(§§ 101-140).
101. _The Significance of the Skeleton._--The skeleton of the Radiolaria is
developed in such exceedingly manifold and various shapes, and exhibits at
the same time such wonderful regularity and delicacy in its adjustments,
that in both these respects the present group of Protista excels all other
classes of the organic world. For, in spite of the fact that the
Radiolarian organism always remains merely a single cell, it shows the
potentiality of the highest complexity to which the process of skeleton
formation can be brought by a single cell. All that has been brought to
pass in this direction by single tissue-cells of animals and plants does
not attain the extremely high stage of development of the Radiolaria. Only
very few Rhizopoda of this very rich and varied class fail to exhibit the
power of forming this firm supporting and protecting organ--indeed, only
ten of the seven hundred and thirty-nine genera which are enrolled in the
list of the Challenger collection, namely, six genera of SPUMELLARIA (five
Thalassicollida, _Actissa_, _Thalassolampe_, _Thalassopila_,
_Thalassicolla_, _Thalassophysa_, Pl. 1, and one genus of Collozoida,
_Collozoum_, Pl. 3), and in addition two genera of NASSELLARIA (the
Nassellida, _Cystidium_ and _Nassella_, Pl. 91, fig. 1), and two genera of
PHÆODARIA (the Phæodinida, _Phæocolla_ and _Phæodina_, Pl. 101, figs. 1,
2). These skeletonless forms of Radiolaria are, however, of extreme
interest, since they include the original stem-forms of the whole class as
well as of its four legions. All Radiolaria which form skeletons have
originated from soft and skeletonless stem-forms by adaptation, and that
polyphyletically, for the skeletal types of the four legions have been
developed independently of each other (§ 108).
102. _The Chemical Peculiarities of the Skeleton._--The chemical
composition of the skeleton shows very marked variations in the different
legions of the Radiolaria. The two legions SPUMELLARIA and NASSELLARIA
(united formerly as "Polycystina") form their skeleton of pure silica (see
note A, below); the legion PHÆODARIA of a silicate of carbon (see note B),
and the ACANTHARIA of a peculiar organic substance--acanthin (see note C).
This explains the well-known fact that the deposits of fossil Radiolaria
(or Polycystine marls) are composed exclusively of the skeletons of
SPUMELLARIA and NASSELLARIA, those of the ACANTHARIA and PHÆODARIA being
entirely absent (in the case of the last group, however, exception must be
made in favour of the Dictyochida, or those PHÆODARIA {lxix}whose skeleton
is made up of isolated scattered tangential siliceous fragments). The
enormous deposits of Radiolarian skeletons in the deep sea of today, which
constitute the Radiolarian ooze, consist, like the fossil Polycystine
marls, almost exclusively of the shells of SPUMELLARIA and NASSELLARIA,
though here the acanthin skeletons of the ACANTHARIA may be present in very
small numbers, and the silicate skeletons of the PHÆODARIA, which offer
more resistance to the solvent action of sea-water, somewhat more
abundantly. Calcareous skeletons do not occur in the Radiolaria (see note
D).
A. The pure siliceous skeletons of the Polycystina were first recognised
in 1833 by Ehrenberg in chalky marls (L. N. 2, p. 117). Since the two
legions ACANTHARIA and PHÆODARIA were entirely unknown to Ehrenberg, his
name Polycystina has reference only to the SPUMELLARIA and NASSELLARIA.
B. The silicate skeleton of the PHÆODARIA was formerly taken by me for a
purely siliceous one. When I described the first PHÆODARIA in my
Monograph in 1862, I was only acquainted with five genera and seven
species, whilst the number of PHÆODARIA here described from the
Challenger amounts to eighty-four genera and four hundred and sixty-five
species. In the vast majority of these (though not in all) the skeleton
becomes more or less intensely stained by carmine, and is also more or
less charred at a red heat, in some even becoming of a blackish-brown. In
many PHÆODARIA, furthermore, the hollow skeletal tubes are destroyed by
the continued action of heat. They are also, for the most part, strongly
acted upon, or even destroyed by boiling caustic alkalis, whilst boiling
mineral acids have no effect upon them. The best method of cleaning the
skeletons of PHÆODARIA from their soft parts is to heat them in
concentrated sulphuric acid, and then add a drop of fuming nitric acid;
in this they are not dissolved even on prolonged heating. From these
facts it would appear that the skeletons of the PHÆODARIA consist of a
compound of organic substance and silica, or a "carbonic silicate." The
more intimate composition yet remains to be discovered, as also the
manifold differences which the various families of PHÆODARIA seem to show
in respect of its composition. The small skeletal fragments of the
Dictyochida (the only remains of PHÆODARIA which occur as fossils) appear
to consist of pure silica.
C. The acanthin skeleton of the ACANTHARIA was first described as such in
my Monograph (1862, pp. 30-32). Johannes Müller, the discoverer of this
legion, took them for siliceous skeletons and defined the #Acanthometra#
as "Radiolaria without lattice-shell, but with siliceous radial spines"
(L. N. 12, p. 46). I formerly supposed that the acanthin skeletons in
some of the ACANTHARIA were partially or wholly metamorphosed into
siliceous skeletons, but, according to the investigations of R. Hertwig,
this does not appear to be the case; he showed that the skeletons of the
most varied #Acanthometra# and #Acanthophracta# are completely dissolved
under the longer or shorter action of acids, and supposes that in all
ACANTHARIA, without exception, the skeleton is composed of acanthin
(1879, L. N. 33, p. 120). Quite recently Brandt has found that the
acanthin spines dissolve not only in acids, alkalis, and "liquor
conservativus" (as I had shown), but also in solutions of carbonate of
soda (1 per cent.), and even of common salt (10 to 20 per cent.); he
concludes from this that they consist of an albuminoid substance
(vitellin) (L. N. 38, p. 400). I am unable to share this view, for I have
never been able to see some of the most important reactions of albumen in
any of the skeletons which I have examined, such for example as the
xanthoproteic reaction, the red coloration with Millon's test, &c. They
do not become {lxx}yellow either with nitric acid or with iodine. In
dilute mineral acids they dissolve more rapidly than in concentrated. My
usual method of cleansing the skeleton of ACANTHARIA (which has been
practised with the same result on thousands of specimens) consists in
heating the preparation in a small volume of concentrated sulphuric acid
and then adding a drop of fuming nitric acid; all other constituents (the
whole central capsule and the calymma) are thus very rapidly destroyed;
the skeleton remains quite uninjured and withstands the combined action
of the mineral acids for a longer or shorter time, though on prolonged
heating it also is dissolved. I do not therefore regard acanthin as an
albuminous substance, but as one related to chitin.
D. Calcareous skeletons have not been certainly demonstrated in the
Radiolaria, and probably do not occur. Sir Wyville Thomson in his
Atlantic (1877, L. N. 31, vol. i. p. 233, fig. 51) described under the
name _Calcaromma calcarea_, a Radiolarian which contained scattered in
its calymma numerous calcareous corpuscles "resembling the rowels of
spurs." These are identical with the "toothed bodies, recalling crystal
balls," which Johannes Müller figured in the Mediterranean _Thalassicolla
morum_ so early as 1858, and compared with the "siliceous asterisks of
_Tethya_" (L. N. 12, p. 28, Taf. vii. figs. 1, 2). I formerly regarded
these peculiar calcareous corpuscles, whose solubility in mineral acids I
had observed, as spicules of a Thalassicollid, and hence described the
species in my Monograph as _Thalassosphæra morum_ (L. N. 16, p. 260). I
have, however, seen reason to change my view, and am now led to suppose
that those peculiar calcareous corpuscles, which may be named
"_Calcastrella_," are not formed by the Radiolarian itself, but are
foreign bodies which have been accidentally incorporated into the calymma
of a Thalassicollid (_Actissa_). These corpuscles occur, often in large
numbers, in many preparations in the Challenger collection, and in the
calymma of other Radiolaria, chiefly #Discoidea#, hence it would appear
that they are foreign bodies taken up by the pseudopodia and carried into
the calymma by the circulation of the sarcode. The Radiolaria which Sir
Wyville Thomson figured as _Calcaromma calcarea_, and Müller as
_Thalassicolla morum_, I regard as species of _Actissa_ (see p. 13),
perhaps _Actissa radiata_ of the Pacific, and _Actissa primordialis_ of
the Mediterranean (compare the description of the Thalassosphærida of the
Challenger collection, pp. 30, 31).
103. _The Physical Properties of the Skeleton._--The skeletons of all
Radiolaria are characterised pre-eminently by a high degree of _firmness_,
which fits them to serve as protective and supporting apparatus. This is
obvious in the case of the pure siliceous shells of the Polycystina; but
the acanthin framework of the ACANTHARIA also possesses a degree of
stiffness but little inferior, whilst the silicate skeletons of the
PHÆODARIA seem on the whole to be not so firm. The hollow skeletal tubes of
the last-named, which are filled with gelatinous material, are very brittle
on account of the delicacy of their walls. Their _elasticity_ also is very
small, whilst that of the acanthin spines is considerable. The thin long
needles of many ACANTHARIA are very elastic, as are also the bristle-like
siliceous spicules of many SPUMELLARIA. The _refractive power_ of the
skeleton in the various legions is very different, depending upon the
chemical constitution. The siliceous skeleton of the Polycystina
(SPUMELLARIA and NASSELLARIA) and the silicate skeleton of the PHÆODARIA
have the same refractive index as glycerine, and hence become invisible
when mounted in that fluid; they then become visible only on addition of
{lxxi}water, and are clearer in proportion to the quantity of water which
is added. The refractive index of acanthin is, however, very different from
that of glycerine, so that the skeletons of ACANTHARIA are readily visible
when mounted in this fluid. In water, the skeletons of all Radiolaria
appear about equally refractive, as also in Canada balsam. The substance of
the skeleton appears almost entirely hyaline, colourless, and transparent.
Very rarely it is faintly coloured (in some ACANTHARIA). A cloudy opaque
constitution is seen in some PHÆODARIA (especially in the "porcellanous
shells" of Tuscarorida and Circoporida, Pls. 100, 114-117); when dried,
these appear by reflected light milky-white or yellowish-white; the cause
of this opacity lies partly in the peculiar "cement-like structure" of
these porcellanous shells, partly in their fine porosity, and the minute
air-bubbles contained in their thick walls.
104. _The Elementary Structure of the Skeleton._--The general constitution
of the skeleton--or more accurately expressed, of the morphological
elements of which the skeleton consists--is of such a nature that it may be
termed structureless. Both the organic acanthin skeletons of the ACANTHARIA
and the silicate skeletons of the PHÆODARIA, as well as the inorganic
siliceous skeletons of the SPUMELLARIA and NASSELLARIA, appear under the
microscope perfectly homogeneous, transparent, colourless, and crystalline.
Only very rarely do they show traces of a concentric striation, which
arises from the deposition of the skeletal substance in layers; as, for
example, the thick spines of some PHÆODARIA (Pls. 105-107, &c.). Some of
the PHÆODARIA, however, form an exception to this rule, inasmuch as their
partially tubular skeletal elements possess a remarkable porcellanous
structure. In the tubular or Cannoid skeleton, which occurs in most
CANNOPYLEA, the lumen of the thin-walled flinty tube is filled with jelly,
and frequently a thin siliceous thread runs in its axis, and is connected
with the wall by transverse threads (§§ 127, 139). The elementary structure
of the opaque porcellanous shells, which distinguish the two families
Circoporida (Pls. 114-117) and Tuscarorida (Pl. 100), is quite peculiar.
Numerous fine siliceous spicules lie scattered irregularly in a finely
granular or porous matrix.
105. _Complete and Incomplete Lattice-Shells._--In the great majority of
Radiolaria (in all four legions) the skeleton has the form of a delicate
lattice-shell or a receptacle in which the central capsule is enclosed. In
a small minority, however, this is not the case. The skeleton then consists
only of isolated rigid pieces (radial or tangential spicules), or of a
simple ring (sagittal ring of the #Stephoidea#), or of a basal tripod with
or without a loose tissue of trabeculæ, &c. (#Plectoidea#); the central
capsule is then not surrounded by a special latticed receptacle, but only
rests upon the skeletal trabeculæ. According to these different
arrangements, two principal groups or sublegions may be distinguished in
each legion, of which one set (Cataphracta) are characterised by a complete
{lxxii}lattice-shell, whilst the others (Aphracta) are without it. The
RADIOLARIA APHRACTA, then, or Radiolaria without a complete skeleton, are
the #Collodaria# (p. 9), the #Acanthometra# (p. 725), the #Plectellaria#
(p. 895), and the #Phæocystina# (p. 1543). On the other hand, the
RADIOLARIA CATAPHRACTA, or Radiolaria with a complete skeleton, are the
#Sphærellaria# (p. 49), the #Acanthophracta# (p. 791), the #Cyrtellaria#
(p. 1015), and the #Phæocoscina# (p. 1590).
Upon this basis the first subdivision of the Radiolaria was made by
Johannes Müller, who recognised three groups:--"I. _Thalassicolla_,
without receptacle, naked or with spicules; II. _Polycystina_, with a
siliceous receptacle; III. _Acanthometra_, without receptacle, but with
siliceous radial spines" (L. N. 12, p. 16).
106. _The Ectolithia and Entolithia (Extracapsular and Intracapsular
Skeletons)._--The relation of the skeleton to the central capsule in the
Radiolaria is very various in many respects; in the first instance two
great groups, _Ectolithia_ and _Entolithia_ (see note A), may be
distinguished topographically by mere external observation; in the former
the skeleton lies entirely outside the central capsule; in the latter,
partially at all events, within it. The _Ectolithia_, with a completely
extracapsular skeleton, include all NASSELLARIA and PHÆODARIA, as well as a
great part of the SPUMELLARIA (all #Collodaria# and the most archaic forms
of #Sphærellaria#); the _Entolithia_, on the other hand, in which the
skeleton lies partly within, partly without the central capsule, include
all ACANTHARIA and the majority of the SPUMELLARIA (most #Sphærellaria#,
see note B).
A. The difference between Ectolithia and Entolithia was applied in my
Monograph in 1862 (p. 222) to separate the Monocyttaria into two main
groups. The arrangement was, however, quite artificial, being contrary to
the natural relations of the larger groups, as was shown seventeen years
later by the discovery of the different structural relations of the
central capsule.
B. Among the ACANTHARIA, which all possess primitively an intracapsular
and centrogenous skeleton, the remarkable _Cenocapsa_ (Pl. 133, fig. 11),
seems to furnish the single exception; in it the skeleton consists of a
simple spherical shell which encloses the concentric central capsule. The
exception is, however, only apparent; the twenty perspinal pores of the
shell show that they were originally in connection with twenty
centrogenous acanthin spines, and that those have disappeared by
retrograde metamorphosis.
107. _Perigenous and Centrogenous Skeletons._--Much more important than the
topographical relation of the skeleton to the central capsule, according to
which the Ectolithia and Entolithia are separated from each other (§ 106),
is the original development of the skeleton within or without the central
capsule, which gives rise to the distinction between perigenous and
centrogenous skeletons. _Centrogenous skeletons_ are found only in the
ACANTHARIA, which are further distinguished from all other Radiolaria by
their skeleton being formed of acanthin; in all ACANTHARIA the formation of
the skeleton begins in the middle of the central capsule, from which twenty
(the number is inconstant only in the {lxxiii}small group #Actinelida#)
radial spines are centrifugally developed. The three other legions, on the
contrary, possess on the whole a _perigenous_ skeleton, which _originally_
develops outside the central capsule and never in its middle. In the
NASSELLARIA and PHÆODARIA the skeleton retains this extracapsular position,
as also in the #Beloidea# and part of the #Sphærellaria# among the
SPUMELLARIA; in the great majority of the latter, however, the primary
perigenous skeleton is subsequently enveloped by the growing central
capsule, so that it lies partially within it (§ 109).
108. _Polyphyletic Origin of the Skeleton._--The skeleton of the Radiolaria
has undoubtedly originated polyphyletically, for it is impossible to derive
its manifold varieties from a single ground-form, or to regard them as
modifications of one type. It is much more probable that the different
skeletonless Radiolaria have entered upon different ways of skeleton
formation quite independently of each other. At the outset it is quite
clear that the skeletons of the _four legions have originated independently
of each other_. Further, it is certain that within the legion of the
SPUMELLARIA the Beloid skeletons of the #Collodaria# are not connected with
the Sphæroid skeletons of the #Sphærellaria# and the forms derived from
them (see § 109). In the same way the skeletons of the PHÆODARIA are
polyphyletic; probably in this legion the Beloid, Sphæroid, Cyrtoid, and
Conchoid skeletons have been developed quite independently (see § 112). In
the NASSELLARIA, on the other hand, it is possible that all the skeletal
forms are to be derived monophyletically from a single simple primitive
form (either the sagittal ring or basal tripod?) (see § 111). Still more
probable is it that the ACANTHARIA have arisen monophyletically, for all
the forms of their acanthin skeleton may be derived without violence from
_Actinelius_ (see § 110).
109. _The Skeleton of the Spumellaria._--The skeletons of the SPUMELLARIA
or PERIPYLEA consist of silica, and are very different and of independent
origin in the two orders of this legion. The first order, #Collodaria#,
have either no skeleton whatever (#Colloidea#, p. 10, Pls. 1, 3), or their
skeleton is _Beloid_, a loose extracapsular envelope of spicules,
consisting of numerous unconnected portions; the separate parts are usually
disposed tangentially, either as simple or compound siliceous spicules
(#Beloidea#, p. 28, Pls. 2, 4). The second order of SPUMELLARIA, on the
other hand (#Sphærellaria#, p. 49), develops a siliceous lattice-shell
which consists of a single piece, and is remarkable for the extraordinary
variety of its forms (pp. 50-715, Pls. 5-50). To this order belong not less
than three hundred genera and seventeen hundred species of the Challenger
Radiolaria (that is, about two-fifths of all the genera and species). In
spite of this extreme richness in different forms this large group must be
regarded as _monophyletic_, since all its forms may be quite naturally
derived from a common stem-form, a _simple lattice-sphere_ (_Cenosphæra_,
p. 61, Pl. 2). The twenty-eight families of #Sphærellaria# may be
distributed in four suborders, among which the #Sphæroidea# constitute the
{lxxiv}stem-forms, since they retain the original spherical shape (Pls.
5-8, 11-30). In the other three suborders a vertical main axis is
developed, which in #Prunoidea# is longer, in #Discoidea# shorter than the
other axes of the shell. Hence the shell of the #Prunoidea# (p. 284, Pls.
13, _bis_, 17, 39, 40) is ellipsoidal or cylindrical, that of the
#Discoidea#, on the other hand, lenticular or discoidal (p. 402, Pls.
31-38, 41-48). Finally, the shell of the fourth suborder, #Larcoidea#, is
lentelliptical; it has the ground-form of a triaxial ellipsoid, and is
characterised by the possession of three unequal dimensive axes, or three
isopolar axes of different lengths perpendicular to each other (p. 599,
Pls. 9, 10, 49, 50).
110. _The Skeleton of the Acantharia._--The skeletons of the ACANTHARIA or
ACTIPYLEA are distinguished from those of all other Radiolaria by two very
important peculiarities; in the first place, they consist not of silica but
of a peculiar organic substance, _Acanthin_, and secondly, their
development is centrogenous, numerous radial spines or acanthin spicules
being formed which are united in the middle of the central capsule. Hence
the ACANTHARIA are the only Radiolaria in which the skeleton originates
from the first in the middle of the central capsule. The number of radial
spines is primitively indefinite, variable, and often considerable (more
than a hundred), but in the great majority it is limited to twenty. In
accordance with this the legion may be divided into two orders, the more
archaic small group Adelacantha, with an indefinite number of spines, and
the more recent group, Icosacantha, which has been developed from them and
possesses twenty regularly disposed spines; of the three hundred and
seventy-two species of ACANTHARIA which have been hitherto described, about
five per cent. belong to the former, about ninety-five per cent. to the
latter division (see note A, below). The numerous genera of Icosacantha may
then be again divided into two suborders, of which the #Acanthonida# (p.
740, Pls. 130-132) produce no complete lattice-shell, and thus agree with
the #Actinelida#, with which they may be united as #Acanthometra# in the
broader sense (or ACANTHARIA without a lattice-shell). The
#Acanthophracta#, on the other hand (p. 791, Pls. 133-140), produce a
complete lattice-shell, usually by means of two opposite or four crossed
transverse processes, which arise from each radial spine and unite with
each other (see note B, below). In most #Acanthophracta# the lattice-shell
remains single; only in the Phractopeltida does it consist of two
concentric lattice-spheres (p. 847, Pl. 133, figs. 1-6). Furthermore, the
whole order #Acanthophracta# may be subdivided into two suborders according
to the different ground-form of the lattice-shell; this remains spherical
in the #Sphærophracta# (the three families Sphærocapsida, Dorataspida,
Phractopeltida, Pls. 133-138). On the other hand, it assumes another form
in the #Prunophracta#; it becomes ellipsoidal in the Belonaspida (Pl. 136,
figs. 6-9), discoidal or lentiform in the Hexalaspida (Pl. 139); and
finally takes the shape of a double cone in the Diploconida (Pl. 140).
{lxxv}A. The group Adelacantha consists only of the suborder
#Actinelida#, with the three families Astrolophida, Litholophida, and
Chiastolida (p. 728, Pl. 129, figs. 1-3); the number of the radial spines
is very different and variable, sometimes only from ten to sixteen, but
usually from thirty to fifty, and often more than one hundred; they are
generally irregularly distributed, and not as in the second main
division. This latter, the Icosacantha, always possesses _twenty_ radial
spines, which are regularly disposed according to a constant law, the
so-called "Müllerian" or "Icosacanthan" law; the twenty spines are always
so placed between the poles of a spineless axis that they form five zones
each of four spines; the four spines of each zone are equidistant from
each other, and also from the same pole, and alternate with those of the
neighbouring zones, so that the whole twenty lie in four meridian planes,
which cut out an angle of 45° (compare pp. 717-722, Pls. 130-140). In
spite of the manifold variations in form which are developed in the
Icosacantha, they may all be derived from a common stem-form,
_Acanthometron_ (p. 742), since the law of distribution of the twenty
spines is constantly inherited.
B. An exception is found in the peculiar family Sphærocapsida (p. 797,
Pl. 133, figs. 7-11; Pl. 135, figs. 6-10). Here the shell is composed of
innumerable small, perforated plates, which arise on the surface of the
calymma independently of the spines.
111. _The Skeleton of the Nassellaria._--The skeletons of the NASSELLARIA
or MONOPYLEA consist of silica, and are never composed of separate
portions, but constitute always a single continuous piece. The ground-form
is originally monaxon, corresponding to that of the central capsule, with a
constant difference between the two poles of the vertical main axis. The
ground-form is never spherical or polyaxon as in the lattice-shells of the
SPUMELLARIA, and the skeleton never consists of hollow tubes, as in the
PHÆODARIA. The legion NASSELLARIA may be divided into two orders; in the
#Plectellaria# (three suborders #Nassoidea#, #Plectoidea#, #Stephoidea#)
the skeleton does not form a complete lattice-shell; in the #Cyrtellaria#,
on the other hand, which are derived from these, the siliceous skeleton
forms a complete lattice-shell enclosing the central capsule. The number of
forms thus developed is astonishingly great, so that among the NASSELLARIA
no less than two hundred and seventy-four genera and sixteen hundred and
eighty-seven species may be distinguished, almost as many as in the
#Sphærellaria#. In spite of this great variety of forms the legion
MONOPYLEA is probably monophyletic; at least all the different skeletal
forms may be derived from three elements which are combined in the most
manifold fashion; (1) the _sagittal ring_, a simple siliceous ring, which
lies vertically in the sagittal plane of the body, encircles the central
capsule and comes into contact with it at the basal pole of the main axis
(§ 124); (2) the _basal or oral tripod_, composed of three diverging radial
spines, which meet in the middle of the basal pole of the central capsule
(or in the centre of the porochora) (§ 125); (3) the _cephalis_, or
lattice-head, a simple ovoid or subspherical lattice-shell, which encloses
the central capsule and stands in connection with it at the basal pole of
its main axis. Any one of these three important structural elements of the
NASSELLARIAN skeleton may possibly be the starting-point {lxxvi}for all the
remaining forms of the MONOPYLEA; the great difficulty in their
phylogenetic derivation lies in the facts that, on the one hand, any one of
the three elements may alone constitute the skeleton, and on the other
hand, in the great majority of the legion, two or three are united together
(compare §§ 182-185).
112. _The Skeleton of the Phæodaria._--The skeleton of the PHÆODARIA or
CANNOPYLEA is always extracapsular, usually consists of a silicate of
carbon (more rarely of pure silica), and in the majority of the legion is
composed of hollow cylindrical tubes, whose siliceous wall is very thin,
and whose lumen is filled with gelatinous material (§ 127). The manifold
and remarkable skeletal forms occurring in this legion are not
monophyletic, since they cannot be derived from a common stem-form; they
are, on the contrary, polyphyletic, various skeletonless PHÆODARIA
(Phæodinida) have independently acquired skeletons of different form and
composition. The legion PHÆODARIA can be subdivided into four orders, the
skeletons of which present the following important distinctions:--(1) The
#Phæocystina# possess only incomplete Beloid skeletons (§ 115), composed of
many separate pieces, sometimes tangentially (Cannorrhaphida, Pl. 101),
sometimes radially arranged (Aulacanthida, Pls. 102-105). (2) The
#Phæosphæria# form Sphæroid skeletons (§ 116), usually only a simple
lattice-shell without special aperture (Pls. 106-111); two concentric
shells united by radial bars occur only in the Cannosphærida (Pl. 112). (3)
The #Phæogromia# are distinguished by the formation of a simple Cyrtoid
skeleton (§ 123) resembling that of the Monocyrtida; the monothalamus
lattice-shell is usually ovoid or helmet-shaped, more rarely polyhedral or
almost spherical; a vertical main axis can always be distinguished, at the
basal pole of which is an aperture usually armed with teeth or spines (Pls.
99, 100, 113-120). (4) The #Phæoconchia# are distinguished from all other
Radiolaria by the possession of a bivalved shell like that of the
Conchifera; the two valves of this Conchoid skeleton must be distinguished
as dorsal and ventral, as in the Brachiopoda (Pls. 121-128). The fifteen
families of PHÆODARIA which are arranged in the four orders just mentioned,
present such great differences among themselves, that the skeleton must be
regarded as probably polyphyletic even within the limits of each order.
113. _Types of Skeletal Formation._--No less than twelve different
principal forms may be distinguished as morphological types of the
formation of the skeleton in the Radiolaria; some of these are peculiar to
a single legion or even to a smaller group; but sometimes the same form
occurs in several legions. Some types occur only in an isolated manner,
independently of the others, but most exist in various combinations with
other types. Of the twelve described below the Conchoid and Cannoid occur
only in the PHÆODARIA; the Plectoid and Circoid only in the NASSELLARIA;
the Astroid only in the ACANTHARIA; the remaining seven types are found in
several legions in the same form and hence are polyphyletic.
{lxxvii}114. _The Astroid Skeleton._--Under the name "Astroid" we place the
peculiar star-shaped skeletons of the ACANTHARIA in opposition to those of
all other Radiolaria, for they are separated from them not only
fundamentally by reason of the chemical nature of their substance
(Acanthin, § 102), but also by their centrogenous origin, and the resulting
stellate form (Pls. 129-140). The ACANTHARIA are the only Radiolaria in
which the skeleton arises within the central capsule by the formation of
numerous rays or radial spines of acanthin which project on all sides from
the centre. Originally these are united at this point, their conical or
pyramidal points meeting and being supported one upon another. In the great
majority of ACANTHARIA this loose apposition is constant, so that when the
soft parts are destroyed the skeleton falls to pieces. Only in a few forms
in this legion are the central ends of the spines fused so that the whole
skeleton forms a connected star (_Astrolithium_). The small group
Chiastolida (or Acanthochiasmida) is characterised by the fact that the two
rays which are opposite to one another in each axis unite and form a
diametral bar. The skeleton is almost always composed of twenty radial
spines, which are regularly disposed (Icosacantha), only in the small
primitive group #Actinelida# is the number variable (Adelacantha, § 110).
115. _The Beloid Skeleton._--As Beloid or spicular skeletons are grouped
together all those which consist of several disconnected portions; these
always lie outside the central capsule, either within the calymma or on its
surface. Such extracapsular Beloid skeletons are entirely wanting in the
ACANTHARIA and NASSELLARIA; they occur only in the #Beloidea# among the
SPUMELLARIA, and in the #Phæocystina# among the PHÆODARIA; the individual
Beloid portions of the former are solid, those of the latter hollow. In
both groups the simplest forms of the separate portions are simple
unbranched needles (_Thalassosphæra_, _Thalassoplancta_, _Physematium_,
_Belonozoum_, among the SPUMELLARIA; _Cannobelos_ and _Cannorrhaphis_ among
the PHÆODARIA); usually these spicules are disposed tangentially over the
surface of the calymma. Among the #Beloidea# branched spicules occur more
commonly than these simple ones; they are either stellate (with many rays
united in a centre) or twin-like, with a tangential bar, from each pole of
which two or three (seldom more) radial branches project (Pls. 2, 4). Among
the PHÆODARIA the subfamily Dictyochida is characterised by the annular
shape of its Beloid portions, either simple rings, or hat-shaped or
pyramidal bodies with a latticed cap over the ring (Pl. 101, figs. 3-14;
Pl. 114, figs. 7-13). The family Aulacanthida among the PHÆODARIA, alone
possesses hollow _radial tubes_, which penetrate the whole calymma, and
project distally over its surface, whilst their proximal ends rest upon the
surface of the central capsule. Although in these cases the enclosed
proximal end is always simple, the free distal end develops the most
various processes in adaptation to its prehensile functions (Pls. 102-105).
{lxxviii}116. _The Sphæroid Skeletons or Lattice-Spheres._--The
"lattice-spheres" or sphæroid skeletons are the simplest and most primitive
forms of lattice shells, and are widely distributed in the three legions
SPUMELLARIA, ACANTHARIA, and PHÆODARIA, whilst they are entirely wanting in
the NASSELLARIA. The round lattice-shell is either a true sphere in the
geometrical sense, or an endospherical polyhedron, _i.e._, a polyhedron,
all whose angles lie in the surface of a sphere (§ 25). In general,
_primary_ and _secondary_ lattice-spheres may be distinguished, of which
the former are secreted on the outer surface of the primary, the latter on
that of the secondary calymma (§ 85). Furthermore, _simple_ and _compound_
lattice-spheres may be distinguished, the latter of which consist of two or
more concentric lattice-spheres firmly united by radial bars; in such cases
the innermost lattice-sphere is always to be regarded as the oldest or
primary, all the succeeding ones as secondary, and the outermost as the
youngest (§ 129). The simple lattice-spheres are usually to be regarded as
primary; they may, however, occasionally be secondary, in which case the
primary shell, originally enclosed, has been lost by degeneration (as, for
example, in the case of the Aulosphærida and some #Sphærellaria#).
117. _The Lattice-Spheres of the Spumellaria._--The lattice-spheres or
Sphæroid skeletons of the SPUMELLARIA exhibit in spite of their simple type
of structure, an extraordinary variety in the formation of the lattice-work
and radial apophyses, so that in the systematic portion of this work no
less than one hundred and seven genera and six hundred and fifty species
are distinguished; these are united in one suborder, the #Sphæroidea# (pp.
50-284, Pls. 5-8, 11-30). It may be divided into two main divisions, the
_Monosphærida_ with a single primary lattice-sphere (Pls. 12-14, 21, 26,
27), and _Pliosphærida_ (or Sphæroidea concentrica) whose skeleton consists
of two or more concentric lattice-spheres united by radial bars. The latter
are subdivided into Dyosphærida with two concentric lattice-spheres (Pls.
16, 19, 20, 22, 28); Triosphærida, with three spheres (Pls. 17, 24, 29);
Tetrasphærida, with four (Pls. 23, 30); Polysphærida, with five or more
(Pls. 15, 23); and Spongosphærida, with spongy lattice-spheres (Pls. 18,
25). A special group is made up of the simple lattice-spheres of the social
Collosphærida (or Sphæroidea polyzoa) (Pls. 5-8); these are usually more or
less irregular, and characterised by the development of peculiar tubular
processes; the latter are generally wanting in the Sphæroidea monozoa,
whose lattice-shell is very regularly formed. This distinction is
interesting and important, inasmuch as the regular lattice-spheres are
explained by the independent development of the free-swimming Monozoa,
whilst the irregular spheres are due to the mutual dependence of the social
Polyzoa.
118. _The Lattice-Spheres of the Acantharia._--The lattice-shells or
Sphæroid skeletons of the ACANTHARIA are immediately distinguishable from
those of all other Radiolaria by their centrogenous development and the
central union of the radial spines by which they are supported; the only
exception is furnished by the remarkable genus _Cenocapsa_ {lxxix}(Pl. 133,
fig. 11), in which the radial spines are absent, not primitively, however,
but in consequence of degeneration; for the twenty cross-shaped perspinal
pores, originally due to the twenty radial spines, are still present. In
the most nearly allied genera, _Porocapsa_ (Pl. 133, fig. 7) and
_Cannocapsa_ (Pl. 133, fig. 8), the proximal part of the twenty radial
spines is still present, while their distal portion has degenerated; hence
in this case they do not stand in direct communication with the spherical
shell. On the other hand, this primitive connection persists in the genera
_Astrocapsa_ (Pl. 133, figs. 9, 10), and _Sphærocapsa_ (Pl. 135, figs.
6-10). The five genera just mentioned form the peculiar family
Sphærocapsida (pp. 795-802); the spherical shell is in these cases composed
of very numerous small plates disposed like a pavement, each plate or aglet
being perforated by a pore canal; in addition to which there are twenty
larger (perspinal) pores (or twenty cross-shaped groups each of four
aspinal pores) at those important points where primitively the twenty
radial spines penetrate the calymma. This peculiar porous "pavement shell"
has probably been developed (independently of the twenty radial spines)
upon the calymma of the #Acanthonida# (_Acanthonia_, p. 749) by the action
of the sarcodictyum; it has, therefore, quite a different morphological
significance from the spherical lattice-shell of the Dorataspida, which is
composed of tangential apophyses of the twenty Acanthonid spines (pp.
802-847, Pls. 134-138). Each radial spine here forms either two opposite or
four crossed transverse processes, and since their branches spread over the
surface of the spherical calymma and are united suturally at their
extremities, the peculiar lattice-sphere of the Dorataspida arises. This
extensive family is again divided into two subfamilies:--the Diporaspida
(Pls. 137, 138) possess always only two opposite apophyses, and form by the
union of their branches two opposite primary apertures or aspinal meshes.
The Tessaraspida, on the other hand (Pls. 135, 138), have always four
crossed transverse processes, and form by their union four primary aspinal
meshes. From the Diporaspida are probably to be derived the Phractopeltida
(p. 847, Pl. 133, figs. 1-6), the only ACANTHARIA which possess a double
lattice-sphere; their double concentric spherical shell may be compared
with that of the Dyosphærida.
119. _The Lattice-Spheres of the Phæodaria._--The lattice-spheres or
Sphæroid skeletons of the PHÆODARIA, which are generally developed quite
regularly, though occasionally in a modified form, fall in the order
#Phæosphæria# into two groups of very different structure, each of which
includes two families. The first group (_Phæosphæria inarticulata_)
contains the families Orosphærida (Pls. 106, 107) and Sagosphærida (Pl.
108); the lattice-work of the former consists of irregular polygonal meshes
and very coarse, partially hollow trabeculæ; in the latter, on the other
hand, it consists of triangular meshes and very slender filiform trabeculæ;
in both families the whole sphæroid skeleton forms a single unsegmented
piece as in most #Sphæroidea#. In the second group of {lxxx}#Phæosphæria#
(_Phæosphæria articulata_), on the other hand, the lattice-sphere is
segmented in quite a peculiar manner, and composed of hollow cylindrical
tangential tubes, which are separated by astral septa at the nodal points
of the network; this remarkable structure characterises the two families,
Aulosphærida (Pls. 109-111) and Cannosphærida (Pl. 112); the segmented
lattice-sphere of the former is simple and hollow; while that of the latter
is connected by centripetal radial tubes with a simple concentric inner
shell, which is sometimes solid, sometimes latticed, and provided with a
main-opening corresponding to the astropyle of the enclosed central
capsule. Since in the Aulosphærida also, hollow centripetal radial tubes
project from the segmented lattice-sphere, it is possible that they have
been derived from the Cannosphærida by the loss of the primitive internal
shell. A special peculiarity of many #Phæosphæria# (_Oroscena_,
_Sagoscena_, _Auloscena_, &c.) consists in the fact that the whole surface
of the lattice-sphere is regularly covered with pyramidal or tent-shaped
prominences (Pl. 106, fig. 4; Pl. 108, fig. 1; Pl. 110, fig. 1). A simple
lattice-sphere quite similar to that of most Monosphærida also constitutes
the skeleton of the Castanellida (Pl. 113), but since it possesses a
special main-opening, it must be referred promorphologically to the Cyrtoid
shells of the #Phæogromia#.
120. _The Prunoid Skeleton or Lattice-Ellipsoid._--The "lattice-ellipsoids"
or Prunoid skeletons have arisen from the lattice-spheres or Sphæroid
skeletons by more energetic growth and elongation of one axis; this is the
main axis of the body and is probably always vertical; its two poles are
commonly equal. The Prunoid skeleton is either a true ellipsoid in the
geometrical sense or an "endellipsoidal polyhedron" (_i.e._, a polyhedron,
all the angles of which lie in an ellipsoidal surface). By further
elongation of the main axis, the ellipsoidal form passes over into the
cylindrical, the polar surfaces of the cylinder being usually rounded,
rarely truncated. The rich order #Prunoidea# (pp. 284-402) contains
numerous modifications of this form of shell which arise on the one hand by
the formation of transverse constrictions, on the other by the apposition
of concentric secondary shells. In respect of the latter, simple and
compound Prunoid shells can be distinguished as in the case of the Sphæroid
shells. In the compound Prunoid shells either all the concentric
lattice-shells may be ellipsoidal or the inner may be spherical. More
important differences are found in the transverse annular constrictions,
which give the Prunoid skeleton a segmented appearance; in this respect,
three principal forms may be distinguished (p. 288):--(A) _Monoprunida_,
with unsegmented shell, having no transverse constriction (Pls. 15-17); (B)
_Dyoprunida_, having a shell with two segments and one (equatorial)
transverse constriction (Pl. 39); (C) _Polyprunida_, with three or more
parallel transverse constrictions, by means of which the shell is divided
into four or more segments (Pl. 40). In the same manner as the #Prunoidea#
have arisen from the #Sphæroidea# among the SPUMELLARIA by greater
{lxxxi}development of the vertical main axis, the ellipsoidal Belonaspida
have arisen from the spherical Dorataspida among the ACANTHARIA (p. 859;
Pl. 136, figs. 6-9; Pl. 139, figs. 8, 9). The main axis of the ellipsoid in
this case is always occupied by the opposite equatorial spines of the
hydrotomical axis (pp. 719, 860). In the legion PHÆODARIA a similar
prolongation of the main axis rarely occurs; it is found, however, in
_Aulatractus_ (Pl. 111, figs. 6, 7), the lattice-shell of this Aulosphærid
being sometimes truly fusiform, sometimes rather ellipsoidal or even
double-conical.
121. _The Discoid Skeletons or Lattice-Discs._--The "lattice-discs" or
Discoid skeletons are characteristic of the SPUMELLARIAN group #Discoidea#,
and have arisen from the lattice-spheres of the #Sphæroidea# by a less
development of one axis, which is the main axis of the body, and is
probably usually vertical; its two poles are always equal. The Discoid
lattice-shell is either a biconvex lens (with a thin margin), or a plane
disc (a shortened cylinder with thick margin), or some form intermediate
between the two. All Discoid shells show a horizontal median plane or
equatorial plane, by which they are divided into two equal halves, an upper
and lower; the margin of the lens itself is originally the equator. The
main axis, the shortest of all the axes of the shell, stands vertically in
the centre of the equatorial plane. Among the PHÆODARIA Discoid shells
rarely occur (_Aulophacus_), as also among the ACANTHARIA (Hexalaspida).
122. _The Larcoid Skeleton or Lentelliptical Lattice-Shell._--The
lentelliptical lattice-shells, which may be shortly designated "Larcoid,"
are especially characteristic of the #Larcoidea#, a large order of
SPUMELLARIA (pp. 599-715; Pls. 9, 10, 49, 50). In addition they recur among
the ACANTHARIA, in the small family Hexalaspida (p. 872, Pl. 139), and the
family Diploconida (p. 881, Pl. 140), which is derived from it. These
lentelliptical lattice-shells are all characterised by the clear
differentiation of three unequal, but isopolar dimensive axes, _i.e._, the
three geometrical axes, perpendicular to one another, which determine the
form of the shell, are of unequal length; the two poles of each are,
however, equal. The geometrical ground-form is, therefore, a triaxial
ellipsoid (§ 34). In the rich order #Larcoidea# the lentelliptical
lattice-shell shows many variations in its development.
123. _The Cyrtoid Skeleton._--Cyrtoid skeletons are those lattice-shells
which possess a vertical main axis with two different poles (Monaxonia
allopola); the upper pole is usually termed the apical, the lower the
basal. Such Cyrtoid shells are characteristic of the great majority of the
NASSELLARIA or MONOPYLEA (and especially of the #Cyrtellaria#); they are
also found in a large division of the PHÆODARIA (the #Phæogromia#), and in
some SPUMELLARIA. In general the manifold Cyrtoid shells may be divided
into two large groups, those with one and those with several chambers. The
_monothalamous_ Cyrtoid shells are usually ovoid, conical, cap- or
helmet-shaped; their {lxxxii}internal cavity is simple, without
constrictions or septa. Among the NASSELLARIA they occur in the Monocyrtida
(Pls. 51-54, 98), where they have received the name "Cephalis." A form of
shell, essentially the same, is found amongst the PHÆODARIA in the order
#Phæogromia#, more especially in the Challengerida (Pl. 99), Medusettida
(Pls. 118-120), and Tuscarorida (Pl. 100), many of these latter closely
resembling many Monocyrtida. Such monothalamous Cyrtoid shells occur much
more rarely among the SPUMELLARIA (_e.g._, among the #Prunoidea# in
_Lithapium_, _Lithomespilus_, _Druppatractus_, Pls. 13, 14, &c.).
Polythalamous Cyrtoid shells (Pls. 55-80) occur exclusively in the
NASSELLARIA, and exhibit in this legion an astonishing variety of
structure; they are distinguished from the monothalamous forms by the
development of internal septa, or of annular incomplete diaphragms, which
usually correspond to the external constrictions; their interior is thus
divided into two or more communicating compartments. Among the
polythalamous Cyrtoid shells may be distinguished three principal groups,
the Stichocyrtid, Zygocyrtid, and Polycyrtid. Zygocyrtid shells are
characteristic of the #Spyroidea# (Pls. 84-90), and are distinguished by a
bilobate cephalis (cephalis bilocularis); the median sagittal ring, or a
corresponding constriction, divides the shell into right and left
compartments. Polycyrtid shells (Pl. 96) are peculiar to the #Botryodea#,
and characterised by a multilobate cephalis (cephalis multilocularis).
Stichocyrtid shells are those in which the primary cephalis remains simple,
and new joints are successively added to its basal pole; such shells occur
in the majority of the #Cyrtoidea#. Secondary chambers are sometimes added
in the other two groups (#Botryodea# and #Spyroidea#). When, as often
happens in these polythalamous Cyrtoid shells, two or three distinct joints
follow each other, the first is called the "cephalis," the second the
"thorax," and the third the "abdomen" (Tricyrtida Pls. 64-75).
124. _The Circoid Skeleton._--This is a very important and remarkable type
of skeletal formation, which occurs exclusively in the legion NASSELLARIA,
where it plays a very prominent part; its characteristic element is the
"sagittal ring," a simple, vertical, siliceous ring, which surrounds the
central capsule in its sagittal plane, and is specially differentiated in
its basal portion. This "primary sagittal ring" whose vertical allopolar
main axis coincides with that of the Monopylean central capsule embraced by
it, is characteristic of all members of the order #Stephoidea# (p. 931,
Pls. 81-83, 92-94); here it forms by itself the skeleton of the Stephanida
(Pl. 81); in the Semantida (Pl. 92) it is combined with a horizontal basal
ring, in the Coronida (Pls. 82, 93) with a vertical frontal ring and in the
Tympanida (Pls. 83, 94) with two horizontal rings, an upper mitral and a
lower basal. In the great majority of these #Stephoidea# there often
develop in definite places characteristic processes or apophyses, whose
branches combine to form a loose tissue or an incomplete lattice-shell.
This becomes complete in the #Cyrtellaria#, the majority of which retain
more or less {lxxxiii}distinct traces of the sagittal ring. Hence the
skeletons of all NASSELLARIA may be derived monophyletically (Hypothesis A,
p. 893) from a simple sagittal ring (_Archicircus_ and _Lithocircus_, Pl.
81). This theory, however, encounters the great difficulty that in many
#Stephoidea# (_Cortina_, _Cortiniscus_, &c.) it is combined in a remarkable
manner with the basal tripod of the #Plectoidea#, whilst in these latter it
is entirely wanting (compare p. 894).
125. _The Plectoid Skeleton._--Those forms are distinguished as Plectoid in
which three, four, or more radial siliceous spines proceed from a common
point, which lies excentrically outside the central capsule and at the
basal pole of its vertical allopolar main axis. This peculiar type of
skeletal formation only occurs in the legion NASSELLARIA, and is specially
characteristic of the order #Plectoidea# (p. 898, Pl. 91). But since the
essential elements of this remarkable skeleton also occur in many other
NASSELLARIA, sometimes combined with the Circoid, sometimes with the
Cyrtoid skeleton, it perhaps has a fundamental significance in this legion;
at all events it is possible to derive monophyletically all the other forms
of this legion from it (Hypothesis B, p. 893). The simplest form of the
Plectoid skeleton is a tripod, the three feet of which either lie in a
horizontal plane (_Triplagia_, Pl. 91, fig. 2), or correspond to the three
edges of a low pyramid (_Plagiacantha_). A fourth ray is sometimes added,
which stands vertically upon the summit of the pyramid (_Plagoniscus_,
_Plagiocarpa_, Pl. 91, figs. 4, 5). In other #Plectoidea# three secondary
rays are intercalated between the three primary (Hexaplagida, &c.); seldom
the number is greatly increased (Polyplagida, &c.). The rays are rarely
simple, but usually branched; in the Plagonida (Pl. 91, figs. 2-6) the
branches remain free; in the Plectanida (Pl. 91, figs. 7-13) they are
united to form a loose wicker-work. From such a web a perfect Cyrtoid shell
may arise. Several forms of Plagonida may also be readily confounded with
the isolated triradiate or quadriradiate spicula of many Beloid skeletons
(_Sphærozoum_, _Lampoxanthium_, &c.).
126. _The Spongoid Skeleton._--From the simple lattice-skeleton which the
majority of Radiolaria possess, some of them develop a spongy shell; the
trabeculæ of the lattice-work, situated in one plane in the former, are
developed in the latter in different planes and cross irregularly in all
directions; thus arises a kind of wicker-work of more or less spongy
structure, usually with very thin trabeculæ and irregular meshes. Such
Spongoid shells are most common among the SPUMELLARIA, especially in the
#Sphæroidea# (Spongosphærida, Pl. 18) and #Discoidea# (Spongodiscida, Pls.
41-47), more rarely in the #Prunoidea# and #Larcoidea#. Lattice-work of
similar spongy structure occurs very seldom among the NASSELLARIA, _e.g._,
in some #Plectoidea# (Pl. 91) and #Cyrtoidea# (_Spongocyrtis_,
_Spongopyramis_, _Spongomelissa_, &c., Pl. 56, fig. 10; Pl. 64, figs. 5-10,
&c.). Among the PHÆODARIA spongy skeletons are very rare; they {lxxxiv}are
to be seen in some #Phæosphæria# (_Oroplegma_, Pl. 107, fig. 1;
_Sagoplegma_, Pl. 108, fig. 2; _Auloplegma_, Pl. 111, fig. 8). No Spongoid
skeletons are known among the ACANTHARIA.
127. _The Cannoid Skeleton._--Cannoid or tubular skeletons are those which
are composed of hollow tubes; they occur exclusively in the PHÆODARIA or
CANNOPYLEA. Tubular processes, nevertheless, occur in some other
Radiolaria, as, for example, among the SPUMELLARIA in a portion of the
Collosphærida (_Siphonosphæra_, _Caminosphæra_, Pls. 6, 7), and of the
#Prunoidea# (_Pipetta_, _Cannartus_, &c., Pl. 39, figs. 6-10, &c.), also
among the NASSELLARIA in _Theosyringium_ (Pl. 68, figs. 4-6), _Cannobotrys_
(Pl. 96, figs. 3, 4, 8-11, 20-22), &c. In all these cases, however, the
tubes are direct processes of the cavity of the shell, the trabeculæ of the
lattice-work being solid. Only in the CANNOPYLEA are the lattice-bars
themselves, the radial spines and appendicular organs, generally tubular
(hence the designation "Pansolenia"). The lumen of the thin-walled
siliceous tubes is filled with jelly, and hence the specific gravity of the
relatively large skeleton is considerably diminished. This peculiarity is
not found in all CANNOPYLEA; it is wanting in all Sagosphærida and
Concharida, as well as in a part of the Orosphærida and Castanellida; in
the latter there are found intermediate stages between hollow and solid
skeletal rods. Very often a fine siliceous thread runs in the axis of the
tubes, which is connected with its wall by lateral branches (Pl. 110, figs.
4, 6; Pl. 115, figs. 6, 7). More seldom the tubes are divided by horizontal
septa into a series of chambers (Medusettida, Pls. 118-120). The two
families Aulosphærida (Pls. 109-111) and Cannosphærida (Pl. 112) are
distinguished from all other PHÆODARIA by the fact that their tubes are
separated by astral septa in the nodal points of the lattice-shell (§§ 112,
134).
128. _The Conchoid Skeleton._--By the name "Conchoid skeletons" are
distinguished the bivalved lattice-shells which occur exclusively in the
legion PHÆODARIA; they are quite characteristic of the #Phæoconchia# or
_Phæodaria bivalvia_, which embrace three families:--Concharida (Pls.
123-125), Coelodendrida (Pls. 121, 122), and Coelographida (Pls. 126-128).
The two valves of the lattice-shell of the Concharida are simple,
hemispherical, or boat-shaped, whilst in the Coelodendrida and
Coelographida tubes grow out from them, which branch and usually give rise
by anastomosis to a second external bivalved shell. In all #Phæoconchia#
the two valves are so disposed about the central capsule that an open slit
remains between them, into which open the apertures of the central capsule;
and since all these _Phæodaria conchoidea_ are TRIPYLEA, with three typical
openings in the central capsule, and since the two lateral accessory
openings lie at either side of the aboral pole, and the unpaired
main-opening at the oral pole of the main axis, it follows that the two
valves are to be regarded as dorsal and ventral as in the Brachiopoda (not
right and left as in the Lamellibranchiata). The dorsal and ventral
{lxxxv}valves are usually equal, but in a portion of the Concharida they
present constant differences. In this family the two valves are attached to
each other by their free edges, just as in the bivalved Mollusca and
Diatoms; and these edges may either be smooth (Conchasmida, Pl. 123, figs.
1-6), or dentate (Conchopsida, Pls. 124, 125); the valvular connection of
the latter is sometimes strengthened by a special ligament which unites the
two valves at the aboral pole (Pl. 123, figs. 8, 9). The form of the valve
is sometimes hemispherical, sometimes boat-shaped, with a sagittal keel.
129. _Medullary and Conical Shells._--In all Radiolaria whose skeleton
consists of a double shell or of two concentric lattice-shells united by
radial bars, an inner medullary shell (testa medullaris) and an outer
cortical shell (testa corticalis) may be distinguished (see note A, below).
The medullary shell is usually to be regarded as a primary, the cortical as
a secondary structure. Such double shells occur among the SPUMELLARIA in
the Dyosphærida (Pls. 19, 20), as well as in many #Prunoidea# (Pls. 39,
40), #Discoidea# (Pls. 33, 34), and #Larcoidea# (Pls. 9, 10); among the
ACANTHARIA only in the family Phractopeltida (Pl. 133); among the
NASSELLARIA only in very few #Cyrtoidea# (_e.g._, _Periarachnium_, Pl. 55,
fig. 11), and finally among the PHÆODARIA in the Cannosphærida (Pl. 112) as
well as in part of the Coelodendrida (Pl. 121) and Coelographida (Pls. 127,
128). In most cases (if not always?) the cortical shell arises by the
growth of radial spines from the surface of the medullary shell; these
become united at equal distances from the centre by transverse apophyses,
the surface of the secondary calymma furnishing the basis for their
secretion (§ 85). Nevertheless, it seems that in many #Sphærellaria# the
formation of the whole cortical shell proceeds simultaneously (at a
definite dictyotic period) like that of the primary medullary shell (see
note B). Whilst in the PHÆODARIA, ACANTHARIA, and NASSELLARIA, at most two
concentric shells are formed, in many SPUMELLARIA their number increases
continuously with additional growth; in many #Sphærellaria# it rises to
four, eight, or even more, as well as in many #Discoidea# (if the
concentric, peripherally disposed rings of chambers be regarded as
incomplete flattened shells). In these cases either only the innermost
primary lattice-shell is to be styled "medullary shell," or at most the two
innermost (inner and outer medullary shells), all the others being
cortical.
A. The distinction between medullary and cortical shells was originally
based in my Monograph (1862, p. 50) upon the topographical relation of
the lattice-shells to the central capsule, inasmuch as I regarded all
intracapsular shells as medullary, all extracapsular as cortical.
Hertwig, however (1879, p. 122), rightly pointed out that this
distinction is unpractical, "because the same lattice-shell in the same
species may lie within or without the central capsule, according to the
size of the latter." He proposes, therefore, to restrict the term
medullary shell to the innermost, and to call all the others cortical; a
course which seems justified by the special significance of the primary
innermost lattice-shell ("as the point of origin of the radial spines").
But in most #Sphærellaria# which form three or more concentric shells,
the two innermost, which lie near together within the {lxxxvi}central
capsule, are very different in size and dictyosis from all the others
which lie outside, and are separated by wider interspaces (compare Pls.
17, 24, 29-32, 40, &c.). In these cases it appears better to regard the
two inner as inner and outer medullary shells, and all the others as
cortical shells. The character of the dictyosis in the intracapsular and
extracapsular shells is often so different that I have made it the basis
of separation of _Thecosphæra_ and _Rhodosphæra_ among the Liosphærida
(p. 60), of Elatommatida and Diplosphærida among the Astrosphærida (p.
208), &c.
B.--R. Hertwig (1879, L. N. 33, pp. 40, 123) separates the true
(simultaneously formed) "cortical shells" (_e.g._, of _Actinomma_,
_Cromyomma_) from the arachnoid "siliceous networks" (_e.g._, of
_Diplosphæra_ and _Arachnosphæra_) which are formed by the successive
union of tangential apophyses of the radial spines. Whether this
principle is right in the theory or not, it cannot be carried out
practically. Compare also Pl. 25, fig. 4.
130. _Dictyosis or Lattice Formation of the Skeleton._--In the great
majority of Radiolaria the dictyosis or formation of lattice-work, and
especially the formation of a variously-shaped "lattice-shell," plays such
an important part that the whole class has long been popularly known in
Germany by the name "lattice animalcules" ("Gitterthierchen" or
"Gitterlinge") (_Protista dictyota_). The old name Polycystina also (1838),
although referring only to the SPUMELLARIA and NASSELLARIA, is derived from
the lattice-work of the siliceous skeleton. The extremely various forms in
which this is manifested furnish the means of distinguishing species. The
specific conformation of the skeletal lattice-work is usually caused by the
special disposition of the sarcodictyum (§ 94), whose exoplasmatic threads
become silicified or (in the ACANTHARIA) converted into bars of acanthin.
In many cases, however, the form of the lattice is mainly dependent upon
the situation and form of the radial spines or of special processes from
them. With respect to their origin, two varieties of lattice may be
distinguished--simultaneous and successive. _Simultaneous dictyosis_ occurs
especially in the simple lattice-shells of the #Sphærellaria# and
PHÆODARIA, where, at a given moment ("dictyotic moment") the _whole_
lattice of the shell is excreted on the surface of the calymma. _Successive
dictyosis_, on the other hand, is found more particularly in the
lattice-shells of the ACANTHARIA (and in the concentric cortical shells of
many #Sphærellaria#), which develop from the separate lattice-plates formed
by the apophyses of the radial spines, and hence not at the same moment.
The lattice-shells of the #Cyrtellaria#, which gradually grow out from a
sagittal ring or a basal tripod, arise by successive dictyosis.
131. _Dictyosis of the Spumellaria._--Siliceous lattice-structures are
wanting in the first section of the SPUMELLARIA, the #Collodaria#, but in
the second section, #Sphærellaria#, they are developed in extraordinary
variety of details. In spite of this extreme richness in different forms,
the lattice-shells of the SPUMELLARIA may all be derived from one and the
same primitive ground-form, a simple lattice-sphere with regular hexagonal
meshes (_Phormosphæra_, p. 61, Pl. 12, figs. 9-11; _Heliosphæra_, &c.).
{lxxxvii}The siliceous bars which bound these regular and subregular meshes
are at first exceedingly then and filiform; afterwards they become thicker
or spread out laterally, whence the meshes often become round with a
hexagonal frame (Pl. 12, fig. 5; Pl. 28, fig. 1). If the latter vanish, a
lattice-shell with simple circular meshes is formed. Very commonly the
regular form of the meshes or pores becomes more or less irregular,
polygonal, or roundish. Hence, in general, four different principal forms
of dictyosis may be distinguished among the SPUMELLARIA; viz. (1) regular
or subregular _hexagonal_ meshes; (2) regular or subregular _circular_
meshes; (3) irregular _polygonal_ meshes; (4) irregular _roundish_ meshes.
The three latter forms are to be regarded as secondary, derived from the
primary first form. In those SPUMELLARIA which possess several concentric
lattice-shells enclosed one within another, either these have all the same
form of dictyosis, or the lattice-work of the innermost primary shell is
different from that of the outer secondary shells (Pls. 19, 20); sometimes
these latter also differ more or less among themselves (§ 129).
132. _Dictyosis of the Acantharia._--The lattice-structures of the
ACANTHARIA differ essentially from those of other Radiolaria in several
particulars. Firstly, they consist not of silica but of acanthin (§ 102);
secondly, they are always secondary formations, usually developed from
transverse processes of the primary centrogenous radial spines; thirdly,
their formation is not simultaneous (at the same time over the same shell),
but successive (proceeding from the individual radial spines tangentially
towards the middle of the intervals); fourthly, the configuration of the
network is due to the relative position of the spines and the mode of union
of their transverse apophyses. Since they are at right angles to the
spines, and since the branches of the apophyses are at right angles to
them, the original ground-form of their dictyosis is a lattice-work with
quadrangular meshes; these are often quite regular and square (Pl. 130,
figs. 5, 6; Pl. 136, figs. 2, 9, &c.); more commonly they are rectangular
or irregularly quadrangular (Pl. 131, fig. 10; Pl. 133, figs. 2, 3, &c.).
In the majority of the ACANTHARIA the quadrangular form of the meshes
passes over into an irregularly polygonal or roundish one (Pls. 137, 138).
Very often the primary meshes of the lattice-shells, which immediately
surround the radial spines, are larger and more regular ("aspinal pores"),
whilst the numerous secondary meshes between them are smaller and irregular
("coronal pores"; Pl. 135, figs. 1-4, &c.).
133. _Dictyosis of the Nassellaria._--The siliceous lattice-structures of
the NASSELLARIA are formed on the whole like those of the SPUMELLARIA, with
which they were formerly united under the name "Polycystina." In this group
also there may be distinguished as two main forms the regular and
irregular. In the NASSELLARIA the regular lattice-structures generally
exhibit hexagonal or circular meshes, whilst the irregular are either
polygonal or roundish; the irregular forms are, however, much more abundant
than the {lxxxviii}regular, and a further distinction from the SPUMELLARIA
consists in the fact that the primary skeletal elements, from which the
lattice is secondarily developed, exercise a predominant influence upon
their form. These primary elements in the majority of the NASSELLARIA are
to be seen in two morphologically most important structures:--first, the
_primary sagittal ring_, which embraces the central capsule in the median
plane (§ 124); and secondly, the _basal tripod_ (§ 125), whose three
diverging rays proceed from the base of the central capsule, whilst
commonly a fourth vertical ray supports the dorsal side of latter (compare
Pls. 81-91, p. 892). In the majority of the NASSELLARIA these two primary
elements appear in combination, whilst in others only one of them is
recognisable. In addition there occur numerous monaxon lattice-shells in
which neither of these elements can be recognised, but a simple ovoid
lattice-shell (cephalis) alone forms the whole skeleton or its primary part
(Pl. 51, fig. 13; Pl. 98, fig. 13). The great difficulty in the
morphological interpretation and phylogenetic derivation of the
NASSELLARIAN skeleton lies in the fact that each of these three
elements--the primary sagittal ring, the basal tripod, and the latticed
cephalis--may form the whole skeleton by itself or be combined with one or
both of the others (p. 893). Even nearly related or at all events very
similar forms may differ very greatly in this respect. With regard to the
manifold forms of their dictyosis it follows that it is partly dependent
upon one of the two first elements, partly independent. In the
#Plectellaria# (or those NASSELLARIA which do not possess a complete
lattice-shell) the lattice-work is usually irregular and arises by union of
the ramifications, which proceed either from the primary sagittal ring
(Pls. 81, 82, 92-94) or from the basal tripod (Pl. 91). In the
#Cyrtellaria# (or NASSELLARIA with a complete lattice-shell, Pls. 51-80),
on the other hand, the lattice-work is sometimes regular, sometimes
irregular, being often very different in the different joints of a
segmented shell (Pl. 72); a great part of it arises independently of the
two chief morphological elements, and develops according to laws similar to
those which regulate the dictyosis of the SPUMELLARIA.
134. _Dictyosis of the Phæodaria._--The lattice-structures of the
PHÆODARIA, which consist of a silicate of carbon (§ 102), are on the whole
not developed in such variety as those of the other Radiolaria, but exhibit
several essentially different types of structure, not reducible to a common
primitive type of lattice-work. In one portion of this legion there occurs
an ordinary simple lattice-work (as in SPUMELLARIA and NASSELLARIA), with
solid trabeculæ; of these the Castanellida (Pl. 113) and Concharida (Pls.
123-125) have usually regular or subregular, circular meshes, sometimes
hexagonally framed; the Orosphærida (Pls. 106, 107) large irregular
polygonal meshes with thick trabeculæ, the Sagosphærida (Pl. 108) large
triangular meshes with thin filiform trabeculæ. The Challengerida (Pl. 99)
are characterised by a very delicate regular lattice-work, with minute
hexagonal pores, like a Diatomaceous frustule. The Medusettida (Pls.
118-120) {lxxxix}show a peculiar alveolar structure, numerous small
compartments being enclosed between two parallel plates. In the Circoporida
(Pls. 114-117) and Tuscarorida (Pl. 100) the opaque porcellanous shell has
a peculiar cement structure (§ 104), and the lattice-structure is confined
for the most part to characteristic rings of pores at the base of the
hollow tubes, which arise from the shell. The most peculiar lattice-work,
however, appears in the segmented shell of the Aulosphærida (Pls. 109-111)
and Cannosphærida (Pl. 112). In the former the large meshes of the
lattice-work are usually subregular and triangular, in the latter
polygonal; the trabeculæ are hollow cylinders, filled with jelly, and
containing usually a central axial thread. In each nodal point of the
lattice, in which three or more tangential tubes meet, these are separated
by stellate or astral septa.
135. _Radial Spines of the Skeleton._--The skeleton in the great majority
of Radiolaria is armed with radial spines, which are of great importance in
the development of their general form and of their vital functions. From a
morphological point of view the number, arrangement, and disposition of the
spines is usually the determining factor as regards the general form of the
skeleton. Physiologically they discharge distinct functions, as organs of
protection and support; they act also, like the tentacles of the lower
animals, as prehensile organs, since their points, lateral branches, barbed
hooks, &c. serve to hold fast nutritive materials. In general main-spines
and accessory spines may be distinguished in most Radiolaria; the former
are of pre-eminent importance in determining the figure of the skeleton;
the latter are merely appendicular organs. The main-spines present such
characteristic and important differences in the various legions of
Radiolaria that they must be considered separately.
136. _Radial Spines of the Spumellaria._--The radial spines, which exhibit
most manifold variations in the large order #Sphærellaria#, present
characteristic differences in its four suborders. In the #Sphæroidea# their
number and disposition serve for the separation into families (p. 59); the
Cubosphærida (Pls. 21-25) always possess six radial main-spines, which
stand opposite to each other in pairs and lie in three diameters of the
shell, which are at right angles to each other and correspond to the axes
of the regular crystallographic system. The Staurosphærida (Pl. 15) have
four spines, which form a regular cross and stand opposite to each other in
pairs, in two axes at right angles. The Stylosphærida (Pls. 13-17) show
only two main-spines, which are opposed to each other in the vertical main
axis of the body. Finally, the Astrosphærida (Pls. 18-20, 26-30) are
characterised by a larger and variable number of radial spines (eight,
twelve, twenty or more), sometimes regularly, sometimes irregularly
arranged. Among the other #Sphærellaria# the #Prunoidea# (Pls. 13-17, 39,
40) are most allied to the Stylosphærida with two opposite main-spines; the
#Discoidea# (Pls. 31-47), on the other hand, to the Staurosphærida with
four crossed spines; there exist, however, #Discoidea# with two opposite,
three marginal, or numerous radial main-spines; it is {xc}characteristic of
this suborder that they all usually lie in the horizontal median plane of
the lenticular shell, arising from its equatorial margin. The #Larcoidea#
(Pls. 9, 10, 49, 50) show a great variety in the number and arrangement of
their radial main-spines, which in the different families of this suborder
stand in direct causal relation to the various forms of growth of the
shell; usually the primary main-spines lie either in the three different
dimensive axes, at right angles to each other, whose differentiation is
characteristic of the lentelliptical Larcoid shell (§§ 34, 122) or in
definite diagonal axes, which cut the former obliquely. The radial spines
of the SPUMELLARIA are _never_ united in the centre of the body, but arise
separately from the surface of the primary central lattice-shell (medullary
shell), more rarely from one of the secondary (cortical) shells, which
enclose it. Their form is originally three-edged (sometimes pyramidal,
sometimes prismatic); the cause of this is to be found in their origin from
the nodal points of the lattice-shell, whose meshes are primitively
hexagonal; hence three trabeculæ unite in each nodal point, and are
produced into three edges of the spine. Very commonly, however, the spines
are round (conical or cylindrical), more rarely polygonal. The three edges
are often delicately toothed, not unfrequently spirally twisted around the
axis of the spine (Pl. 21, figs. 1, 12).
137. _Radial Spines of the Acantharia_.--The radial spines of this legion
have a much greater significance than in the other three classes of
Radiolaria, since here alone they are the primary determining factors in
the skeletal structure, and grow outwards from the middle of the central
capsule. This centrogenous origin of the radial spines is as characteristic
of the ACANTHARIA as their chemical constitution, which is not siliceous
but acanthinic (§ 102). Furthermore, their form is in most cases so
peculiar that even an isolated ACANTHARIAN spine can be generally
distinguished from one belonging to either of the other three legions. In
the great majority of the ACANTHARIA (all #Acanthonida# and
#Acanthophracta#) twenty radial spines are constantly present, which,
disposed according to a definite geometrical law, make up the skeleton
(compare § 110 above and p. 717). The twenty spines are generally simply
apposed to each other in the centre (either by the surfaces or the edges of
their pyramidal base); more rarely they are completely united and form a
single star-like piece of acanthin (_Astrolithium_). Very rarely
(_Acanthochiasma_) each two opposite spines are united so that ten
diametric bars cross in the middle of the central capsule. Whilst in the
great majority of ACANTHARIA these twenty radial spines are present, the
small group #Actinelida# is characterised by the possession of an
inconstant, often very large number, sometimes over one hundred. Among
these #Actinelida# are probably to be found the stem-forms of the whole
legion. The variously modified spines of the ACANTHARIA may be grouped in
three main categories: (1) round (cylindrical or conical); (2) four-edged
(prismatic or pyramidal); (3) two-edged (leaf- or sword-shaped). The latter
very commonly bear two {xci}opposite transverse processes, the former four
crossed ones. By ramification and union of these apophyses arise the
lattice-shells of the #Acanthophracta# (excepting the Sphærocapsida).
138. _Radial Spines of the Nassellaria._--The radial spines in this legion
show as great a variety in their form as in the SPUMELLARIA, and, as in
them, are solid, siliceous bars, usually three-edged (prismatic or
pyramidal), or round (cylindrical or conical); more seldom they are
polygonal in section. The great majority of the NASSELLARIA are, however,
distinguished by a triradial structure, three primary radial bars diverging
from the base of the central capsule (usually from the centre of the
porochora); there is usually in addition a fourth apical spine, which
passes upwards vertically or obliquely on the dorsal aspect of the central
capsule. These three or four typical radial spines of the NASSELLARIA may
be derived with great probability from the basal tripod of the #Plectoidea#
(_Plagoniscus_, _Plectaniscus_, &c., Pl. 91); and since this tripod is very
characteristically combined in _Cortina_ and _Cortiniscus_ with the primary
sagittal ring of the #Stephoidea#, the three typical rays may be generally
designated "cortinar feet," in contradistinction to the other radial
processes of the NASSELLARIAN skeleton. One of the three descending basal
feet ("pes caudalis," Pls. 91-95, C) is always unpaired, and lies in the
vertical median plane (or sagittal plane), just as does the vertically
directed apical spine, which originally forms the dorsal bar of the
sagittal ring, and is produced upwards into the "apical horn," (marked _a_
on the plates). The other two basal feet are paired, and diverge right and
left, forwards and downwards ("pedes pectorales," _p.p._). Six-rayed
NASSELLARIA, in which three secondary (interradial) feet are intercalated
between the three primary (perradial) cortinar feet, are less common than
the three-rayed forms. In some groups the number rises still higher, nine,
twelve, or even more secondary feet being intercalated between the three
primary. Besides, accessory radial spines may be developed on different
parts of the shell, which have sometimes a definite relationship to the
typical radial spines, sometimes not. Their form and ramification are very
various (Pls. 51-98).
139. _Radial Spines of the Phæodaria._--The radial spines of the PHÆODARIA
are very clearly distinguished from those of other Radiolaria by the fact
that they are usually hollow tubes, rarely solid bars. As a rule, the tubes
are cylindrical, often slightly fusiform or conical, their siliceous wall
is very thin, and their lumen filled with jelly; a fine thread of silica
usually runs in the axis, and in several families is connected by fine
transverse threads with the wall of the tube (Pl. 110, figs. 4, 6; Pl. 115,
figs. 6, 7). The peculiar family Medusettida is characterised by a very
remarkable segmentation of the hollow spines (Pls. 118-120). Each tube is
divided by a series of septa into chambers, which communicate by a central
or excentric opening in each septum, an arrangement resembling the
siphuncle of the chambered Cephalopod shells. The number and
{xcii}arrangement of the radial tubes in most PHÆODARIA is indefinite and
very variable; only in a few families is the number constant in each
species and genus, and the disposition regular. The Medusettida (Pls.
118-120) resemble the NASSELLARIA, inasmuch as equal radial feet diverge
from the base of their shell, sometimes three in number (_Cortinetta_, Pl.
117, fig. 9), sometimes four (_Medusetta_, Pl. 120, figs. 1-4), sometimes
six (_Gazelletta_); _Gorgonetta_ is specially distinguished by the
possession of six ascending and six descending spines regularly alternating
(Pl. 119). The Tuscarorida (Pl. 100) usually have three or four equidistant
feet. The Circoporida (Pls. 115-117), on the other hand, rather approach
the #Sphæroidea#, their spherical or regular polyhedral shell having a
definite number of tubular radial spines, which arise at regular intervals
from their angles; _Circoporus_ has six, _Circospathis_ nine, _Circogonia_
twelve, and _Circorrhegma_ twenty radial tubes. Very rarely the tubes of
the PHÆODARIA are angular, usually they are round, more or less
cylindrical, though they are often bifurcated or even ramified, and exhibit
a great wealth of the most delicate appendages; siliceous hairs, bristles,
spines, barbed or anchor-like hooks, spathillæ, brushes, circlets, &c.
(compare Pls. 99-128).
140. _Main-Spines and Accessory Spines._--As accessory spines (Paracanthæ)
we oppose to the main-spines (Protacanthæ), just described, all those
processes which have no determining influence upon the formation of the
skeleton as a whole, but are to be regarded as secondary constituents of
the skeleton, or appendicular organs of inferior significance. They are
developed in the utmost variety, sometimes as hairs or bristles, sometimes
as thorns or clubs, either straight or curved (often zigzag), smooth or
barbed; sometimes standing vertically upon the shell, or directed towards
the centre, sometimes obliquely, or rising at a definite angle. In those
SPUMELLARIA whose lattice-shell consists of several concentric spheres, the
accessory spines generally arise from the outermost, the main-spines, on
the contrary, from the innermost. In the NASSELLARIA, multifarious forms of
accessory spines are especially developed in the order #Plectellaria#. In
the PHÆODARIA they are often furnished with delicate appendages, _e.g._,
anchor-hooks, spathillæ, coronets, &c. Among the ACANTHARIA the accessory
spines which arise from the surface of the shell in the #Acanthophracta#
are very characteristic. They are not radially disposed (like the similar
superficial spines of the SPUMELLARIA), but parallel to the radial
main-spines from whose transverse processes they arise. Since in all these
#Acanthophracta# the twenty radial main-spines are opposite to each other
in pairs, all the accessory spines (often several hundred) are parallel to
ten different regularly disposed axes of the lattice-shell (Pls. 134-138).
The skeletons of the Radiolaria, in addition to the general relations
which have been discussed above, present numerous and important special
differences in the various larger and smaller groups. These are indicated
in detail in the descriptions of the legions, orders, and families in the
systematic portion of this Report.
{xciii}BIOGENETICAL SECTION.
A SKETCH OF OUR KNOWLEDGE OF THE DEVELOPMENT OF THE RADIOLARIA IN THE YEAR
1884.
----