during the years 1873-1876.

By ERNST HAECKEL, M.D., Ph.D., Professor of Zoology in the University of Jena.

SECOND PART.--OSCULOSA.

(#NASSELLARIA AND PHÆODARIA.#)

CONTENTS.

SECOND PART.

II. Subclass OSCULOSA, 889
Legion III. NASSELLARIA vel MONOPYLEA, 889
Order 11. Nassoidea, 895
" 12. Plectoidea, 898
" 13. Stephoidea, 931
" 14. Spyroidea, 1015
" 15. Botryodea, 1103
" 16. Cyrtoidea, 1126

Legion IV. PHÆODARIA vel CANNOPYLEA, 1521
Order 17. Phæocystina, 1542
" 18. Phæosphæria, 1590
" 19. Phæogromia, 1642
" 20. Phæoconchia, 1710

Note on the Dimensions and Measurements, 1760

ADDENDA, 1761

ERRATA, 1763

INDEX, 1765

{889}LEGION III. NASSELLARIA,

VEL MONOPYLEA, VEL MONOPYLARIA (PLS. 51-98).

_Nassellaria_ (_inclusis_ Spyridinis), Ehrenberg, 1875.
_Monopylea_, Hertwig, 1879.
_Monopylaria_, Haeckel, 1881.
_Cyrtida_ et _Acanthodesmida_, Haeckel, 1862.
_Cricoidea_, Bütschli, 1882 (L. N. 40, p. 537) = NASSELLARIA.

_Definition._--Radiolaria with simple membrane of the central capsule, which is monaxon or bilateral, and bears on one pole of the main axis a porous area (porochora), forming the base of a peculiar intracapsular cone (podoconus). Extracapsulum without phæodium. Skeleton siliceous, very rarely wanting. Fundamental form originally monaxon, often dipleuric or bilateral.

The legion NASSELLARIA vel MONOPYLEA, in the extent here defined, was constituted in 1879 by Richard Hertwig in his work Der Organismus der Radiolarien (pp. 133-137). He gave to this large group the rank of an order, and united in it the two families Acanthodesmida and Cyrtida, which I had constituted first in 1862 in my Monograph (pp. 237, 265, 272); but he added, too, as a third family the Plagiacanthida, united by me with the former. In the first system of Ehrenberg (1847, _loc. cit._, pp. 53, 54), four families belonging to the MONOPYLEA were enumerated, the Halicalyptrina, Lithochytrina, Eucyrtidina, and Spyridina. He united the three former under the name "Polycystina solitaria," which he afterwards changed into NASSELLARIA (1875, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 157).

In my Monograph of the Radiolaria (1862, pp. 265-345) forty-four genera of NASSELLARIA were enumerated (six Acanthodesmida and thirty-eight Cyrtida), whilst the total number of genera in the whole class of Radiolaria at that time amounted to one hundred and thirteen. But owing to the astonishing number of new and interesting forms of this legion which I afterwards detected in the collection of the Challenger, in 1881 I distinguished in my Prodromus not less than three hundred and seventeen genera. These were disposed in five large main groups, retained in the present Report, with twenty-six families, viz., (1) #Plectoidea# (with three families), (2) #Stephoidea# (with four families), (3) #Spyroidea# (with four families), (4) #Botryodea# (with three families), and (5) #Cyrtoidea# (with twelve families). The first two groups have an incomplete or rudimentary skeleton, and may be united in the order #Plectellaria#, whilst the other three families possess a complete latticed shell, and {890}may be united as #Cyrtellaria#. The former correspond to the Acanthodesmida, the latter to the Cyrtida in my Monograph.

The character common to all MONOPYLEA or NASSELLARIA, which separates them from all other Radiolaria, was first recognised by Richard Hertwig in 1879, and consists in the singular structure of the monaxonian central capsule, bearing on the basal pole a peculiar porous area or operculum, the "Porenfeld;" we call it shortly the "porochora." It represents a circular or elliptical porous plate on the basal pole of the vertical main axis of the central capsule, and bears a peculiar "podoconus" or "Pseudopodien-Kegel," a conical body of singular structure, protruding inside the membrane into the capsule. The pseudopodia arising from this peculiar "podoconus" proceed from the capsule piercing the "porochora," whilst the other parts of the capsule are not perforated.

The MONOPYLEA are therefore "Merotrypasta," like the following fourth legion, the PHÆODARIA or CANNOPYLEA. But in these latter we find on the basal pole of the monaxonian capsule only one single large main opening, prolonged into a peculiar tube, and there is no trace of the typical "podoconus," characteristic of all NASSELLARIA. The latter agree, however, with the former in the possession of a basal opening, serving for the emission of the pseudopodia, and in the monaxonian fundamental form, arising from this structure. Therefore these two legions of "Merotrypasta" exhibit a wider divergence from the ACANTHARIA and SPUMELLARIA, the two legions of "Holotrypasta," in which the central capsule is everywhere perforated by innumerable small pores (compare above, pp. 5, 6, 716).

_The Skeleton_ in all NASSELLARIA consists either of pure silica, or of a peculiar silicate; never of acanthin (as in all ACANTHARIA). The siliceous bars and beams constituting it are invariably solid (as also in the SPUMELLARIA); never hollow (as in the PHÆODARIA). In the small family of Nassellida alone (with the two genera _Cystidium_ and _Nassella_) the skeleton is entirely absent. In all other genera of NASSELLARIA the siliceous skeleton is more or less developed; imperfect, or quite rudimentary in the #Plectellaria# (#Plectoidea# and #Stephoidea#), but perfect and forming a regular lattice-shell in the #Cyrtellaria# (#Spyroidea#, #Botryodea#, and #Cyrtoidea#). The different forms of this skeleton exhibit an extraordinary variety, but may be reduced to a few very simple fundamental forms, or even to a single, most simple original form. The comparative morphology is more interesting, but also more difficult to understand than in any other Radiolaria.

The geometrical fundamental form of the skeleton is in all NASSELLARIA monaxonial, the vertical main axis of the body, which is already indicated by the axis of the central capsule with two different poles, being also expressed constantly in the form of the skeleton. The lower or basal pole of the latter always exhibits a different shape from that of the upper or apical pole. This difference is so striking in nearly all MONOPYLEA, that the two poles may be determined on the first view.

{891}In the great majority of NASSELLARIA not only is the monaxonial fundamental form expressed, but also the dipleuric or bilateral, so that the symmetrical halves of the body may be easily determined; the right and left halves exhibit the same symmetry as in the vertebrates, so that we may distinguish an anterior ventral and a posterior dorsal face of the body. The whole form is in this case determined by three dimensive axes, perpendicular to one another, two of which are heteropolar, the third is homoeopolar. The apical pole of the vertical main axis (principal or longitudinal axis) is different from the basal pole. The ventral pole of the horizontal sagittal axis (or dorso-ventral axis) is different from the dorsal pole. The right pole of the horizontal transverse axis (lateral or frontal axis) is equal to the left pole. Therefore the sagittal or median plane of the body (in which the principal and the sagittal axis are crossed) divides it into symmetrical equal halves and is perpendicular to the frontal axis.

Three different original elements of structure are recognisable in the majority of NASSELLARIA, viz., (1) a vertical simple ring, the primary or sagittal ring, placed vertically in the sagittal plane and enclosing the median plane of the central capsule; (2) a basal tripod, composed of three diverging radial rods, which are united on the basal pole of the central capsule and are either expanded horizontally or descend; (3) an ovate or subspherical, simple lattice-shell, the cephalis or capitulum, which surrounds the central capsule and exhibits a peculiar structure on its basal pole.

These three important original elements of structure--the sagittal ring, the basal tripod, and the latticed cephalis--are so united in the majority of NASSELLARIA that the cephalis rests upon the tripod and includes the sagittal ring wholly or partially. The simplest realisation of this typical union is afforded by the Archiperida and Tripospyrida, and these may be derived from the simpler important Stephanid _Cortina_ (Pl. 83, fig. 9; Pl. 92, fig. 21; Pl. 97, fig. 1). In this and in all other tripodal NASSELLARIA, the three basal rods or the "cortinar feet" are constantly so arranged that an odd or posterior rod, the "caudal foot" (_c_) is opposed to the two anterior paired rods, the "pectoral feet" (one right, _p_", and one left, _p_'). The caudal foot lies in the sagittal plane, and is prolonged upwards into the dorsal rod of the sagittal ring (_b_), and over this in a free ascending spine, the "apical horn" (_a_). The curved ventral rod of the ring (_r_) is united above with the base of the apical horn, below with the common centre of the tripod or the "cortinar centrum." The characteristic position of the central capsule in this skeleton of _Cortina_ is such that its basal pole (with the porochora) rests upon the centre of the tripod, whilst its sagittal perimeter is separated from the surrounding ring by the calymma; the numerous pseudopodia arising from its base diverge downwards and are supported and protected by the three basal feet of the tripod (Pl. 97, fig. 1). Compare also Pls. 51, 53, 84, 95, 98.

The typical skeleton of _Cortina_, a tripodal ring, becomes more developed in the Semantid _Cortiniscus_, in which the basal parts of the three diverging feet are united {892}by a second horizontal ring, the cortinar or basal ring (Pl. 92, figs. 11-13). The pores between the former and the latter, or the "cortinar pores," may be regarded as the first beginning of the lattice-plate, composing the "cephalis" or the simple primordial shell in the Archiperida and Tripospyrida, and transmitted from these by heredity to the great majority of NASSELLARIA.

The "cephalis or capitulum" (the "Köpfchen" of the German authors) is therefore the most important part of the skeleton in all #Cyrtellaria#, or in all NASSELLARIA possessing a complete lattice-shell. In the #Plectellaria#, however, or in those NASSELLARIA which do not possess a complete fenestrated shell, the "cephalis" is either imperfect or totally wanting. The cephalis surrounds the enclosed central capsule on all sides in the form of an ovate or subspherical lattice-shell, and is separated from it only by the jelly-like calymma. The sagittal ring is either enclosed in the wall of the cephalis (whole or partially), separating its two lateral halves, or it is enclosed in the cavity of the cephalis and connected with its sagittal perimeter by short beams. The base of the cephalis (with the cortinar plate) often rests immediately upon the centre of the tripod; in the majority of NASSELLARIA, however, this near relation is altered by reason of later changes and secondary modifications.

The number of various forms, developed from these simple original structural elements of the skeleton, is astonishing, and there are described more than three hundred genera and nearly two thousand species of this legion in the following pages. This large number may be easily increased by subsequent observers. Since in all these MONOPYLEA the characteristic structure of the central capsule is identical, and also the structural elements of the siliceous skeleton are similar, it is very probable that they may have arisen from a single common stock. But it is very difficult (and at present impossible) to explain a natural monophyletic system of this large legion. The greatest difficulty is presented by the fact that the three structural elements mentioned above, viz., the sagittal ring, the basal tripod, and the latticed cephalis, are not constantly united, but each alone may also constitute the skeleton by itself. In this respect the following seven cases are possible, and are actually realised.

A. The skeleton is composed of the sagittal ring only and of its spiny appendages, without basal tripod and without latticed cephalis. This is the case in the majority of #Stephoidea# (Stephanida, Semantida, Coronida, and Tympanida).

B. The skeleton is composed of a basal tripod only (_Plagiacantha_), or of a tripod in the centre of which arises a vertical apical horn (_Plagoniscus_), and often of an irregular framework, arising from the rods of the tripod; but there is neither a trace of a sagittal ring nor a complete latticed cephalis. This is the case in the remarkable suborder #Plectoidea# (Plagonida and Plectanida).

C. The skeleton is composed of a latticed cephalis only, a simple ovate or subspherical fenestrated shell, which encloses the monopylean central capsule; there is no {893}trace of a sagittal ring nor of a basal tripod. This is the case in the remarkable family of Cyrtocalpida (Archicorida and Archicapsida), in numerous #Botryodea# and in other #Cyrtellaria#.

D. The skeleton is composed of a sagittal ring and a basal tripod, without latticed cephalis. This is the case in a few, but very important forms of #Stephoidea#: _Cortina_, _Stephanium_, _Cortiniscus_, _Stephaniscus_, _Podocoronis_, and some allied genera.

E. The skeleton is composed of a sagittal ring and a latticed cephalis, but without basal tripod. This is the case in numerous #Cyrtellaria#, in the Circospyrida (or Zygospyrida apoda: _Dictyospyris_, _Circospyris_) and some other #Spyroidea#; and in a large number of #Botryodea# and Cyrtoidea eradiata (a part of the Sethocyrtida, Theocyrtida, Lithocampida, and others).

F. The shell is composed of a basal tripod and a latticed cephalis, but without any trace of the sagittal ring. This is the case in numerous Cyrtoidea triradiata and multiradiata, and perhaps in the majority of the following families--Tripocalpida, Tripocyrtida, Podocyrtida, and Podocampida.

G. The shell is composed of all three above-mentioned elements, of a sagittal ring, a basal tripod, and a latticed cephalis. This is the case in the great majority of #Spyroidea# (with a few exceptions only), and perhaps also in the majority of #Cyrtoidea#.

The survey of these seven groups, A to G, each of which is represented by numerous living forms, shows clearly how difficult and complicated the morphology and phylogeny of the numerous NASSELLARIA must be. For all possible combinations of the three original structural elements are realised abundantly, and in such complicated relations, and so intermingled in the different orders and families, that it seems nearly hopeless to answer the question of their true origin. The identity in the structure of the central capsule, however, in all these MONOPYLEA, makes it probable that they have all arisen originally from the skeletonless Nassellida (_Cystidium_, _Nassella_), either in a monophyletic or in a polyphyletic way. In this respect the following phylogenetical hypotheses are possible.

1. Monophyletic hypothesis, deriving all NASSELLARIA from a simple sagittal ring (_Archicircus_, _Lithocircus_, &c., Pl. 81). The groups A, D, E, and G may be derived easily from such a ring, but the groups B, C, and F only by means of the hypothesis that the original ring may be completely reduced and finally lost. This hypothesis was stated by me in the years 1877 to 1879, when I had got the first general survey of the astonishing number of new NASSELLARIA in the Challenger collection, and as I had found the sagittal ring in the majority of them. This, my former hypothesis, is mentioned by Richard Hertwig (1879, _loc. cit._, pp. 68, 126). It was afterwards supported with particular energy by O. Bütschli (1882, Zeitschr. für wiss. Zool., Bd. XXXVI.).

2. Monophyletic hypothesis, deriving all NASSELLARIA from a basal tripod (_Triplagia_, _Plagoniscus_, &c., Pl. 91). The groups B, D, F, and G, all triradiate, may {894}be derived easily from such a tripod; but the groups A, C, and E only by means of the hypothesis that the original tripod may be completely reduced and finally lost. This hypothesis was employed in 1881 in my Prodromus, since I had convinced myself that the "triradial structure" is prevalent in the great majority of NASSELLARIA, and is perhaps more important than the sagittal ring.

3. Monophyletic hypothesis, deriving all NASSELLARIA from a latticed cephalis, a simple ovate or subspherical fenestrated shell without ring and tripod (_Cyrtocalpis_, _Archicapsa_, &_c._). The groups C, E, F, and G may be derived easily from such a cephalis, but the groups A, B, and D only by means of the hypothesis that the sagittal ring as well as the basal tripod may remain as the last remnants of a reduced cephalis. This hypothesis was given in 1862 in my Monograph, where I constructed the first pedigree of Radiolaria (p. 234). I there derived all the Cyrtida from the #Sphæroidea# (_Cyrtidosphæra_), supposing that _Cyrtocalpis_ and some other Monocyrtida may form a direct phylogenetical passage from the #Sphæroidea# to the #Cyrtoidea#.

4. Polyphyletic hypothesis, deriving the different groups of NASSELLARIA from different skeletonless Nassellida, by development of simple siliceous skeletons in different ways. Among the numerous polyphyletic hypotheses which are possible, one of the simplest would be the supposition that three different fundamental forms of skeleton may have arisen independently one from another: (1) a simple sagittal ring as original form of the #Stephoidea# and #Spyroidea# (A); (2) a simple basal tripod as original form of the #Plectoidea# (B); (3) a simple latticed cephalis as original form of the #Botryodea# and #Cyrtoidea# (C). This triphyletic hypothesis is supported by R. Hertwig (1879, _loc. cit._, p. 136); he assumes that the original skeletonless Nassellida (_Cystidium_) have produced three different branches, his "Acanthodesmida" (= #Stephoidea# and #Spyroidea#) with a primary ring, his "Plagiacanthida" (= #Plectoidea#) with a primary tripod, and his Cyrtida (= #Botryodea# and #Cyrtoidea#) with a primary cephalis. This hypothesis seems rather probable on the first view; but it meets with the greatest difficulties in view of the fact that these three original elements of the skeleton are more or less evidently combined in the great majority of NASSELLARIA. The greatest difficulty arises from the fact that often among very similar and closely allied forms the first exhibits all three elements (A, B, C) combined, whilst the second has a combination of A and B, the third of B and C, the fourth of A and C; and there are other forms, very similar to the former, in which one element only is recognisable. Another difficulty arises from the fact that the intimate structure of the cephalis in the majority of #Cyrtellaria# is not perfectly known, and often exhibits structures which are difficult to explain with regard to the three elements A, B, C. Under these circumstances further researches on the numerous imperfectly known NASSELLARIA are required, and chiefly accurate observations on their more minute structure and on their important ontogeny.

{895}We divide the immense legion of NASSELLARIA into two large orders, the #Plectellaria# without complete lattice-shell, and the #Cyrtellaria# with a complete lattice-shell or a "cephalis," including the central capsule; the latter, of course, have arisen from the former. The #Plectellaria# comprise three different suborders, the #Nassoidea# (without skeleton), the #Plectoidea# (with a tripodal skeleton, without ring), and the #Stephoidea# (with a primary sagittal ring, with or without tripod). The #Cyrtellaria# again also comprise three different suborders, the #Spyroidea# (with bilocular cephalis and a sagittal constriction), the #Botryodea# (with multilocular and lobate cephalis, exhibiting two or more constrictions and three or more lobes), and the #Cyrtoidea# (with simple, unilocular cephalis, without constriction).

_Synopsis of the Orders and Suborders of_ NASSELLARIA.

I. Order {No skeleton, 1. #Nassoidea#.
PLECTELLARIA. {
Nassellaria without {Skeleton with a basal tripod,
complete lattice-shell. { without ring, 2. #Plectoidea#.
{
{Skeleton with a sagittal ring
{ (usually without tripod), 3. #Stephoidea#.

{Cephalis bilocular, with a
{ sagittal constriction, 4. #Spyroidea#.
II. Order {
CYRTELLARIA. {Cephalis multilocular, with
Nassellaria with a { two or more constrictions
complete lattice-shell. { and lobes, 5. #Botryodea#.
{
{Cephalis simple, without
{ constriction and lobes, 6. #Cyrtoidea#.

----

Order V. PLECTELLARIA, Haeckel, 1883.

_Definition._--NASSELLARIA without complete lattice-shell, usually with an incomplete skeleton, formed of a ring, a tripod, or an irregular framework.

Suborder I. NASSOIDEA, Haeckel.

_Definition._--NASSELLARIA without skeleton. Single family Nassellida.

{896}Family XLV. #NASSELLIDA#, Haeckel.

_Cystidina_, Haeckel, 1883, Sitzungsb. Jena Ges. für Naturw., Februar 16.

_Definition._--NASSELLARIA without skeleton. The soft body composed of a monopylean central capsule (with porochora and podoconus) and of a surrounding jelly-veil or calymma.

The family #Nassellida# comprises the simplest and most primitive forms of NASSELLARIA, the only group which is entirely without a skeleton. The central capsule is therefore perfectly free and naked, enveloped by the calymma only, as in the #Colloidea# among the SPUMELLARIA, in the Phæodinida among the PHÆODARIA. Probably these naked and skeletonless MONOPYLEA must be regarded as surviving remnants of the common ancestral group of this legion; but the possibility is not excluded that the few observed forms are either young NASSELLARIA which have not yet secreted a skeleton, or older NASSELLARIA which have lost their original skeleton.

We distinguish in this small family two genera only: _Cystidium_ with hyaline, not foamy calymma, without extracapsular alveoles, and _Nassella_, with a very voluminous foamy calymma, including numerous large alveoles; the former corresponds to _Actissa_ and the latter to _Thalassicolla_ among the #Colloidea# or the skeletonless SPUMELLARIA. But in these two latter genera, as in all PERIPYLEA, the central capsule is perforated everywhere by innumerable small pores; the two former genera, however, exhibit the same characteristic podoconus in the central capsule, and the same porochora at its base, as all the other MONOPYLEA. The pseudopodia are protruded from the central capsule through the porochora only.

_The Central Capsule_ is in the two observed genera either ovate or nearly spherical, usually slightly tapering towards the basal mouth. Its transverse section is constantly circular. The membrane of the capsule is usually rather thick and double-contoured, and bears on the truncate basal pole a circular "porochora" or area porosa, through which the pseudopodia are protruded. The _porochora_ is either quite simple, circular, or in some species trilobed, with three equal circular lobes, each of which is surrounded by a girdle of small granules. The _podoconus_, or "pseudopodial-cone," arising vertically from the horizontal basal porochora, is half as long as the central capsule, or longer, simply conical and finely striped longitudinally. The _nucleus_ lies usually in the uppermost part of the central capsule, above or behind the podoconus, and is either spherical or ovate, sometimes kidney-shaped. It includes one or more nucleoli.

_Synopsis of the Genera of Nassellida._

Calymma hyaline, without alveoles, 382. _Cystidium_.

Calymma foamy, with numerous large alveoles, 383. _Nassella_.

{897}Genus 382. _Cystidium_,[1] R. Hertwig, 1879, Organismus d. Radiol., p. 87.

_Definition._--#Nassellida# with hyaline calymma, without extracapsular alveoles.

The genus _Cystidium_ is the simplest and most primitive among all NASSELLARIA, and may be regarded as the common ancestral form of this legion, for which it has the same high importance as _Actissa_ for the SPUMELLARIA. The central capsule is quite simple, ovate or nearly spherical, and included in a voluminous hyaline calymma, which contains no large alveoles. _Cystidium_ differs from _Actissa_, its probable ancestral form, in the possession of the "podoconus" and its basal "porochora," characteristic of all NASSELLARIA or MONOPYLEA.

1. _Cystidium princeps,_ n. sp. (Pl. 91, fig. 1).

Central capsule ovate, one and a third times as long as broad. Podoconus with simple circular porochora, about half as long as the capsule, surrounded at its base by red granules of pigment. Nucleus spherical. Three equal oil-globules in the endoplasm. No pigment in the voluminous calymma, which includes numerous xanthellæ.

_Dimensions._--Length of the central capsule 0.1, breadth 0.075; nucleus 0.035.

_Habitat._--Indian Ocean, Maldive Islands (Haeckel), surface.

2. _Cystidium lecythium,_ n. sp.

Central capsule ovate, one and a half times as long as broad. Podoconus with simple circular porochora, three-fourths as long as the capsule. Nucleus ellipsoidal. No oil-globules in the endoplasm. No pigment and no xanthellæ in the calymma.

_Dimensions._--Length of the central capsule 0.12, breadth 0.08; nucleus 0.04.

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

3. _Cystidium inerme,_ R. Hertwig.

_Cystidium inerme,_ R. Hertwig, 1879, Organismus d. Radiol., p. 87, Taf.
vii. figs. 1-1_b_.

Central capsule subspherical, a little longer than broad. Podoconus about two-thirds as long as the capsule, with trifid porochora, which is composed of three equal circular lobes. Nucleus spherical. Numerous oil-globules in the endoplasm. The calymma includes numerous xanthellæ and brown pigment around the mouth.

_Dimensions._--Length of the central capsule 0.06, breadth 0.05; nucleus 0.03.

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

{898}Genus 383. _Nassella_,[2] nov. gen.

_Definition._--#Nassellida# with foamy calymma, containing numerous large extracapsular alveoles.

The genus _Nassella_ differs from the preceding _Cystidium_, its probable ancestral form, in the development of numerous large alveoles in the extracapsular calymma, and therefore exhibits the same relation to it that _Thalassicolla_ bears to _Actissa_ among the SPUMELLARIA. The foamy calymma is very voluminous, and includes numerous symbiotic xanthellæ.

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.

The triradial structure, common to the Triplagida and Triplectida, is replaced by the quadriradial structure in the Tetraplagida and Tetraplectida. Probably the latter have been derived from the former by development of a fourth spine, and then this latter would correspond to the "apical horn" of the other NASSELLARIA. But possibly also both structures have originated independently from one another. We may distinguish not less than four different kinds of the quadriradial structure. In the first case all four spines are equal, and diverge from a common central point at equal angles in different directions, corresponding to the four axes of a regular tetrahedron (_Tetraplagia_ and _Tetraplecta_, Pl. 91, figs. 3, 8).

In the second case all four spines are also equal, but they are not united in a common central point, but opposite in pairs on the two poles of a common central rod (_Plagonidium_). Therefore the skeleton possesses here the same form as in the "geminate-biradiate" spicula of many #Beloidea# (_e.g._, _Thalassoxanthium bifurcum_ and _Sphærozoum furcatum_). The development of the short horizontal middle rod, connecting the two divergent pairs of spines, is here probably effected by the porochora of the central capsule resting upon it.

Whilst in these two cases of quadriradial structure all four spines are equal, in two other cases they become differentiated in a very remarkable manner. One spine is vertically directed upwards, in shape and size different from the three others, which are directed downwards; the former corresponding probably to the "apical horn," the latter to the three "basal feet," which are found in the great majority of the #Spyroidea# {902}and #Cyrtoidea#. Therefore we encounter here for the first time that characteristic "cortinar structure" which is complete in _Cortina_ and _Cortiniscus_ (Pl. 92, figs. 11-13, 21), and which may be regarded as the strongest argument for a close relationship, or even for a common monophyletic origin of all NASSELLARIA.

The four spines, which we regard therefore as "cortinar spines," exhibit a twofold kind of central junction. In the simpler case they are united in a common central point, on which rests the porochora of the central capsule (_Plagoniscus_ and _Plectaniscus_, Pl. 91, figs. 4, 9). These forms are nearer to _Cortina_, and may be derived immediately from _Tetraplagia_ and _Tetraplecta_ by differentiation of the four equal spines. In the other case the four cortinar spines are separated in pairs, diverging from the two poles of a short horizontal common "central rod" (_Plagiocarpa_ and _Periplecta_, Pl. 91, figs. 5, 10). These forms may be compared with the spicula of some #Beloidea# and derived from _Plagonidium_; but their basal central rod may be compared again with the basal part of the sagittal ring of _Cortina_, and this comparison becomes very important in those forms like _Plagiocarpa procortina_ (Pl. 91, fig. 5). Here the four spines approach very nearly to those of _Cortina_; the two ventral spines (or pectoral feet) on the anterior pole of of the middle rod are equal, but very different from the two dorsal spines, arising from the posterior pole; the lower odd spine of the latter corresponds to the "caudal foot," the upper spine to the "apical horn" of _Cortina_ and of the #Cyrtellaria#. The vertical plane, determined by these two dorsal spines, is the sagittal plane, and two opposite curved branches which lie in this plane (an upper arising from the basal part of the apical spine and a lower arising from the anterior pole of the middle rod) may be regarded as ventral parts of an incomplete sagittal ring. This interesting form and some other similar Tetraplagida may be regarded either as beginning #Stephoidea# (_Cortina_, with incomplete sagittal ring) or as retrograde #Stephoidea# (_Cortina_, with partly reduced sagittal ring). In every case they seem to indicate the near relationship between the #Stephoidea# and #Plectoidea#.

Another argument for this close relationship may be found in the position of the central capsule in the interesting genus _Plagiocarpa_ (Pl. 91, fig. 5). Its basal part (with the porochora) rests upon the common central rod, its ventral face upon the ventral prolongation of the latter, its dorsal face upon the apical horn; its axis lies in the sagittal plane. The three basal spines (the odd caudal and the paired pectoral feet) diverge from its basal pole downwards in the same manner as in the Cortina, the Zygospyrida and the Monocyrtida.

Less important than those quadriradial Tetraplagida and Tetraplectida, are the sexradial #Plectoidea#, the Hexaplagida and Hexaplectida. These may be derived immediately from the triradial #Plectoidea# by prolongation of the three primary original spines (of _Plagiacantha_) over the common central point. Here also two {903}different kinds of central junction are found. In the simpler case all six radial spines arise from a common central point (_Hexaplagia_ and _Hexaplecta_). In the other case the six radial spines arise from the two poles of a short horizontal common central rod, opposed in two groups, each of three spines (_Plagonium_ and _Plectanium_, Pl. 91, figs. 6, 11). In this latter case the single corresponding spines of the two opposite groups are usually parallel, and exhibit therefore exactly the same characteristic "germinate-triradiate" form which is found in many #Beloidea# (_e.g._, in the common _Sphærozoum punctatum_ and the similar _Lampoxanthium punctatum_).

The fourth and last group of this suborder contains the multiradiate #Plectoidea#, the Polyplagida and Polyplectida. Here the number of radial spines, diverging from the common centre, exceeds six, and is commonly seven to nine, at other times ten to twelve or more (_Polyplagia_ and _Polyplecta_, Pl. 91, fig. 12). When these two genera are better known from further accurate observations, they may probably be divided into several different genera (as already proposed in my Prodromus, 1881), since not only the number, but also the central junction and the arrangement of the numerous radial spines in the few observed species seems to be very different. In some seven-radiate species (_e.g._, _Polyplecta heptacantha_) four larger spines seem to be true "cortinar spines," the three smaller secondary productions of the former. In the nine-radiate species the nine spines seem to be sometimes basal branches of three primary spines, at other times six secondary intercalated between the three primary (like _Enneaphormis_, Pl. 57, fig. 9). In those multiradial #Plectoidea#, in which the number of spines amounts to ten or twelve or more, the laws of disposition are not yet recognised.

Comparing these different productions of the skeleton in the numerous #Plectoidea#, we find expressed two remarkable and very different affinities. On the one hand many #Plectoidea# exhibit exactly the same peculiar forms, which are only found besides in the #Beloidea# (as many species of _Triplagia_, _Plagiacantha_, _Tetraplagia_, _Plagonidium_, _Hexaplagia_, and _Plagonium_). On the other hand many #Plectoidea# bear the same characteristic composition of the skeleton (or the "cortinar structure") which is found in the Cortinida among the #Stephoidea#, and in numerous #Spyroidea# and #Cyrtoidea#, which all agree in the possession of three divergent basal feet and a vertical apical horn. A most important argument for the close affinity of all these "cortinar NASSELLARIA" seems to be given by the fact that the sagittal ring, which in _Cortina_ is combined with the quadriradial structure, exhibits in the #Cyrtellaria# the most different stages of development; in one group it is complete, in the second incomplete, and in the third it has completely disappeared.

The form of the radial spines composing the skeleton is usually three-sided prismatic, gradually tapering from the thicker central base towards the distal apex; sometimes they are slender pyramidal. More rarely the spines are cylindrical or slender conical. In the majority of species the spines are straight, in the minority more or {904}less curved. In very few species only are they quite simple, without branches. They are nearly always more or less branched, in many larger species very richly ramified. The modes of ramification are rather variable. In the majority of #Plectoidea# the spines are rather regularly verticillate, bearing an increasing number of verticils, each of which is composed of three divergent branches. These arise from the three edges of the spine, and all the branches of one edge are usually parallel, either perpendicular to the spine, or directed at an acute angle towards its apex. When the verticils are numerous (five to ten or more), their size commonly tapers gradually towards the apex. Pinnate spines occur more rarely than verticillate ones; in this case the two paired lateral edges only of the prismatic spine bear opposite or alternate branches, whilst the odd middle edge bears no ramules. In some species the spines are singly or doubly forked. In many species (mainly those with cylindrical spines) the ramification of the spines is more or less irregular.

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#).

{905}_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.

{906}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.
_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 {907}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).

{908}_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.
-------------------------------------------------------------------------
{All four spines equal, 386. _Tetraplagia_.
Four spines arising from {
one common central point. {One apical spine
{ opposed to three
{ basal spines, 387. _Plagoniscus_.

{All four spines equal, 388. _Plagonidium_.
Four spines arising in two {
pairs from the poles of a {One apical spine
common central rod. { 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_.

Subfamily 1. TRIPLAGIDA, Haeckel, 1881, Prodromus, p. 423.