SPUMELLARIA or PERIPYLEA is usually distinguished by a more or less
complete radial arrangement, which does not occur in the same form in other
Radiolaria; it may be regarded as characteristic of this legion, for it
probably occurs in all the species at some period of life or other, and
stands in a direct causal relationship with the typical structure of the
capsule-membrane in all the "PERIPYLEA" (see note A). For as this is
commonly perforated by very numerous pores distributed at equal intervals
over the whole surface of the capsule, and since a communication between
the intra- and extracapsular sarcode takes place through these, the radiate
structure of the endoplasm may be readily explained as due to the influence
of radial currents which take place continuously or intermittently in the
endoplasm. This radiate structure is most obvious when the endoplasm
contains no secondary products or only an insignificant amount of these,
and thus appears colourless and almost homogeneous, or only finely
granular. Under these circumstances, an optical section of the central
capsule usually reveals a distinct radial striation; numerous narrow,
straight, dark streaks alternating regularly with still narrower clear
ones; the latter consist of homogeneous, the former of more or less
granular protoplasm (Pl. 20, fig. 1_a_). Often there may be distinguished
in each darker streak a single straight row of strongly refracting (fat?)
granules, sometimes several such rows. Occasionally the whole endoplasm
becomes divided up into a number of large "radial wedges," club-shaped,
conical or pyramidal masses of granular protoplasm, separated by clear
divisions of hyaline plasma (_e.g._, in _Actissa radiata_, p. 14, where in
the optical section of the central capsule, between the membrane and the
nucleus, twenty-five dark radial wedges of equal size were separated by
thick clear partitions of hyaline protoplasm). In the majority of the
SPUMELLARIA this radial striation is partially or entirely concealed by the
formation of pigment or of other products. Very often it is only visible in
the cortical layer, which lies immediately below the capsule-membrane (Pl.
1, figs. 1, 3). The remarkable "centripetal cones" which characterise the
Thalassicollid genus _Physematium_, and were formerly described as
"centripetal cell-groups," are probably a special development of these
cortical radial wedges; they are conical cortical bodies, regularly
distributed on the inner surface of the membrane of the central capsule,
and disposed with the apex turned towards the centre (see note B). More
rarely than in the cortical layer, a similar radial structure is to be
found in the innermost medullary layer immediately surrounding the nucleus.
Here the endoplasm sometimes breaks up into fine radial threads, which are
anatomically separable and hang down from the free nucleus as thin
processes (see note C). In some cases it is also possible to isolate radial
rods from the cortical layer of teased out central capsules.
{xlvii}A. The radial structure of the endoplasm was first described in my
Monograph (1862, p. 74), though R. Hertwig (1879, p. 112) was the first
to indicate its typical significance in the case of the PERIPYLEA, and to
demonstrate its causal relation with the radial currents in the central
capsule of this legion. More recent investigations have led me to the
conviction that this phenomenon is more widespread, and often more
strongly developed, than was formerly imagined, and that it is probably
one of the typical characters of all SPUMELLARIA (at least of the
Monozoa).
B. The centripetal cones of _Physematium_, which have hitherto been known
only in these colossal Thalassosphærida, were fully described in my
Monograph under the name "conical centripetal cell-groups"; by their
first discoverer, A. Schneider (L. N. 13), they were termed "nests," and
compared with the "nests" (central capsules) of the Polycyttaria. In the
_Physematium mülleri_ of the Mediterranean (hitherto only observed by
Schneider and myself at Messina) it appeared as though each centripetal
cone were composed of a group of from three to nine (usually four or
five) slender wedge-shaped cells, whose common centripetal apex was
produced into a radial thread of sarcode (L. N. 16, p. 258, Taf. iii.
fig. 7). Since then (1866) I have observed at Lanzerote, in the Canary
Islands, a nearly related form, which I take to be _Physematium
atlanticum_, Meyen. In this, however, the "centripetal cell-groups" were
wanting, and the whole cortical layer of the endoplasm was cleft into
numerous radial portions, each enclosing a nucleus (probably the
mother-cells of flagellate spores, see p. 35).
C. The radial fibres of the medullary endoplasm which cling to an
extracted nucleus have been observed by Hertwig in certain #Sphæroidea#
(_Diplosphæra_, _Arachnosphæra_) (L. N. 33, p. 40).
78. _The Endoplasm of the Actipylea._--The intracapsular protoplasm of the
ACANTHARIA or ACTIPYLEA is often distinguished by a partial or complete
radial arrangement like that of the PERIPYLEA, but differing in the number,
size, form, and distribution of the radial portions into which the
endoplasm is differentiated. For since the pores of the capsule membrane
are distributed at equal distances all over the surface in the SPUMELLARIA,
whilst in the ACANTHARIA they are arranged in definite groups, and since
the number and arrangement of the pores has a direct influence upon the
internal currents of the endoplasm, it follows that the radial structure in
the latter legion must be very different from that in the former. In
addition to this there must not be forgotten the important influence which
the early centrogenous formation of the skeletal rods exercises upon the
disposition and growth of the intracapsular structures. Hence the endoplasm
of the ACANTHARIA does not separate into innumerable thin, closely packed
radial wedges or cortical radial rods, but into a small number of large
pyramidal portions between which run the radially disposed heterogeneous
portions of the contents of the capsule, viz., the radial bars of acanthin
and the peculiar intracapsular "axial threads." As a direct consequence of
the regular disposition of these heterogeneous radial portions, which is
often characteristic of the various families of the ACANTHARIA, a
corresponding differentiation of the endoplasm is brought about; it divides
into a number of conical or pyramidal portions (radial pyramids), whose
bases rest upon the capsule-membrane and whose apices are directed towards
the centre of {xlviii}the capsule (the central star of the skeleton).
These radial pyramids are, however, but rarely visible, being usually more
or less concealed by a dark pigment.
The differentiations of the endoplasm in the central capsule of the
ACTIPYLEA have been but little investigated, but they appear to vary
somewhat in the different groups of this legion. In all ACANTHARIA in
which the twenty radial bars are regularly arranged according to the
Müllerian law (see p. 717) and in which axial threads constant in number
and disposition run between them from the central star to the
capsule-membrane, it obviously follows that the endoplasm must be divided
into more or less distinct radial pyramids, and this must the case
whether these take the form of continuous tracts or of actually separable
portions. The regular polygonal figures, often seen on the surface of the
central capsule (with special distinctness in _Acanthometron elasticum_
and _Acanthometron pellucidum_) separated by a network of granular
threads, are the bases of such radial pyramids (see Hertwig, L. N. 43, p.
12, Taf. i. figs. 1-7).
79. _The Endoplasm of the Monopylea._--The intracapsular protoplasm of the
NASSELLARIA or MONOPYLEA is distinguished from that of any of the other
three legions by the development of a quite peculiar fibrillar structure,
the axial "pseudopodial cone," which may shortly be termed the "podoconus"
(foot-cone). Since this is in direct correlation with the peculiar
structure of the capsular opening, the large "porochora," which is situated
at the basal pole of the main axis, it is quite as characteristic of the
legion as the latter itself (see note A). The podoconus is primitively a
vertical regular cone whose circular base occupies the horizontal porochora
or "basal porous area" of the central capsule, while its vertical axis
coincides with that of the latter. The apex of the cone, usually somewhat
rounded off, is therefore directed towards the aboral or apical pole of the
central capsule and separated from it by a larger or smaller interval. In
this interval the nucleus originally lies (as in Pl. 51, fig. 13; Pl. 98,
fig. 13); but it is usually displaced subsequently and lies excentrically.
The cone is of very variable height; on an average its vertical height is
about equal to the diameter of its horizontal base; these dimensions are,
however, dependent upon the form of the central capsule; the height being
greater in slender ovoid or conical capsules, and less in depressed
sphæroidal or discoidal ones, than the diameter of the base. The podoconus
consists of differentiated endoplasm, which becomes more deeply stained by
carmine and offers greater resistance to solvents than the surrounding
finely granular protoplasm. The apex, especially, becomes very intensely
stained. It always exhibits a very characteristic fine but distinct
striation, numerous straight radial lines diverging from the apex of the
cone towards the base. The number of these striæ appears to correspond with
that of the vertical rods in the porochora, and each of these latter stands
apparently in direct communication with the basal end of an apical stria (§
59). These threads are probably differentiated constant contractile threads
of endoplasm, or even myophanes, comparable with the contractile cortical
threads of the CANNOPYLEA and the permanent axial threads of the ACTIPYLEA.
The numerous modifications, {xlix}undergone by the form and contents of
the central capsule in the different groups of MONOPYLEA, especially those
due to the formation of the skeleton, are not without influence upon the
podoconus. The most important divergencies from the above described primary
form are the following:--(1) The vertical axial cone becomes oblique, its
axis inclining in the sagittal plane and approaching either the dorsal or
the ventral wall of the capsule; the cause of this appears to be usually
the excentric development of the growing nucleus or the formation of a
large oil-globule. (2) The smooth mantle of the podoconus becomes divided
by three longitudinal furrows into three equal prominent ridges, which
correspond to three circular lobes in the porochora; the cause of this
basal triradial lobular formation lies probably in the triradial
development of the skeleton in many NASSELLARIA or in the cortinar
structure of the collar septum. (3) The simple podoconus splits into three
or four elongated lobes, which eventually become almost completely
separated and correspond to the lobes of the central capsule, in the axial
wall of which they lie as longitudinally striated bands. The behaviour of
these bands justifies the hypothesis that the podoconus is a muscular
differentiated portion of the endoplasm and is composed of myophane
fibrillæ, whose contraction determines the opening of the central capsule.
A. The podoconus of the MONOPYLEA was first described by R. Hertwig in
1879, and recognised as a characteristic component of the central capsule
in the most various groups of this legion (in #Plectoidea#, #Stephoidea#,
#Spyroidea#, and #Cyrtoidea#; see his figures, _loc. cit._, Taf. vii.,
viii., and the description, pp. 71, 73, 83, 106). Hertwig called it the
"pseudopodial cone," and regarded it as a conical process of the
capsule-membrane, which is developed from this latter and projects from
the porous area into the interior of the central capsule; "it is
penetrated by fine canals which arise at the apex of the cone, diverge
towards the base, and terminate there in the rods of the pseudopodial
area. The intracapsular protoplasm penetrates at the apex of the
pseudopodial cone into its fine canals, runs along them and emerges from
the rods of the porous area in the form of slender threads" (_loc. cit._,
p. 19). I cannot agree with this view of Hertwig, although I have been
able to confirm the accuracy of his description by my own observations
upon numerous excellently stained and preserved preparations in the
Challenger collection. As I have proved by numerous teased out
preparations, and as Hertwig himself correctly states, "the cone is more
readily detached from the membrane than from the protoplasm, when the
capsule is teased" (_loc. cit._, p. 73). Hence I regard the podoconus not
as a differentiated portion of the capsule-membrane but as endoplasm, and
believe that it is composed of myophanes or "contractile muscular
fibrils" in the same manner as the cortical layer of the CANNOPYLEA.
Probably the contraction of these fibrils serves to raise the opercular
rods and hence to allow the exit of the endoplasm through the pores which
lie between these opercular rhabdillae (compare § 59).
80. _The Endoplasm of the Cannopylea._--The intracapsular protoplasm of the
PHÆODARIA or CANNOPYLEA is distinguished from that of the other three
legions by several characteristic peculiarities, which are very important,
since they stand in causal relation to the typical structure of the
capsule-membrane and in particular of its {l}remarkable aperture. In the
case of many and perhaps of all PHÆODARIA the endoplasm is differentiated
into a granular medullary and a thin fibrillar cortical layer, the former
of which usually encloses numerous small vacuoles, while the latter
contains muscular fibrillæ. In the voluminous central capsule of large
PHÆODARIA the whole cortical layer of the endoplasm, which lies immediately
below the delicate inner capsule-membrane, sometimes appears delicately and
regularly striated, and most distinctly so under the apertures, towards the
centre of each of which the dark striæ are radially directed (see note A,
below). These striæ are probably contractile muscular fibrillæ; or
"myophanes," by whose contraction the openings are voluntarily widened. In
the Tripylea this fibrillar star is much more strongly developed under the
astropyle (the main opening) than under the parapylæ (or accessory
openings); and probably the peculiar radial structure of the operculum of
the former is due to the stronger development of these radial fibrils
(being their impression). In many PHÆODARIA, indeed, the fine myophane
fibrils are only visible under the apertures, whilst in others they form a
continuous fibrillar cortical layer on the whole inner surface of the inner
capsule-membrane; the fine fibrillæ run meridionally from one pole of the
main axis to the other; perhaps the whole central capsule may change its
form in consequence of their contractions. The medullary portion of the
endoplasm, which lies below this thin cortical layer, is usually finely
granular in the PHÆODARIA, and permeated by numerous spherical vacuoles,
which are noteworthy from their equal size and regular distribution. Each
clear vacuole usually contains a dark shining fat-granule, more rarely a
group of such granules (see note B). Compare § 60, and Pl. 101, figs. 1-3;
Pl. 104, figs. 1, 2; Pl. 111, fig. 2; Pl. 128, fig. 2, &c.
A. The fine fibrillæ in the cortical layer of the endoplasm were first
described by Hertwig in 1879 (L. N. 33, p. 98, Taf x. figs. 6-10). He
found them, however, only below the three openings in the capsule of the
Tripylea, where they form three stellate groups of fibrils. I find them
very clearly shown, and with especial distinctness, under the astropyle
in most PHÆODARIA of which I have had the opportunity of examining
well-stained and preserved central capsules. In many cases, also, the
striation is not confined to the apertures, but spreads over the whole
cortical layer. Perhaps this constitutes in all PHÆODARIA a thin
myophane-sheet, whose contractile fibrils run from one pole of the main
axis to the other and cause by their contraction changes in the form of
the spheroidal central capsule.
B. The granular medullary portion of the endoplasm of the PHÆODARIA, with
its numerous clear spherical vacuoles, was first described in my
Monograph (1862), in the case of _Aulacantha_ (p. 263), _Aulosphæra_ (p.
359), and _Coelodendrum_ (p. 361) as a "finely granular, mucous substance
(intracapsular sarcode), packed more or less closely with clear spherical
vesicles from 0.005 to 0.015 mm. in diameter, each of which contains one
or two, rarely three, dark shining granules." That these clear spheres
are true vacuoles was first clearly proved by Hertwig (L. N. 33, p. 98).
As a rule all the vacuoles of the same central capsule are of equal size
(generally from 0.008 to 0.012 mm. in diameter), and are distributed at
equal intervals throughout the finely granular endoplasm.
{li}CHAPTER III.--THE EXTRACAPSULUM.
(§§ 81-100).
81. _The Components of the Extracapsulum._--The extracapsulum or
extracapsular malacoma, under which name are included all those parts of
the soft body which lie outside the central capsule, consists of the
following constant, and important constituents:--(1) The _calymma_ or
extracapsular jelly-veil; (2) the _sarcomatrix_ or layer of exoplasm
immediately surrounding the membrane of the central capsule; (3) the
_sarcodictyum_ or network of exoplasm, covering the surface of the calymma;
(4) the _pseudopodia_ or radial fibres of exoplasm, which may again be
subdivided into intracalymmar pseudopodia, uniting the sarcomatrix and
sarcodictyum, and extracalymmar pseudopodia, radiating freely into the
water outside the calymma.
82. _The Calymma._--The calymma or extracapsular jelly-veil of the
Radiolaria is always the most voluminous portion of the extracapsulum, and
in spite of its simple structureless constitution is of great morphological
and physiological importance. In all Radiolaria this gelatinous mantle
completely surrounds the central capsule, but is separated from its outer
surface by a continuous, though thin, layer of exoplasm, the sarcomatrix.
The pseudopodia radiating from the latter pierce the calymma, form the
sarcodictyum at its surface, and radiate from its nodal points freely into
the surrounding water. The calymma is rarely visible in living freshly
captured Radiolaria, examined in sea-water, for its gelatinous substance is
perfectly hyaline, colourless and pellucid, and possesses the same
refractive index as sea-water; but when the object is removed from this
fluid and transferred to carmine solution or some other colouring matter,
the extent and figure of the calymma become apparent, for the staining
fluid does not at first penetrate into the gelatinous material. When this
has taken place, however (after a longer or shorter time), and the
gelatinous material has become coloured, its form and size may be observed
by the converse experiment; the object is transferred once more to water
and the outlines of the calymma become as clear as those of the central
capsule. The same is the case with dead specimens in which the sticky
surface of the calymma has become covered with dust.
The jelly-veil of the Radiolaria was recognised even by the earliest
observers of the group, Meyen (1834), and Huxley (1851), and compared
with that of the Palmellaria; the former noticed it in _Physematium_ and
_Sphærozoum_ (L. N. 1, p. 283), and the latter in _Thalassicolla_ and
_Collosphæra_ (L. N. 5, p. 433). In all these SPUMELLARIA, both in the
monozootic _Thalassicolla_ and in the polyzootic _Sphærozoum_ and
_Collosphæra_, the calymma is very voluminous and filled with large
alveoli. Meyen called them "muco-gelatinous masses, in the interior of
which are contained small equal-sized vesicles"; Huxley likewise found
clear vesicles in the jelly and compared them with Dujardin's vacuoles.
Johannes Müller observed the jelly-veil in many different Radiolaria, in
particular in the #Acanthometra#, first discovered by him, but
erroneously believed that it only originated {lii}after death by
liquefaction of the sarcode (L. N. 12, p. 6). This mistake is, however,
easy to understand, since in living Radiolaria the calymma is usually
invisible on account of its perfect transparency, whilst in dead
specimens it is usually quite distinct on account of the dust clinging to
its adhesive surface. I myself believed that the formation of the
voluminous hyaline jelly-veil was only partially due to liquefaction
after death, but that it was to some extent present in the living
organism and that it might vanish and subsequently reappear by means of
imbibition (L. N. 16, pp. 109, 110). R. Hertwig was the first to
demonstrate, in 1879, that the jelly-veil is constantly present in living
Radiolaria, that it forms the basis of the extracapsular malacoma and
surrounds the central capsule as a second protective sheath (L. N. 33, p.
114).
83. _The Structure of the Calymma._--The extracapsular jelly-veil appears
structureless in most Radiolaria, inasmuch as it represents a homogeneous
pellucid excretion of the exoplasm and contains neither fibres nor other
formed structures. In some groups, however, definite structural characters
become secondarily developed. The most common and striking of these is the
formation of alveoles, which takes place in the extracapsulum (see § 86).
In consequence of this the calymma assumes a remarkable frothy consistency
and appears to be composed of large, clear, thin-walled vesicles; this is
especially the case in the #Collodaria# (#Colloidea#, Pls. 1, 3, and
#Beloidea#, Pls. 2, 4), and in many large PHÆODARIA, especially among the
#Phæocystina# (Phæodinida and Cannorrhaphida, Pl. 101, and Aulacanthida,
Pls. 102-104). More rarely the calymma is not permeated by vacuoles, but
there appear in it fine striæ parallel to the surface as though it were
composed of thin concentric laminæ like an onion; perhaps these are the
expressions of a different quantity of water in the various layers. In the
calymma of many Radiolaria thin, straight, radial lines are to be seen,
which are probably pseudopodia, and not to be attributed to any structural
modification, or they may be slender canals which serve for the exit of the
pseudopodia. On the outer surface of the calymma of different Radiolaria,
and especially in the ACANTHARIA, a peculiar network of fibres is to be
found, composed of polygonal meshes, like elastic fibres, probably due to a
local thickening of the jelly. These polygonal meshes are often very
regularly distributed between the radial spines of the #Acanthometra#, and
stand in a definite relation to them. The fibres which form the meshes are
often rather strong, resembling elastic fibres, as above-mentioned, and
either simple or composed of bundles of very fine fibrillæ (L. N. 33, p.
15, Taf. i. fig. 1, Taf. ii. fig. 4).
84. _The Consistency of the Calymma._--The gelatinous material of which the
calymma of the Radiolaria consists is a pellucid mass, rich in water and
usually quite hyaline and structureless; its consistency is very variable.
In the majority of the Radiolaria it may perhaps be about equal to that of
the jelly which composes the umbrella of most Medusæ; but as in these
latter it may vary between very wide extremes, constituting on the one hand
a very soft jelly-mantle, offering but little {liii}resistance to
mechanical influences and almost disintegrating under the eyes of the
observer, and on the other hand forming a firm gelatinous shell, comparable
to cartilage in hardness, elasticity, and power of mechanical resistance.
In many Radiolaria of large dimensions with an alveolar calymma (especially
in numerous #Collodaria# and PHÆODARIA) this may be split by means of
dissecting needles and the central capsule extracted like the stone from a
cherry, and then it is easy to ascertain that the firmness and elasticity
of this jelly-veil are not less than those of a cherry. The different
degrees of consistency in the various Radiolaria may be dependent either
upon the relative amount of water which they contain, or upon qualitative
or quantitative variations in the organic substance of which the jelly
consists. Great importance is to be attached to the considerable
consistency of the calymma, because it furnishes the indispensable
groundwork for the deposition of many parts of the skeleton and
particularly of the lattice-shells.
85. _The Primary and Secondary Calymma._--In most Radiolaria the external
form and volume of the calymma are different at different stages of growth,
and this difference is mainly dependent upon the development of the
skeleton. Hence it is advisable to distinguish in general the primary from
the secondary calymma. The _primary calymma_ is in the great majority of
Radiolaria a perfect sphere, in the middle of which lies the concentric
central capsule; on the surface of this gelatinous plate the primary
spherical lattice-shell is secreted in most SPUMELLARIA and
#Acanthophracta#, as well as in those PHÆODARIA which possess a spherical
shell; in the remaining PHÆODARIA also and in the NASSELLARIA, where the
lattice-shell is not spherical but monaxon, it is secreted on the surface
of the primary calymma. This takes place at a definite time, very important
in the development of the Radiolarian, which for the sake of brevity we
shall term the "_lorication-period_." Since the firm surface of the primary
calymma furnishes the necessary foundation for the deposition of the
primary lattice-shell, it is of the greatest mechanical significance in all
shell-bearing Radiolaria. The _secondary calymma_ arises only after the
lorication-period by further growth of the primitive jelly-mantle and in
the fully developed Radiolarian usually encloses wholly or partially the
external parts of the skeleton, in consequence of which it assumes the most
various forms. Very often the secondary calymma is polyhedral, being
stretched between the radial spines of the skeleton, the distal ends of the
latter then forming the fixed points of the gelatinous polyhedron.
86. _The Extracapsular Vacuoles and Alveoles._--The calymma of the
Radiolaria usually appears completely homogeneous and hyaline without any
structure; sometimes it encloses numerous clear vesicles, vacuoles or
alveoles, and then assumes a frothy appearance, the expression of a more or
less distinct alveolar structure. {liv}The clear vesicles to which this is
due are either spherical, or polyhedral from mutual pressure, and like the
similar ones in the central capsule may be divided into membraneless
vacuoles and vesicular alveoles. The _vacuoles_ are simple drops of fluid,
without a special envelope, and immediately surrounded by the gelatinous
substance of the calymma, in which they appear as simple cavities. The
_alveoles_ on the contrary are true vesicles, with a thin envelope, which
encloses a drop of fluid or a globule of jelly; in the latter case its
contents are different in refracting power and amount of contained water
from the substance of the surrounding calymma. A sharp boundary between the
membraneless vacuoles and the vesicular alveoles cannot be drawn in the
case of the extracapsular hyaline spheres any more than in the
intracapsular; the envelope of the alveoles is sometimes very distinct and
even anatomically separable, whilst at other times it is very thin and
scarcely recognisable; it may occasionally arise and disappear within a
very short time (see note A). There is no doubt that in the calymma as in
the central capsule the vesicular alveoles are secondary products, which
have arisen from the vacuoles by the secretion of an enveloping membrane.
This membrane is either a delicate sheath of exoplasm, or a firmer and more
resistant skin, distinct from the exoplasm, and probably an excretion from
it (_e.g._, Pl. 4, figs. 2, 3). In many cases the outer surface even of the
vacuoles is covered by a network of pseudopodia, which form a sarcoplegma
similar to a fenestrated alveolar membrane. The colourless pellucid fluid
in the vacuoles and alveoles is usually simple sea-water, more rarely it
contains a small quantity of albumen ("albumen-spheres") or jelly
("gelatinous spheres"). The size of these spheres is very variable. Quite
small vacuoles may be found in the calymma of many Radiolaria. Large
vacuoles, on the other hand, producing the appearance of an alveolar
structure, are confined to but few groups, to a part of the SPUMELLARIA
(#Colloidea#, #Beloidea#, and a few #Sphæroidea#), and to the #Phæocystina#
(PHÆODARIA with incomplete skeleton); besides they occur only rarely in
individual genera, _e.g._, _Nassella_ among the skeletonless NASSELLARIA.
Since the volume of the calymma is much increased by the development of
vacuoles, and the power of mechanical resistance is at the same time much
increased, the fact is explained that the vacuoles occur mainly in
Radiolaria which have no skeleton or only an incomplete one (see note B).
Among the monozootic #Collodaria# the alveolar structure is especially well
developed in the following genera; _Thalassicolla_ (Pl. 1, figs. 4, 5),
_Thalassophysa_, _Thalassoplancta_, _Lampoxanthium_ (Pl. 2, figs. 1, 2);
among the PHÆODARIA in most genera of the Phæodinida, Cannorrhaphida and
Aulacanthida (Pls. 101-104), and probably also in other voluminous
PHÆODARIA (_e.g._, #Phæosphæria#). The alveoles or vacuoles in the calymma
of these large Radiolaria lie usually in several layers, one above another,
and increase in size from within outwards. The Polycyttaria or social
Radiolaria (the three families Collozoida, Sphærozoida and Collosphærida)
without exception have an alveolar structure, and the special form of
{lv}their colonies or coenobia is to a great extent determined by the
development, number, size and arrangement of the alveoles in their calymma
(compare Pls. 3-8). In these cases there is not unfrequently developed a
large central alveole (see note C) whose thickened wall encloses a globe of
jelly and serves as the central support of the whole colony (Pl. 5, fig.
1). Still more striking, however, is the arrangement of certain
Polycyttaria, where each individual of the colony (or each central capsule
with its calymma) is enclosed in a large alveole, whose firm wall often
attains considerable thickness (Pl. 4, figs. 2, 3). The whole colony then
appears as an aggregate of numerous cells, each of which possesses two
envelopes, the inner central capsule and the outer alveolar membrane;
between these lies in the Collosphærida the siliceous lattice-shell (Pl. 6,
fig. 2). These pericapsular alveoles may be regarded as an outer cell-wall
more correctly than the membrane of the central capsule itself, but the
arrangement may also be compared to the temporary encystation of other
Protista (see note D).
A. The extracapsular vacuoles in the calymma were first observed in 1851
by Huxley, in _Thalassicolla_ and _Sphærozoum_, and compared with
Dujardin's sarcode vacuoles (L. N. 5). Afterwards J. Müller noticed that
generally these "large clear vesicles are covered by a fine membrane,"
and hence he called them "alveoles" (L. N. 12, pp. 3, 7, &c.). In my
Monograph I have described them more in detail as "extracapsular
alveoles" (1862, p. 88, Tafs. i.-iii. xxxii.-xxxv.). Ever since then the
point has been debated whether these clear spaces are simple vacuoles in
the sense of Huxley or vesicular alveoles as stated by J. Müller. This
contention is unnecessary, for both varieties are present, and often no
sharp line can be drawn between them. R. Hertwig has recently come to the
conclusion that they are as a rule "membraneless vacuoles," but that they
"sometimes become surrounded by a special envelope" (L. N. 33, p. 31). He
even succeeded "in extracting from a _Collosphæra_ the large vesicle
which lies in the centre of many colonies and removing its covering of
central capsules and jelly."
B. The _mechanical importance_ of the alveolar structure, which certainly
increases the elasticity and mechanical resistance of the voluminous
calymma, has not yet been sufficiently realised; in the case of those
Radiolaria which have no skeleton, or at all events no lattice-shell, it
may take the place of this as a protective envelope. Furthermore, by
taking in and giving out water it may discharge a hydrostatic function,
causing the organism to rise or sink in the water.
C. The _large central alveole_ found in the colonies of many Polycyttaria
(especially Collosphærida) and first described in my Monograph (Taf.
xxxiv. fig. 1), has since then been observed by Hertwig, Bütschli, and
other investigators, and recognised as the "central support of the whole
colony, surrounded by a delicate membrane" (compare L. N. 33, p. 31, and
L. N. 41, p. 436). In a colony of _Trypanosphæra transformata_ (Pl. 5,
fig. 1), which I observed living while in Ceylon in 1881, the membrane of
the large central alveole was surrounded by a firm network of
sarcoplegma, and could be mechanically isolated from the central
jelly-sphere which it enclosed.
D. The _pericapsular alveoles_, figured in Pl. 4, figs. 2, 3, from a
_Sphærozoum_, and in Pl. 6, fig. 2, from a _Siphonosphæra_, were very
well preserved in some preparations in the Challenger collection; perhaps
their development coincides with the formation of spores, and may be
regarded as an encystation.
{lvi}87. _The Extracapsular Fat-Globules._--Fat is probably as widely
distributed in the exoplasm as in the endoplasm of the Radiolaria; a
considerable proportion of the small, dark, highly refractive granules
appear to consist of fat; most likely they are for the most part direct
products of metastasis. These widely-spread granules, which are sometimes
coloured, and which by their passive motion produce the phenomenon of
granular circulation in the exoplasm, are not the only fatty structures in
the extracapsulum; larger globules sometimes occur. In certain large
#Collodaria# (_e.g._, _Thalassicolla melacapsa_, Pl. 1, fig. 5;
_Thalassophysa sanguinolenta_, &c.) radial series of oil-globules are found
in the calymma, especially in its proximal portion; in others the central
capsule is surrounded by a layer of oil-globules (situated in the
sarcomatrix). In the PHÆODARIA a part of the phæodium appears to consist of
fat-globules.
88. _The Extracapsular Pigment._--The formation of colouring matters in the
extracapsulum is on the whole rare in the Radiolaria, apart from the
"yellow cells" (see § 91) and from the peculiar phæodium of the PHÆODARIA,
which will be separately treated of in the next paragraph. Considerable
masses of extracapsular pigment, usually black or blue, rarely brown or
red, are found only in a few Radiolaria belonging to the first three
legions; most often in the SPUMELLARIA. Some large #Collodaria#, _e.g._,
the common _Thalassicolla nucleata_ and a few other species of this genus
(Pl. 1, fig. 4), are characterised by a rich deposit of black or blue
pigment in the sarcomatrix and in the proximal portion of the calymma.
Brown pigment is deposited in the calymma of many #Sphæroidea# and
#Discoidea#, as well as of some NASSELLARIA (_Cystidium_, _Tridictyopus_,
&c.). In a part of the ACANTHARIA red pigment granules are thickly strewn
in the sarcoplegma and pass along the free pseudopodia, as for example in
_Actinelius purpureus_ and _Acanthostaurus purpurascens_. The composition
and significance of these extracapsular pigments are not completely known.
On the extracapsular pigment of _Thalassicolla nucleata_, compare my
Monograph, pp. 87, 251. On the red extracapsular pigment-granules of the
ACANTHARIA, see L. N. 19, pp. 345, 364, &c.
89. _The Phæodium of the Phæodaria._--The PHÆODARIA, which are
distinguished from the other three legions of Radiolaria by the double
membrane of the central capsule, and the peculiar structure of the
main-opening (astropyle), differ also in other points, the most important
of which is the constant presence of a voluminous mass of extracapsular
pigment. This possesses a peculiar constitution and special significance,
and is not to be confounded with the extracapsular pigment-granules of
other Radiolaria (_e.g._, _Thalassicolla_), and hence it has been
distinguished by the name "Phæodium," and the individual granules which
compose it as "Phæodella" (see note A). The phæodium is always excentric in
position relatively to the central capsule, of which it {lvii}surrounds the
oral half in the form of a voluminous concavo-convex cap, hiding the
astropyle at its basal pole so completely that the latter is rarely visible
until the phæodium has been removed (Pls. 99-104; Pl. 115, fig. 8; Pl. 123,
&c.). The central capsule is generally almost completely embedded in the
phæodium, so that only its aboral pole (with the two parapylæ in the
TRIPYLEA) projects. In the #Phæogromia#, in which the lattice-shell
possesses a special opening and the central capsule lies excentrically in
the aboral position of its interior, the phæodium occupies the oral aspect,
between the capsule and the aperture (Pls. 99, 100, 118-120, &c.). In the
peculiar family Coelographida (Pls. 126-128) a special receptacle (galea
with its rhinocanna) for the phæodium is developed outside the bivalve
shell, within which the central capsule lies. The proboscis, which in all
PHÆODARIA arises from the centre of the astropyle, lies in the vertical
axis of the phæodium and is entirely surrounded by it. The volume of the
phæodium in the majority of the PHÆODARIA may be said to be about as great
as that of the central capsule, although in some species it is considerably
larger. Its colour is always dark, usually between green and brown,
commonly olive-green or blackish-brown, rarely reddish-brown or black. The
phæodellæ or pigment-granules which make up the greater part of the
phæodium (see note B) are irregular in form and unequal in size and show no
definite structure; usually they are spherical or ellipsoidal, and exhibit
fine parallel striæ which run transversely or obliquely (Pl. 101, fig. 3,
6, 10; Pl. 103, fig. 1, &c.). Between the larger granules is usually found
a thick dust-like mass of innumerable very small grains. The physiological
significance of this peculiar phæodium is still unknown, but is probably
considerable, judging from its large size and especially from its constant
topographical relation to the astropyle; the latter consideration would
lead to the supposition that it plays an important part in the nutrition
and metastasis of the PHÆODARIA (see note C).
A. The phæodium of _Aulacantha_, _Thalassoplancta_, and _Coelodendrum_
was first described in 1862, in my Monograph, as an excentric
extracapsular mass of pigment of blackish-brown or olive-green colour
(pp. 87, 262, 264, 361, Taf. ii. iii. xxxii.). Since then John Murray,
who investigated many living PHÆODARIA during the Challenger expedition,
has shown its general distribution in this legion (Proc. Roy. Soc. Lond.,
vol. xxiv. p. 536, 1876). From the constancy of its presence I gave the
legion the name PHÆODARIA in 1879 (L. N. 34).
B. With regard to the special composition of the phæodium and the
constitution of the phæodellæ, see the general description of the
PHÆODARIA, pp. 1533-1537.
C. Perhaps the phæodellæ are to some extent symbiontes with the
PHÆODARIA; the xanthellæ present in most other Radiolaria are absent in
this legion.
90. _The Extracapsular Xanthellæ._--Xanthellæ or Zooxanthellæ, symbiotic
"yellow cells," are very commonly found in the extracapsulum of the
Radiolaria, especially in many SPUMELLARIA and NASSELLARIA; whilst in the
ACANTHARIA similar yellow cells usually only occur within the central
capsule, and in the PHÆODARIA their {lviii}presence has not been certainly
demonstrated. The extracapsular Xanthellæ are found most abundantly in the
#Collodaria#, both in the monozootic Thalassicollida and in the polyzootic
Sphærozoida. They occur in smaller numbers in the #Sphærellaria#, and in
many divisions of the latter they seem to be entirely absent. Also it
sometimes happens that, though present in large numbers in some
SPUMELLARIA, they are entirely absent in others nearly related to them;
indeed, this has also been observed in the case of different individuals of
the same species. This fact alone is sufficient to show that the Xanthellæ
are not an integral part of the Radiolarian organism (as was formerly
believed) but parasites or more correctly symbiontes, which live as
inhabitants of the calymma. More recent investigations have shown, that
besides the yellow pigment-grains they contain starch or an amyloid
substance, that is to say, vegetable reserve materials, that their thin
envelope contains cellulose, and that their yellow colouring-matter
resembles chlorophyll and is related to that of the Diatomaceæ
("Diatomin"). Hence they are now generally regarded as unicellular Algæ,
nearly related to those which occur as symbiontes in other marine animals
(_Exuviella_, &c.). The starch, which they develop with the formation of
oxygen, may serve as nutriment to the Radiolaria, while the carbonic acid
yielded by the latter is also beneficial to the Xanthellæ. The form of the
Xanthellæ is usually spherical and elliptical, often also sphæroidal or
discoidal. Their diameter is usually between 0.008 and 0.012 mm., rarely
more or less. The differences exhibited by Xanthellæ which live in
different groups of Radiolaria demand further investigation, which will
perhaps lead to the establishment of several species of the genus
_Zooxanthella_. At present _Zooxanthella extracapsularis_, in the calymma
of SPUMELLARIA and NASSELLARIA, may be clearly distinguished from
_Zooxanthella intracapsularis_, in the central capsule of the ACANTHARIA.
The "yellow cells" were first described in 1851 by Huxley, in the
#Collodaria#, and afterwards by J. Müller (1858) in many SPUMELLARIA and
NASSELLARIA. In my Monograph (1862, pp. 84-87) I gave a detailed account
of their structure and increase by division, and laid special emphasis on
the fact that they are the only elements in the Radiolarian organism
which "are _undoubtedly cells_ in the strict histological sense of the
word." Afterwards, in my Beiträge zur Plastiden-Theorie, I showed the
constant presence of "starch in the yellow cells of the Radiolaria"
(1870, L. N. 21). Shortly afterwards Cienkowski observed that the yellow
cells live independently and reproduce themselves after the death of the
Radiolaria, and in consequence first put forth the hypothesis that they
do not belong to the Radiolarian organism, but that they are unicellular
Algæ parasitic upon it (1871, L. N. 22). This view was ten years later
more fully established by Karl Brandt, and elucidated by comparison with
the symbiosis of the gonidia of Algæ, and the hyphæ of Fungi in the
formation of Lichens, which had in the meantime become known (1881, L. N.
38). Brandt gave this unicellular yellow Alga the name _Zooxanthella
nutricola_, and afterwards gave fuller details regarding its remarkable
vital relations (L. N. 39). Patrick Geddes, who named it _Philozoon_,
supplemented this account and showed experimentally that it gives off
oxygen under the influence of sun-light (1882, L. N. 42, 43). In
consequence {lix}of this there is no doubt that all Xanthellæ (the
_Zooxanthella extracapsularis_ of SPUMELLARIA and NASSELLARIA, and the
_Zooxanthella intracapsularis_ of the ACANTHARIA, and possibly also the
_Zooxanthella phæodaris_ of the PHÆODARIA) do not originally belong to
the Radiolarian organism, as was believed up to the time of Cienkowski,
but penetrate actively into it from without, or are taken in passively by
means of the pseudopodia. In any case their symbiosis, when they are
associated with the Radiolarian cell in large numbers, may be of great
advantage to both parties, since the metastasis of the Xanthella is
vegetable, that of the Radiolarian animal in character. In any case their
symbiosis is to a large extent accidental, by no means as necessary as in
the case of the Lichens. See on these points in addition to Brandt and
Geddes (_loc. cit._) also Geza Enz, Das Consortial-Verhältniss von Algen
und Thieren, Biol. Centralbl., Bd. ii. No. 15, 1883, Oskar Hertwig, Die
Symbiose oder das Genossenschaftsleben im Thierreich, Jena, 1883, and
Bütschli, Die Radiolarien, in Bronn's Klass. u. Ord. d. Thierreichs, 1882
(L. N. 41, pp. 456-462).
91. _The Exoplasm or Extracapsular Protoplasm._--The extracapsular
protoplasm, which may be shortly termed the "exoplasm" (or ectosarc), is
primitively in all Radiolaria (and especially in their earliest development
stages) the only important constituent of the extracapsulum, besides the
calymma. Although the extracapsular and intracapsular protoplasm of the
Radiolaria are everywhere in direct communication, and although the
openings in the membrane of the central capsule bring about an interchange
between them, still the two portions of sarcode show certain constant and
characteristic differences, which are due to the physiological division of
labour between the central and peripheral parts of the body and their
corresponding morphological differentiation. The extracapsular, like the
intracapsular, protoplasm is originally homogeneous, but may afterwards
become differentiated in various ways, producing the special constituents
of the extracapsulum. Such "external protoplasmic products" are vacuoles,
pigment-bodies, &c. More important, however, are the topographically
different sections into which the exoplasm may be divided according to its
relations to the central capsule and the calymma. In this respect the
following parts may be generally distinguished--(1) the _Sarcomatrix_, or
fundamental layer of the exoplasm, which surrounds the central capsule as a
continuous sheath of sarcode and separates it from the calymma; (2) the
_Sarcoplegma_, an irregular network of the exoplasm, which spreads
throughout the gelatinous material of the calymma; (3) the _Sarcodictyum_
or network of sarcode on the outer surface of the calymma; and (4) the
_Pseudopodia_, which project outwards from the latter and radiate into the
water.
92. _The Sarcomatrix._--The sarcomatrix, being "the fundamental layer of
the pseudopodia" (or "matrix of the exoplasm"), constitutes the proximal
innermost section of the extracapsular sarcode, and in all Radiolaria forms
a thin continuous mucous layer, which covers the whole outer surface of the
central capsule and separates it from the surrounding calymma (see note A,
below). The sarcomatrix communicates internally {lx}through the openings of
the central capsule with the endoplasm, whilst externally the pseudopodia
or mucous threads arise from it, which by their union form the sarcoplegma.
The sarcomatrix is only interrupted in the SPUMELLARIA and ACANTHARIA by
those parts of the skeleton which perforate the membrane of the central
capsule. In all NASSELLARIA and PHÆODARIA, as in the #Collodaria#, it
appears as a perfectly continuous sarcode-envelope of the central capsule.
Its thickness is variable; in general it is most strongly developed in the
SPUMELLARIA and PHÆODARIA, less so in the NASSELLARIA, and is thinnest in
the ACANTHARIA. The thickness seems, however, to vary even in one and the
same individual, the difference depending partly upon the different stages
of development and partly upon nutritional conditions. After abundant
inception of nutriment the thin protoplasmic layer of the matrix is
thickened and turbid, rich in granules and irregular masses, which are
probably due to enclosed but only half-digested food; xanthellæ also, as
well as foreign bodies taken up with the nutriment, such as frustules of
Diatoms and shells of smaller Radiolaria, and of pelagic infusoria, larvæ,
&c., are often, especially in large individuals, aggregated in considerable
quantities in the matrix. After long fasting, on the contrary, this is poor
in these enclosed bodies and in granules; it then forms a thin colourless
more or less hyaline mucous coating to the central capsule. From a
physiological standpoint the sarcomatrix is to be regarded as the _central
organ of the extracapsulum_, and as of pre-eminent significance. Probably
it is not only the most important organ for the nutrition of the Radiolaria
(especially for digestion and assimilation in particular), but perhaps is
also the central organ of perception. On the other hand the sarcomatrix
belongs to those components of the Radiolarian organism which take no part
in the formation of the skeleton.
A. The sarcomatrix was first described in my Monograph in 1862 (p. 110)
as the "Mutterboden der Pseudopodien," possessing a pre-eminent
physiological importance. Compare also my paper on the sarcode elements
of the Rhizopoda (Zeitschr. f. wiss. Zool., Bd. xv. p. 342, 1865).
93. _The Sarcoplegma._--By the name sarcoplegma, as distinguished from the
remaining extracapsular sarcode, is understood the intracalymmar web of
exoplasm or "ectosarcode network," which ramifies within the gelatinous
mass of the calymma. Internally it is in direct connection with the
continuous sheath (sarcomatrix), which encloses the central capsule, whilst
externally it is in contact with the superficial sarcode network
(sarcodictyum) which surrounds the calymma. The configuration of this
exoplasmic web, which penetrates the jelly-veil in all directions, is
exceedingly variable; in most Radiolaria it is extremely irregular in form,
like the protoplasmic network in the ground-substance of many kinds of
connective tissue. In some groups, however, it assumes a rather regular
shape which it appears to retain (_e.g._, in many ACANTHARIA). It must be
assumed also that in those instances where the consistency {lxi}of the
calymma approaches that of cartilage, the tracks of the exoplasmic threads
remain constant, but accurate observations are wanting as to how far the
configuration of the sarcoplegma is constant or variable in the different
groups, as well as regarding its peculiar behaviour in those Radiolaria
whose calymma is characterised by the formation of vacuoles or alveoles
(see § 86). Usually it envelops the larger alveoles in the form of a
reticulate veil. In many #Collodaria# the exoplasm is aggregated at certain
points of the intracalymmar web, so that large balls or amoeboid bodies
appear to be distributed between the alveoles, _e.g._, in _Thalassophysa
pelagica_ and _Thalassicolla melacapsa_ (Pl. 1, figs. 4, 5). The
sarcoplegma is metamorphosed directly into silex in the Radiolaria
spongiosa, or those genera which possess a spongy cortical skeleton, and
were formerly known as Spongurida; to this category belong the
Spongosphærida (Pl. 18) and Spongodiscida (Pl. 47) as well as certain
NASSELLARIA and PHÆODARIA. The single siliceous spicules, which are
irregularly interwoven to form the spongy web, are to be regarded as the
silicified threads of the intracalymmar sarcode network. From a
physiological point of view the sarcoplegma is of importance both for the
nutrition and motion of the Radiolaria, since it brings the sarcomatrix and
the sarcodictyum, with the pseudopodia which radiate from it, into direct
communication.
94. _The Sarcodictyum._--The sarcodictyum may be defined as the
extracalymmar network of exoplasm, and is a reticular covering which lies
upon the outer surface of the gelatinous calymma. Internally, the
sarcodictyum is in direct communication with the sarcoplegma, or the web of
exoplasmic threads which ramifies in the gelatinous substance of the
calymma; externally, on the other hand, the pseudopodia radiate freely from
it; thus its relation to these is similar to that which the sarcomatrix
bears to the roots of the sarcoplegma. Relations similar to those which
have led to the separation of the primary from the secondary calymma,
induce us to distinguish also a primary and secondary sarcodictyum. The
original or _primary sarcodictyum_ ramifies over the surface of the
original or primary calymma, and like this is of pre-eminent importance in
the formation of the primary lattice-shell; if we regard the surface of the
primary calymma as the indispensable foundation for the deposition of this
latter, then the primary sarcodictyum furnishes the material from which it
is developed: silex in the SPUMELLARIA and NASSELLARIA, a silicate of
carbon in the PHÆODARIA, and acanthin in the ACANTHARIA. It may indeed be
said that the primary lattice-shell of the Radiolaria arises by a direct
chemical metamorphosis of the primary sarcodictyum, by a chemical
precipitation of the dissolved skeletal material (silex, silicate, or
acanthin), which was stored up in the exoplasm of the sarcodictyum. Hence a
deduction from the special conformation of the former to that of the latter
is permissible. The particular form of the primary lattice-sphere with its
regular or irregular meshes is due to the corresponding form of the primary
sarcodictyum; both regular and irregular forms of this {lxii}commonly
occurring. The form of the _regular sarcodictyum_ with circular or regular
polygonal, usually hexagonal, meshes is constantly maintained during the
formation of the regular lattice-shells (_e.g._, Pl. 12, figs. 5-10; Pl.
52, figs. 8-20; Pl. 96, figs. 2-6; Pl. 113, figs. 1-6). The form of the
_irregular sarcodictyum_, on the other hand, with irregular polygonal or
roundish meshes, persists during the development of the irregular
lattice-shells (_e.g._, Pls. 29, 70, 97, 106). All this is true also of the
_secondary sarcodictyum_, or the exoplasmic network which ramifies over the
surface of the secondary calymma. The secondary lattice-shells, which are
deposited on the surface of the latter, retain the configuration of the
secondary sarcodictyum, by the chemical metamorphosis of which they have
originated; this is the case in many SPUMELLARIA which develop several
concentric lattice-shells (Pl. 29), in some NASSELLARIA (Pl. 54, fig. 5),
in the Phractopeltida among the ACANTHARIA (Pl. 133), and in the
double-shelled PHÆODARIA, Cannosphærida, and part of the Coelodendrida and
Coelographida (Pls. 112, 121, 128). In those Radiolaria which form no
lattice-shell whatever, the conformation of the sarcodictyum is usually
irregular, with meshes of irregular form and unequal size; sometimes,
however, they seem to be very regular, as in many #Acanthometra# (Pl. 129,
fig. 4).
95. _The Pseudopodia._--On the whole the pseudopodia or thread-like
processes of the exoplasm exhibit in the Radiolaria the same characteristic
peculiarities as in all true Rhizopoda; they are usually very numerous,
long and thin, flexible and sensitive filaments of sarcode, which show the
peculiar phenomena of granular movement. Their physiological significance
is in several respects very great, for they serve as active organs for the
inception of nutriment, for locomotion, sensation, and the formation of the
skeleton (see note A, below). The presence of a calymma, however, which
distinguishes the Radiolaria from the other Rhizopoda, brings about certain
modifications in the behaviour of the pseudopodia. If in general all the
threads, which arise from the sarcomatrix or fundamental layer and radiate
outwards, be called "pseudopodia," then that part of them which is included
in the gelatinous substance of the calymma and forms the sarcoplegma may be
termed the "collopodia" (or intracalymmar pseudopodia), and the remaining
portion, which passes outwards from the sarcodictyum freely into the water,
may be described as "astropodia" (or extracalymmar pseudopodia). In many
Radiolaria these two portions present some differences in morphological and
physiological respects, and certain distinctions are probably generally
present (see note B). Apart from this universal differentiation in the
different groups of the Radiolaria, specially modified forms of pseudopodia
may be recognised as the axopodia and myxopodia of the ACANTHARIA (see §
95, A), and the sarcode-flagellum of certain SPUMELLARIA (see note C).
A. The pseudopodia of the Radiolaria have been so fully described in my
Monograph, in 1862, both morphologically and physiologically, that I need
only refer to the account there given {lxiii}(pp. 89-127); for
supplementary observations see R. Hertwig (1879, L. N. 33, p. 117) and
Bütschli (1882, L. N. 41, pp. 437-445).
B. The _Astropodia_, or free radiating pseudopodia, are in many
Radiolaria more or less clearly distinguishable from the collopodia,
which form the sarcoplegma within the calymma; how far these distinctions
depend upon a permanent differentiation (especially in the ACANTHARIA and
PHÆODARIA) needs further investigation.
C. The _sarcode-flagellum_ (perhaps better termed _axoflagellum_) was
first described in my Monograph (1862, p. 115) in the case of various
#Discoidea# (Taf. xxviii. figs. 5, 8; Taf. xxx. fig. 1). Hertwig has
given a substantially similar account of the organ in some other
#Discoidea# (L. N. 33, p. 67, Taf. vi. figs. 10, 11); probably this
peculiar structure is confined to the order #Discoidea# among the
SPUMELLARIA, but is widely distributed within its limits. The
axoflagellum is a thick cylindrical thread of sarcode, finely striated
and pointed towards its free end. It always lies in the equatorial plane
of the discoidal body, and always unpaired in one of its axes; in the
triradiate #Discoidea# it is in the axis of the unpaired principal arm
and opposite to it (Pl. 43, fig. 15). In the Ommatodiscida (p. 500, Pl.
48, figs. 8, 19, 20) the axoflagellum probably passes out through the
peculiar marginal ostium of the shell. Perhaps it is always connected
with the central nucleus by intracapsular axial fibres, and is to be
regarded as a specially differentiated bundle of pseudopodia (or
axopodia?).
95A. _The Myxopodia and Axopodia._--The two forms of pseudopodia which we
distinguish as myxopodia and axopodia differ markedly from each other both
morphologically and physiologically. The _myxopodia_, or ordinary free
pseudopodia, which are found in large numbers in all Radiolaria, and
constitute their most important peripheral organs, are simple homogeneous
exoplasmic threads, which arise from the sarcodictyum or extracalymmar
sarcode network, and radiate freely into the water; here they may branch
and combine by anastomosis to form a changeable network, but they never
contain an axial thread. The _axopodia_, on the other hand, are
differentiated pseudopodia, which consist of a firm radial thread, and a
soft covering of exoplasm; they penetrate the whole calymma in a radial
direction and project freely from its surface, and generally (if not
always) they are produced inwards to the middle of the central capsule,
perforating its membrane; their proximal end is lost in a dark central heap
of granules. Such axopodia are at present known with certainty only in the
ACANTHARIA, where they are widely, and perhaps universally, distributed.
Their development in this legion probably stands in direct causal relation
to the peculiar structure of the central capsule and the centrogenous
formation of the skeleton. Since the radial skeletal rods of the
#Acanthometra# possess originally a thin coating of protoplasm, it may be
said that the centrogenous axopodia of this group became differentiated in
two ways, the firm axial threads of one section remaining very thin and
covered by protoplasm, whilst those of the other section became
metamorphosed into radial bars of acanthin. This hypothesis acquires more
probability from the regular distribution and arrangement of the axopodia
in the ACANTHARIA; they usually stand at fixed intervals {lxiv}between the
radial bars, singly or in groups; sometimes their number seems to be not
greater than that of the bars, whilst in other cases a circlet or group of
axopodia corresponds to each radial bar. Perhaps their fine axial thread
consists of acanthin. At all events the axopodia are constant organs
(probably sensory, like the "palpocils") and not retractile like the
movable myxopodia.
The axial threads in the pseudopodia of the #Acanthometra# were first
discovered by R. Hertwig, who accurately described their peculiar
structure and arrangement (L. N. 33, pp. 16, 117).
96. _The Myophriscs of the Acanthometra._--The #Acanthometra# are
characterised by a very peculiar differentiation of the exoplasm, namely,
by the formation of myophriscs or contractile threads from the
sarcodictyum. In most (and perhaps in all) ACANTHARIA of this order each
radial bar is surrounded by a circlet of such contractile threads, which
was first described as a "ciliary corona" (see note A, below). The number
of contractile threads in each circlet usually amounts to from ten to
twenty, rarely being more than thirty and less than eight; it often appears
to be constant in the individual species (see note B). In the living state
the myophriscs are long, thin filaments, the pointed distal end of which is
inserted into the radial bar, whilst the thicker proximal end is attached
to the surface of the calymma, which is elevated round the base of each rod
into the form of a gelatinous cone or skeletal sheath (see note C).
Probably the myophriscs lie on the outer surface of the apical portion of
this gelatinous cone, and are hence to be regarded as exoplasmic threads
differentiated from the sarcodictyum. Sometimes, however (as in
_Acanthochiasma_), they fuse into a contractile membrane and form the
envelope of a cone, whose interior is occupied by a gelatinous papilla of
the calymma. On mechanical irritation the myophriscs contract rapidly and
suddenly, like muscle-fibrillæ, becoming at the same time thicker, and
hence are very different from pseudopodia. Their distal point of insertion
being fixed to the firm acanthin rod, they raise by their contraction the
skeletal sheath, to which their bases are attached or in the surface of
which they lie. The result of their contraction is therefore a distention
and increase in volume of the calymma, with which is no doubt connected an
inception of water into the gelatinous mass, and hence a diminution in its
specific gravity. Probably the #Acanthometra# contract their myophriscs
voluntarily when they wish to rise in the water; when these relax the
calymma collapses owing to its elasticity, water is then expelled and the
specific gravity increases. From a physiological point of view, then, the
myophriscs are to be regarded as a hydrostatic apparatus, morphologically
as myophanes or muscular fibrillæ, such as also occur in the intracapsular
protoplasm (see §§ 77-80). On more violent irritation and after the death
of the #Acanthometra# the myophriscs separate from the radial bars and
remain attached to the distal ends of the conical gelatinous sheaths as
free "ciliary coronas." At the same time, {lxv}they melt into short, thick,
hyaline rods, the so-called "gelatinous cilia." The myophriscs are found
only in the order #Acanthometra#, and are wanting in the #Acanthophracta#,
as well as in the other three legions of Radiolaria.
A. The "_ciliary coronas_" on the skeletal rods of dead #Acanthometra#
were first described by the discoverer of this order, Johannes Müller,
and referred to as "the stumps of the contracted, thickened threads" (L.
N. 12, p. 11, Taf. xi.).
B. The "_number of the gelatinous cilia_" I found constant in certain
species of #Acanthometra#, and stated in my Monograph (L. N. 16, p. 115)
"that here is to be found the first differentiation of the diffuse
sarcode into definite organs of regular definite number, size, and
position, which deserve the name tentacles rather than pseudopodia."
C. The nature of the myophriscs as fibrillæ allied to muscles was first
discovered by R. Hertwig, who described them as "structures of peculiar
nature," under the name of "contractile threads," and pointed out in
detail their histological and physiological peculiarities (L. N. 33, pp.
16-19, Taf. i.).
97. _The Exoplasm of the Peripylea._--The extracapsular protoplasm of the
SPUMELLARIA or PERIPYLEA is in communication with the intracapsular sarcode
by the innumerable fine pores of the capsule-membrane, and like these pores
is evenly distributed over the whole surface. The sarcomatrix which
immediately surrounds the central capsule is moderately strong, and sends
out innumerable long, thin pseudopodia, which probably correspond to the
pores of the membrane. Their number is markedly greater in the SPUMELLARIA
than in the other three legions. The ramifications and communications which
the radiating fibres of the sarcomatrix undergo within the calymma,
apparently present the most manifold variations, so that the sarcoplegma or
intracalymmar network thus formed has very diverse forms. On the surface of
the calymma the exoplasmic threads constitute a variously disposed
sarcodictyum, a regular or irregular exoplasmic network, by the
silicification of which a primary lattice-shell arises in the majority of
the SPUMELLARIA. The free ends of the pseudopodia, which arise from this
extracalymmar network and radiate out into the water, appear in most
SPUMELLARIA to be relatively short, but exceedingly numerous. Specially
modified pseudopodia and axial threads in particular do not seem to occur
in this legion. Perhaps, however, among the latter may be reckoned the
remarkable pseudopodia which combine to form the sarcode flagellum in many
#Discoidea# (and perhaps in other SPUMELLARIA). This axoflagellum is a
particularly strong thread of sarcode, arising from a definite point in the
central capsule; it is cylindrical or slenderly conical in form, much
longer, stronger, and more contractile than the ordinary pseudopodia; it
contracts in a serpentine fashion on mechanical irritation and seems to
originate by the fusion of a bundle of pseudopodia (compare § 95, C).
98. _The Exoplasm of the Actipylea._--The extracapsular protoplasm of the
ACANTHARIA or ACTIPYLEA differs in several important respects from that of
other {lxvi}Radiolaria, and appears to undergo more significant
differentiations than that of the three other legions. Since the pores in
the wall of the central capsule are not distributed evenly and at equal
intervals over its whole surface (as in the PERIPYLEA), but rather exhibit
a regular disposition in groups at unequal intervals, the number of
projecting pseudopodia is much less and the law of their arrangement
different from that which obtains in the PERIPYLEA (§ 58). In many and
probably in all ACANTHARIA they are divided into two groups, those which
arise from the centre of the capsule and possess firm axial threads, and
those which have not these characters (compare § 95, A). The axopodia, or
stiff pseudopodia with axial threads, arise from the centre of the capsule,
are present in much smaller numbers than the soft and flexible myxopodia,
and are regularly disposed between the radial bars of acanthin, usually so
that they are as far removed from them as possible, _i.e._, in the centre
between each three or four bars; these latter may indeed be regarded as
strongly developed axial threads, which have become changed into acanthin
(§ 95, A). The soft myxopodia, or pseudopodia without axial threads, are
much more numerous than the others, and arise from the sarcodictyum or
exoplasmic network which ramifies over the surface of the calymma. Their
number and arrangement seem, however, in many (if not in all) ACANTHARIA to
be regular and not to possess the extraordinary variability seen in the
other three legions. In many #Acanthometra# the sarcodictyum exhibits a
symmetrical conformation, with regular or subregular, polygonal (mostly
hexagonal) meshes, and generally the stronger threads of the sarcodictyum
secrete a firm, homogeneous or fibrillar, striated substance, which forms a
network of ridges on the surface of the calymma. In the #Acanthophracta#
the place of this is taken by the acanthin network of the primary
lattice-shell. The axopodia of the #Acanthometra# are usually about as long
as the radial spines between which they stand; their stiff axial thread is
surrounded by a soft sheath of protoplasm, communicating with the thin
sarcomatrix which surrounds the central capsule. Numerous branches pass
into the calymma from the exoplasmic sheath of the axial threads, and form
by their interweaving a loose sarcoplegma. The most peculiar differentiated
products of the exoplasm of the ACANTHARIA, however, are the myophane
fibrillæ of the #Acanthometra#, which have already been described under the
name of myophriscs (§ 96).
99. _The Exoplasm of the Monopylea._--The extracapsular protoplasm of the
NASSELLARIA or MONOPYLEA arises only from the porochora, or the
intracapsular podoconus, the oral base of which is formed by this porous
area. The pseudopodia or protoplasmic threads which pass through the pores
of the latter, united into a bundle, are not very numerous (in most
NASSELLARIA probably between thirty and ninety), and unite just outside it
to form a thick discoid sarcomatrix; this covers the porochora completely
below, and spreads out in the form of a thin envelope of exoplasm over the
whole {lxvii}surface of the central capsule; at the apical portion of the
latter the sarcomatrix is often so thin that it can only be recognised by
the aid of reagents; it separates the membrane of the central capsule from
the surrounding calymma. The pseudopodia, which penetrate the latter and by
loose anastomoses from a wide-meshed sarcoplegma within it, are usually not
very numerous. The greater part of them radiate in a bunch downwards from
the basal disc of the sarcomatrix, and a smaller number arise from the
thinner envelope which covers the remainder of the central capsule (Pl. 51,
fig. 13; Pl. 65, fig. 1; Pl. 81, fig. 16). On the outer surface of the
calymma the collopodia, which have passed through it, unite to form the
sarcodictyum, and through the silicification of this the primary
lattice-shell arises in the great majority of the NASSELLARIA. From the
surface of the sarcodictyum arise the astropodia, or free pseudopodia which
radiate outwards into the water. Their number in most MONOPYLEA is
relatively small, but their length appears to be very great.
100. _The Exoplasm of the Cannopylea._--The extracapsular protoplasm of the
PHÆODARIA or CANNOPYLEA is much better developed as regards volume than in
the other three legions, and is connected with the intracapsular sarcode by
only a few apertures in the capsule-membrane. In most PHÆODARIA three of
these are present, the astropyle or main-opening at the oral pole of the
main axis, and the two lateral parapylæ or accessory openings on either
side of the aboral pole (§ 60). In several families the latter appear to be
wanting, whilst in others their number is increased; these families have
not yet, however, been observed during life. The protoplasm projects both
from the oral main-opening and from the two aboral accessory openings in
the form of a thick cylindrical rod; the tube into which each opening is
produced in many PHÆODARIA (longer in the case of the astropyle, shorter in
the parapylæ) being regarded as an excretion from this protoplasmic
cylinder. The sarcode threads within the tube appear like a bundle of
fibrils, either quite hyaline or finely striated. After issuing from the
mouth of the aperture they pass over into a thick sarcomatrix, which
surrounds the central capsule entirely and separates it from the enclosing
calymma. In the neighbourhood of the basal astropyle the sarcomatrix is
usually swollen into a thick lenticular disc, which is in direct contact
with the peculiar phæodium of this legion (§ 89). The pseudopodia, which
radiate from the sarcomatrix, and form by anastomosis a wide-meshed
sarcoplegma within the calymma, are usually not very numerous in the
PHÆODARIA, but are very strong. Sometimes two stronger bundles of
collopodia may be distinguished at the two poles of the main axis, an oral
bundle (in the direction of the proboscis of the astropyle) and an aboral
bundle (at the opposite pole between the parapylæ). The collopodia of the
sarcoplegma unite at the surface of the calymma into a regular or irregular
sarcodictyum, which, in most PHÆODARIA produces by the secretion of a
peculiar silicate the primary lattice-shell. {lxviii}The free astropodia,
which pass outwards from the sarcodictyum into the water, are in most
PHÆODARIA very numerous (Pl. 101, fig. 10). Since, however, only a few
species of this great legion have been observed in a living state, their
pseudopodia require further accurate examination.