Chapter II: The Central Capsule (2)

A. The "_spherical hyaline vesicles_," which I described in my Monograph
(1862, p. 71) as among the most important and constant contents of the
central capsule, are partly vacuoles, {xlii}partly homogeneous nuclei.
Most recent investigators, Bütschli in particular (1882, L. N. 41), have
pointed out and rightly criticised this confusion. The criticism might,
however, have been more justly expressed by stating that, in the
preparation of my Monograph (1859-1862), I did not make use of modern
methods of demonstrating the nucleus by staining fluids, which were quite
unknown at the time, and only discovered a decade later. In fact, without
the aid of such reagents, it is quite impossible to distinguish between
the various "spherical transparent vesicles," of which those found in the
central capsule of the PHÆODARIA and many monozootic #Collodaria# are
simple vacuoles lying in the endoplasm, whilst, on the other hand, those
of the Polycyttaria and many other Radiolaria are true homogeneous
nuclei. For not only are the general appearance of the small clear
spheres, their refractive power, and regular distribution in the
endoplasm quite similar, but they are also of much the same size, for the
diameter ranges from 0.005 to 0.015 mm., being generally between 0.008
and 0.012 mm. In addition to this there is generally in each hyaline
sphere a dark brightly shining granule, which, in the case of the
vacuole, is simply a fat-granule, whilst in the case of the nucleus, it
is a true nucleolus. The small hyaline spheres in the young uninucleate
capsules of the Polycyttaria are simple vacuoles (Pl. 3, fig. 12), whilst
in the ripe multinucleate capsules they are true nuclei (Pl. 3, figs. 3,
8, 9), and it is quite impossible to discriminate between these two
conditions without the use of reagents. This has been expressly
recognised by R. Hertwig, who has the merit of having been the first to
clearly distinguish, by the aid of staining fluids, between these two
different constituents (1879, L. N. 33, p. 108).

B. The "_albumen spheres_," which were first observed by A. Schneider in
1858 in the common cosmopolitan _Thalassicolla nucleata_ (L. N. 13, p.
40), and which appear to occur in only a few other Thalassicollida, are
distinguished from the ordinary hyaline spheres of about the same size by
their higher refractive power and by certain albuminoid reactions,
especially the coagulation of a membranous envelope under the influence
of certain reagents (see my Monograph, p. 250, and Hertwig, L. N. 26,
1876, p. 46). They often enclose various formed contents, and require
further investigation.

C. The _gelatinous spheres_ of various sizes, found in the endoplasm of
the Radiolaria, agree in their reactions (especially in staining by
certain reagents) with the common extracapsular jelly of the calymma, and
are hence distinguishable both from the true (coagulable) "albumen
sphere," and from the ordinary watery vacuoles.

D. The _alveoles_, which are only accurately known in the case of certain
large monozootic #Collodaria#, but which also seem to occur in the
central capsule of other remarkably large Radiolaria, were described in
my Monograph in the case of _Thalassolampe margarodes_ and _Physematium
mülleri_, under the name "intracapsular alveolar cells" (1862, pp. 77,
254, 257). They are not, however, true nucleated cells, and the body
described as a nucleus is not such in reality. Nevertheless these large
hyaline spheres do possess a special envelope, as I have recently
convinced myself by the examination of ruptured central capsules of
_Thalassolampe maxima_, _Thalassopila cladococcus_, and _Physematium
atlanticum_ (Pl. 1, figs. 2, 3). The central capsule of these
#Collodaria# becomes distended to most unusual dimensions (2 to 12 mm. in
diameter) by the great development of these large hyaline vesicles, each
of which measure from 0.1 to 0.5 mm. in diameter.

73. _The Intracapsular Fat-Globules._--Fat is present in the central capsule of all Radiolaria in larger or smaller quantities, and generally appears in the form of very {xliii}numerous, small, spherical granules, which are either distributed evenly in the endoplasm (as an emulsion) or enclosed in the vacuoles; the latter, in particular, is the case in most PHÆODARIA, perhaps generally. In this group each vacuole contains as a rule a single dark, shining fat-granule, and sometimes also an irregular bunch composed of from two to five or more granules. In addition to these small fat-granules (_granula adiposa_) which are always present, the central capsule of many Radiolaria contains also larger fat-globules (_globuli adiposi_). These appear to be generally wanting in the PHÆODARIA, and are on the whole rare in the ACANTHARIA; whilst, on the contrary, they are very common in the NASSELLARIA and SPUMELLARIA. The Polycyttaria or social Radiolaria are as a rule distinguished by the possession of a single large central oil-globule, which lies in the centre of the central capsule, and is on an average about one-third of it in diameter (Pl. 3, figs. 4, 5). This is absent, however, in those young capsules of the Polycyttaria in which the primary nucleus is centrally situated (Pl. 3, fig. 12). Those species of Polycyttaria whose central capsule reaches a considerable size, often enclose numerous oil-globules, and in _Collophidium_ (species of _Collozoum_ with an elongated cylindrical capsule, Pl. 3, figs. 1, 3) the axis of each capsule is occupied by a row of numerous oil-globules. In the monozootic SPUMELLARIA, in which the nucleus is always centrally situated, the large oil-globules are, of course, excentric, being in apposition to the inner surface of the capsule-membrane (Pl. 1, fig. 3; Pl. 2, figs. 2, 5). In the #Discoidea# the oil-globules, which are often present in large numbers, form elegant concentric rings around the central nucleus, and in those species with segmented arms, there are one or more transverse rows in each segment (Pl. 43, fig. 15). In the NASSELLARIA the number and distribution of the oil-globules are dependent upon the form of the central capsule. When this is simple, without lobes, and ovoid or conical, they generally lie in its aboral half above the podoconus (Pl. 51, figs. 5, 13; Pl. 97, fig. 1). When, on the contrary, the basal portion of the capsule sends out three or four dependent processes (as in the majority of the #Cyrtoidea#), a large globule may generally be seen in the swollen distal part of each conical or ovoid lobe (Pl. 53, fig. 19; Pl. 60, figs. 4-7). In many #Stephoidea# and #Spyroidea#, whose central capsule is separated into two lateral portions by the constriction corresponding to the sagittal ring, each of these contains either a single large globule or a group of small ones (Pl. 90, figs. 7, 10). These oil-globules are usually colourless and highly refractive; rarely they are yellow or brown, sometimes rose-coloured, or an intense blood-red (_e.g._, in _Thalassophysa sanguinolenta_) or even orange (in _Physematium mülleri_). In many SPUMELLARIA, and particularly in the Polycyttaria, an albuminous substratum may be recognised in them, which is sometimes disposed in layers, and after extraction of the fat presents the appearance of a laminated sphere. The physiological significance of the oil-globules is twofold; in the first place they tend to diminish the specific gravity of the organism; in the second they may be utilised as a reserve store {xliv}of nutriment. In the latter respect they are of special importance in the process of spore-formation, each flagellate spore usually containing a fat-granule.

74. _The Intracapsular Pigment-Bodies._--In the majority of Radiolaria when observed alive, the central capsule is coloured, only in the minority is it colourless. The colour is never diffuse, but always due to the formation of definite pigment granules or vesicles, which are sometimes distributed evenly throughout the endoplasm, sometimes aggregated in the central or peripheral regions. Their form may be either spherical, irregularly rounded, or polyhedral. They vary much in dimensions, but in most cases are immeasurably small, and appear under a high magnifying power as fine dust; occasionally, however, their diameter may amount to from 0.001 to 0.005 or more. The chemical constitution of the intracapsular pigment is unknown in most Radiolaria, and is probably very various. In many instances the pigment-granules consist of fat, in others not. The commonest colours are yellow, red, and brown; violet and blue are rare, and green still rarer. Sometimes a definite tone of colour prevails throughout a whole group, and may then be attributed to inheritance, _e.g._, red is found in most #Sphæroidea#, and blue in the Polycyttaria (see note A). One colour is almost always constant in the members of the same species. True pigment-cells, belonging to the Radiolarian organism, do not occur within the central capsule. The peculiar yellow cells which are found in the central capsule of many ACANTHARIA are symbiotic xanthellæ (see § 76).

A. The number of Radiolaria whose pigment has been examined in the living
state, is too small to allow of any general conclusions being drawn.
Regarding the different colours known, see my Monograph, L. N. 16, p. 76.

75. _The Intracapsular Crystals._--The crystals found in the central capsule of many Radiolaria may be divided into two groups, of very different significance; small crystals, which are very widely distributed, and large crystals, which occur in only a few genera. The _small crystals_ may also be termed "spore-crystals," since each swarm-spore often contains such a crystal. They are rod-like or spindle-shaped, and consist of an organic substance which probably serves as a reserve of nutriment for the developing spores. Such spore-crystals have been observed in numerous SPUMELLARIA and ACANTHARIA belonging to various families, and are probably present throughout the two legions which make up the Porulosa. On the other hand, they have not been noticed in the Osculosa (NASSELLARIA and PHÆODARIA), the few swarm-spores belonging to these groups which have been observed not exhibiting any crystals. The _large crystals_, which occur in small numbers in the endoplasm, have hitherto only been observed in a few species of SPUMELLARIA, belonging to the Polycyttaria. They were first noticed in the common _Collosphæra huxleyi_, and regarded as coelestin. They are also found in the central capsule of many other Collosphærida, _e.g._, _Buccinosphæra_ (Pl. 5, figs. 11, 12). Crystal-masses, crystal-sheaves, or spherical masses of radiating acicular crystals are enclosed in {xlv}the vacuoles or "albumen globules" of _Thalassicola nucleata_ and other Thalassicollida, as well as in the central capsule of _Coelographis_ and some other PHÆODARIA (Pl. 127, figs. 4-7). All these large crystals are probably to be regarded as excretory products.

75A. _The Intracapsular Concrements._--Concretions, either mineral or organic, of varying form and constitution, are to be found in the endoplasm of Radiolaria belonging to very different families. They are most abundant and multiform in _Thalassicolla nucleata_, being usually circular or elliptical discs, which are concentrically laminated and highly refractive, resembling starch-grains. Among them twin forms may frequently be observed, as though the concrements were in process of division (see note A). Similar amyloid concretions are to be seen in the central capsule of different SPUMELLARIA and NASSELLARIA, _e.g._, in _Cephalospyris triangulata_ (Pl. 96, fig. 28). Violin-shaped, highly refractive concrements have been observed in the central capsule of numerous SPUMELLARIA, NASSELLARIA, and ACANTHARIA, _e.g._, _Thalassosphæra_, _Spongosphæra_, _Plegmosphæra_, _Cyrtocalpis_, _Peripyramis_, _Botryocella_, &c. (see note B). The chemical constitution of these concrements is insufficiently known.

A. The amyloid concretions of _Thalassicolla nucleata_ have been
described in detail in my Monograph (pp. 80, 250, Taf. iii. figs. 2, 3),
and by R. Hertwig in the Histologie der Radiolarien (1876, p. 47, Taf.
iii. figs. 9-13).

B. The violin-shaped concretions of _Thalassosphæra bifurca_ have been
figured in my Monograph (pp. 80, 261, Taf. xii. fig. 1).

76. _The Intracapsular Xanthellæ._--The xanthellæ, zooxanthellæ, or symbiotic "yellow cells" are found within the central capsule only in the ACANTHARIA, whilst in other Radiolaria they only occur in the extracapsulum. They are most frequent in the #Acanthometra#, rarer in the #Acanthophracta#, but even in the former they are often wanting. Their number is very variable, but usually small, from ten to thirty in one capsule. They lie for the most part immediately below the capsule membrane, in the cortical layer of the endoplasm. The form of the yellow cells is either spherical or ellipsoidal, often also spheroidal or even lentiform. The diameter varies from 0.01 to 0.03 mm. They possess a distinct membrane and an excentric nucleus, and contain numerous yellow pigment-granules in the endoplasm. This yellow pigment dissolves in mineral acids to form a green fluid, and in other respects also behaves somewhat differently from the yellow pigment in the extracapsular yellow cells of the SPUMELLARIA and NASSELLARIA. In both cases, however, the xanthellæ are not integral portions of the organism, but unicellular algae, living as parasites or symbiontes in the body.

A. The yellow cells in the central capsule of the ACANTHARIA were first
observed by Joh. Müller (L. N. 12, pp. 14, 47). In my Monograph I
described them at greater length, and indicated their differences from
the extracapsular yellow cells of other Radiolaria (L. N. 16, pp. 77,
86). Since then, R. Hertwig has demonstrated their cellular nature (L. N.
33, pp. 12, 113), and still more recently {xlvi}Brandt has given further
accurate information regarding their occurrence, constitution, and
physiological significance (L. N. 39, ii. Art., p. 235, figs. 62-73).

77. _The Endoplasm of the Peripylea._--The intracapsular protoplasm of the 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.