Chapter XI: Literature and History (32)

Although the number of genera and species in this legion is much increased by the rich collection of the Challenger, we can divide all ACANTHARIA into two different orders: #Acanthometra# (without complete lattice-shell) and #Acanthophracta# (provided with a complete lattice-shell).

The ACANTHARIA agree with the SPUMELLARIA in the structure of the simple capsule-membrane, which is perforated by numerous small pores (but constantly devoid of the large main opening, which the NASSELLARIA and PHÆODARIA possess, being hence united as "Merotrypasta"). We can therefore unite both former legions as "Holotrypasta" (compare above, pp. 5, 6); but in many ACANTHARIA (if not in all?) the numerous small pores of the capsule-membrane exhibit a certain peculiar arrangement not observed in the SPUMELLARIA; therefore the latter can be regarded as true "Peripylea" in opposition to the former as "Actipylea."

The peculiar main character of all ACTIPYLEA or ACANTHARIA is determined by the chemical constitution of their skeleton, which is not silex, but a peculiar organic substance, called by me in 1862 "acanthin" (Monogr. d. Radiol., pp. 30, 32). In all other Radiolaria the skeleton is composed of silex or of a silicate. But besides this {717}chemical difference, an important morphological character of the skeleton also separates the ACANTHARIA from all other Radiolaria: in the latter the skeleton is never centrogenous or arising from the centre of the capsule; in strict opposition to this general fact the skeleton of all ACANTHARIA is centrogenous, composed of radial spines, which arise from the central point of the capsule and pierce its membrane. These characteristic "radial spines of acanthin," arising from the centre, are never hollow (as formerly was supposed), but constantly solid. Their form is extremely variable, and most important for the distinction of genera and species; but more interesting from a general point of view is their peculiar arrangement or disposition.

The regular disposition of twenty radial spines has general value almost for all ACANTHARIA, with the exception only of the small group of #Actinelida#. In this latter group the number of radial spines is either more or less than twenty, and their disposition is either quite irregular or follows a peculiar rule. The number of individuals of these #Actinelida#, compared with that of the other ACANTHARIA, may be scarcely 1 per cent., whilst the latter have more than 99 per cent.; the number of observed species is in the former about 5 per cent., in the latter about 95 per cent. Nevertheless the small group of #Actinelida# is very important, being probably the ancestral group from which all other ACANTHARIA have been phylogenetically derived. These other ACANTHARIA, with twenty regularly disposed radial spines, represent the two large groups of #Acanthonida# and #Acanthophracta#. For short and clear distinction of these two groups of ACANTHARIA, we will call the #Actinelida# (with irregular number and disposition of radial spines) Adelacantha, in opposition to the Icosacantha (#Acanthonida# and #Acanthophracta#), which all possess twenty regularly disposed radial spines.

Johannes Müller, the great zoologist, to whom we are indebted for the first detection and accurate knowledge of the #Acanthometra#, already recognised the regularity in the peculiar disposition of their twenty radial spines (Abhandl. d. k. Akad. d. Wiss. Berlin, 1858, pp. 12, 37). In honour of my great master I have called this regular disposition the "Müllerian law of spine disposition," and have given a full explanation of it in my Monograph (1862, pp. 40-45, 371, 372). With regard to its general value for all Icosacantha (#Acanthonida# and #Acanthophracta#), we might also call this promorphological Müllerian law "the Icosacanthan law."

In 1862 I had already given the following precise definition of this "Icosacanthan law" (_loc. cit._, p. 40):--"Between two poles of a spineless axis are regularly disposed five parallel zones, each with four radial spines; the four spines of each zone are equidistant one from another, and also equidistant from each pole; and the four spines of each zone are so alternating with those of each neighbouring zone, that all twenty spines together lie in four meridian planes, which intersect one another at an angle of 45°." For the clear conception of this remarkable Müllerian law, and for the complete understanding of its high value for the complicated {718}morphology of all Icosacantha, it is the most profitable way to retain constantly in mind for comparison the figure of a terrestrial globe with its axis and zones. The axis of the globe is the spineless axis of all Icosacantha, around which all twenty spines are symmetrically disposed; it is perpendicular to the bisecting equatorial plane, in which lies the middle of the five parallel zones; therefore the four spines, crossed perpendicularly in this equatorial plane, are called the equatorial spines (_c_1 to _c_4 in the figures of Pls. 131-140); often, and mainly in the family Quadrilonchida (Pl. 131), these four equatorial spines are much larger or of a peculiar form, different from that of the sixteen other spines. Each pair of the four equatorial spines lies in one equatorial axis, and this latter is perpendicular to the crossing axis, in which lies the other pair of opposite spines. We may regard these two equatorial diameters, perpendicular one to another and to the spineless axis, as the two perradial axes or primary axes. Correspondingly the two meridian planes, which are determined by one perradial axis and the spineless axis, may be called the two primary or perradial meridian planes.

The globe is divided by the equatorial plane into two equal halves, the northern and the southern hemisphere. In each hemisphere there are disposed quite symmetrically eight radial spines, the distal ends of which fall in two parallel circles, a larger tropical circle (nearer to the equator) and a smaller polar circle (nearer to the pole of the spineless axis). Therefore we call the four spines of the former the "tropical spines" and the four spines of the latter the "polar spines." The angle between the former and the equatorial plane is about 30°, the angle between the latter and that plane about 60°.

The eight polar spines (four northern and four southern) lie in the same two meridian planes as the four equatorial spines. Therefore in each of these two perradial planes lie six radial spines, opposite in pairs; two equatorial and four polar spines. Commonly all eight polar spines are of the same size and form; and often they are also equal to the eight tropical spines; but in some cases (e.g., in some species of Quadrilonchida) they are much smaller than the twelve other spines, and sometimes even rudimentary. In all figures of the Pls. 131-140 (and also in my Monograph, 1862, Taf. xv.-xxii.) the polar spines of the northern circle are marked by the characters _a_1 to _a_4, the polar spines of the southern circle by the characters _e_1 to _e_4. In the first perradial meridian plane lie _a_1 and _a_3, _e_1 and _e_3, in the second _a_2 and _a_4, _e_2 and _e_4.

The eight tropical spines lie between the eight polar and the four equatorial spines, four in each hemisphere; their distal points fall in two parallel circles, which correspond exactly to the two tropics of the globe. Therefore the four northern tropical spines may be called "canceral spines" (as their ends fall in the Tropic of Cancer) and the four southern correspondingly "capricornal spines" (as their points lie {719}in the Tropic of the Capricorn). In the figures of the Pls. 131-140 (as well as in my Monograph, 1862, Taf. xv.-xxii.) the four northern or canceral spines are marked by the characters _b_1 to _b_4, and the four southern or capricornal spines by the characters _d_1 to _d_4. Also the eight tropical spines lie (crossed in pairs) in two meridian planes; they do not lie, however, in those perradial planes, in which are placed the twelve other spines; but in two different meridian planes, crossing the former at angles of 45°; we call these the "secondary" or "interradial" meridian planes. Each of these planes is determined by the spineless axis and by two crossed interradial or secondary axes; in each of the latter lie two opposite tropical spines. In the first interradial meridian plane lie _b_1 and _b_3, _d_1 and _d_3, in the second _b_2 and _b_4, _d_2 and _d_4.

It is a most interesting and important fact, that in all Icosacantha (#Acanthonida# and #Acanthophracta#) this regular disposition of the twenty spines (in five parallel zones and four meridian planes) becomes constantly preserved by heredity, whilst the form and size of the different spines are extremely varied by adaptation.

Only in a minority of the Icosacantha are all twenty spines perfectly equal or nearly equal in size and form; and then it is often very difficult to distinguish the different zones in their disposition. But in far the greater part the size or the form of the twenty spines becomes different in different zones; and then we can commonly distinguish easily the five different zones. Firstly, in all Quadrilonchida and Dorataspida, the four equatorial are distinguished from the sixteen other spines either by form or by size, and often in a very remarkable degree. As soon as these four principal spines are recognised, it is easy to determine also the sixteen others; for the eight polar spines lie in the same two (perradial) meridian planes as the former, whilst the eight tropical spines lie in two different (interradial) meridian planes, intersecting the two former at angles of 45°. Commonly, therefore, this distinction is rather easy.

In the majority of the Icosacantha all four equatorial spines are exactly of the same form and size. But in four families the two opposite spines of one equatorial axis are much larger, or of another form, than those of the crossing axis. This is the case in the Amphilonchida, Belonaspida, Hexalaspida, and Diploconida. Therefore we here call the major equatorial axis (with larger spines) the "hydrotomical axis," and the minor axis (with smaller spines) the "geotomical axis." Correspondingly, the meridian plane, in which the two larger equatorial spines are placed (_c_1, _c_3) and the appertaining four polar spines (_a_1, _a_3, _e_1, _e_3) may be called the "hydrotomical plane"; in the remarkable family of Hexalaspida (Pl. 139) all six spines of this hydrotomical plane are much larger than the other fourteen. Perpendicular to this plane is the second perradial meridian plane, which we call the "geotomical plane"; in it lie the two smaller equatorial spines (c2, c4) and the corresponding four polar spines (_a_2, _a_4, _e_2, _e_4). In some Hexalaspida (_Hexonaspis_ and _Hexacolpus_) the six spines of the hydrotomical plane become so preponderant that {720}the other fourteen spines appear rudimentary; and in some of them the two equatorial spines of the hydrotomical plane are much larger than the four polar spines of the same plane. This curious relation reaches its maximum in the Diploconida (Pl. 140).

The different development of the two equatorial axes (of the larger hydrotomical and the smaller geotomical axis) is the first and most important cause of the peculiar forms, which are produced in the four cited families. We derive these terms also from the metaphor of the terrestrial globe. The hydrotomical plane is that meridian plane of the globe which intersects almost only the water-hemisphere (the island of Ferro in the Atlantic, the island of Pandora in the Pacific). Perpendicular to this is the geotomical plane, the meridian of which intersects great land-masses in both hemispheres (Bombay in India, Athabasca in Canada). Both poles of the smaller geotomical axis are everywhere equal (the East Indian and the Western American). However, both poles of the larger hydrotomical axis (the eastern Atlantic and the western Pacific) are in some genera very different, _e.g._, in _Amphibelone_ among the Amphilonchida, and in _Zygostaurus_ among the Quadrilonchida. In this case we call the anterior (commonly more developed) pole of the hydrotomical axis the frontal pole, the opposite posterior (commonly smaller) the caudal pole (Pl. 131, figs. 7, 8; Pl. 132, figs. 9, 10). On both sides of these (right and left) lie symmetrically the two equal poles of the geotomical lateral axis.

The promorphology of the ACANTHARIA demonstrates that the geometrical fundamental form in those groups is different. In the majority of the ACANTHARIA, where the two equatorial axes are equal, that form is a double square-pyramid or a "quadrate octahedron"; the four equal equatorial spines indicate the two diagonals of the square, which is the common base of the united regular four-sided pyramids; their common axis is the spineless axis of the body; the ends of the polar spines fall on the edges of the pyramids, while the ends of the tropical spines fall on the halving lines of their faces. However, in those ACANTHARIA in which the two equatorial axes become different, the square double pyramid becomes changed into a rhombic double pyramid; the common base of the united pyramids is thus a rhombus; the hydrotomical axis is the larger, the geotomical axis the smaller diagonal of the rhombus.

Opposed to the Icosacantha, under the name "Adelacantha," is the small group of Actinelida, in which the number and disposition of the radial spines is variable, not determined by the Müllerian law. Probably this group is the common ancestral stock, from which the Icosacantha have been derived by gradual development of their peculiar disposition. Probably the oldest and most primitive form of all ACANTHARIA is _Actinelius_, in which a variable and undetermined (often very large) number of radial spines is united in one common central point, and therefore forms a needle-sphere. Whilst here all spines (often more than a hundred) are of equal size and form, in the nearly allied _Astrolophus_ large and small spines are intermingled. Both genera together form the small ancestral family of Astrolophida. In the strange family of Litholophida the radial spines do not {721}radiate within a spherical space (equally disposed in all directions), but within a quadrant or even an octant, forming a conical brush or pencil.

One very remarkable form of #Actinelida# is _Actinastrum_, forming the transition from these Adelacantha to the common regular Icosacantha. In the two observed species of _Actinastrum_ we find thirty-two radial spines, twenty of which are disposed after the Müllerian law, as in the Icosacantha. The other twelve are four interradial equatorial spines (lying in the two secondary meridian planes) and eight perradial tropical spines (lying in the two primary meridian planes). Therefore here in each primary meridian plane are placed ten spines (two equatorial, four tropical, and four polar spines), whereas in each secondary meridian plane are placed six spines (two equatorial and four tropical). But here also all thirty-two spines are so regularly placed that their free distal ends fall into five parallel zones, four in each polar zone, eight in each tropical zone, and eight in the equatorial zone.

_The Central Junction_ of the radial spines in the ACANTHARIA becomes effected in four different ways:--(1) by simple apposition of the pyramidal central ends or bases; (2) by a basal leaf-cross, or by broad wings, four on each spine, supported one upon the other; (3) by a central concrescence of the meeting bases of all the twenty spines, growing perfectly together; and (4) by a concrescence in pairs of every two opposite spines. The most common and probably the original mode of junction is the first--by pyramidal apposition; the spines at the central base are pointed in the form of a pyramid, and the triangular faces of the neighbouring pyramids are simply placed upon one another. Often the small basal pyramids are imperfectly separated from the spines by an annular constriction. Commonly the basal pyramids of the four equatorial spines are six-sided, those of the sixteen other spines five-sided.

The second mode of junction, by a basal leaf-cross, is developed from the first and appears as a strengthening or a mechanical elaboration of it. Immediately above the basal pyramid arise from its radial edges four thin and broad triangular leaves or wings, and the meeting edges of the neighbouring wings are in apposition one with the other, so that between the bases of every three or four neighbouring spines a hollow pyramidal space remains open. The apex of such a pyramidal space is directed towards the centre of the body, but separated from it by the small basal pyramid; its open base is directed outwards. The twenty-two hollow pyramidal spaces are disposed regularly in four different groups:--(A) Four equatorial spaces, four-sided, each limited by two equatorial and two tropical spines (one canceral and one capricornal); (B) eight perizonal spaces (four northern and four southern), four-sided, each limited by one equatorial, two tropical, and one polar spine; (C) eight peripolar spaces (four northern and four southern), three-sided, each limited by one tropical and two polar spines; (D) two polar spaces (one northern and one southern), four-sided, each limited by four neighbouring polar spines.

{722}The third mode of junction, by central concrescence of all twenty spines, was formerly regarded by me as an important peculiarity, sufficient for the separation of subfamilies and genera (Monogr. d. Radiol., 1862, pp. 399, 401; Prodromus, 1881, p. 466). But I found afterwards that in many species where the twenty spines commonly remain separated, accidentally they grow perfectly together and form one single piece of acanthin--a starrulet with twenty rays. Therefore I now think it is more natural to divide those species only into different subgenera.

A fourth and a very different mode of junction, quite sufficient for the distinction of different families, is the concrescence in pairs of every two opposite spines, lying in one diameter (in _Acanthochiasma_ and _Chiastolus_). Here we obtain a number of "diametral spines" (each composed of two originally opposed radial spines) and all these diametral spines are crossed loosely near the central point of the body without any solid and permanent apposition (Chiastolida). However, in some species of this peculiar family the central part of the diametral spines is twisted like a screw or spirally convoluted (Pl. 129, figs. 2, 3).

_The Form of the Radial Spines_ in the ACANTHARIA is extremely varied, and constitutes the main characters for the distinction of nearly four hundred species. But all these different forms may be reduced phylogenetically to three different fundamental forms:--(_a_) the cylindrical (with circular transverse section), (_b_) the two-edged (with elliptical or lanceolate transverse section), and (_c_) the four-edged (with square transverse section). No doubt the first (_a_) is the original primitive form, from which the two others are secondarily derived. Triangular spines never occur in the ACANTHARIA, whilst, however, they are common in the #Sphærellaria#. The first and original form, the cylindrical spine, is either a true cylinder of equal thickness in its whole length, or it is more or less conical. Rarely the spine is in the distal half spindle-shaped, and thicker than in the basal half. The second form, the two-edged spine, is more or less compressed from two opposite sides; its two edges are either more blunt, rounded, or more acute, sharp; its transverse section in the former case is elliptical, in the latter case lanceolate or rhomboidal. Sometimes the two edges are broader and in the form of two thin opposite wings. The two-edged spines may be occasionally shorter, triangular or lanceolate, at other times longer sword-shaped or linear. The third form, the four-edged spine, has constantly a square transverse section; the sides of this square are either even or concave; in the latter case the four edges are broadened and wing-like, but in the former case not. The quadrangular spines are either prismatic (of equal breadth throughout their whole length) or pyramidal (becoming gradually thinner towards the distal apex).

_The Apex of the Radial Spines_, or their free distal end, is in the majority of ACANTHARIA simple, conical. In the minority it is either truncated or bifid, or four-sided pyramidal, often with two, rarely with four prominent parallel teeth. In some forms {723}the bifid spines are so deeply cleft that they become forked. Much more interesting and more varied than these different forms of the distal end are those of the apophyses of the radial spines.

_The Apophyses of the Radial Spines_, or their "lateral transverse processes," are of the greatest importance for the morphological development of the whole subclass. Only in sixteen among the sixty-five genera of ACANTHARIA are the apophyses perfectly wanting; in the other genera they determine in the first place their general character. In the #Acanthometra# the apophyses remain perfectly free, whilst in the #Acanthophracta# their meeting ends or branches compose the latticed shell. All differences in form and shape of the apophyses can be reduced to only two primary modes; either the spine bears two opposite or four crossed apophyses; correspondingly all Acantharia apophysaria may be divided into two different main groups, the Zygapophysica (with two opposite lateral processes) and the Staurapophysica (with four crossed lateral processes opposite in pairs). Both groups have probably no direct phylogenetic connection, but seem to be derived independently from different stocks, and produce different families. The Zygapophysica are probably derived from Astrolonchida with two-edged spines (_Zygacantha_), and from this group arise the Diporaspida, the ancestral group of the majority of #Acanthophracta#. On the other hand the Staurapophysica are probably derived from Astrolonchida with four-edged spines (_Acanthonia_), and from this group arise the Tessaraspida. The apophyses of the #Acanthonida# are partly simple, partly branched or even latticed; the apophyses of the #Acanthophracta# are never simple, constantly branched and commonly latticed.

_The Malacoma_ (or the whole soft body of the ACANTHARIA as opposed to the skeleton) exhibits some peculiarities which distinguish them from the other Radiolaria, as well in the structure of the central capsule and its nucleus as in that of the enveloping extracapsular body and the pseudopodia.

The _Central Capsule_ is constantly spherical in the far greater number of the ACANTHARIA, viz., in the following six families:--Astrolophida, Chiastolida, Astrolonchida, Dorataspida, Sphærocapsida, and Phractopeltida. Among these six families the Astrolonchida and Dorataspida are far greater and far richer in different forms than all the other families. The central capsule becomes ellipsoidal or cylindrical, prolonged in one axis, in the three families, Amphilonchida, Belonapsida, and Diploconida; it becomes discoidal or lenticular, by the shortening of one axis, in two families, viz., in the Quadrilonchida and Hexalaspida. Finally, the peculiar family Litholophida is distinguished by the conical form of its central capsule.

_The Membrane_ of the central capsule in all ACANTHARIA is simple, commonly thin, sometimes very delicate; in some species it seems to be developed late, just immediately before the formation of the spores; but in no species is it completely missing. The membrane is constantly pierced by innumerable fine pores, for the emission of the {724}pseudopodia; but in many species (and probably more or less in all ACANTHARIA) there is recognisable a certain regularity in the disposition of the numerous pseudopodia and of the pores by which they radiate from the capsule. Sometimes these pores are disposed in a regular network of ramified lines, whilst the meshes of this network are devoid of pores; in other cases they form regular tufts or bushes between the radial spines. Probably in no ACANTHARIA are the pores of the capsule membrane so numerous and so equally distributed throughout as in the SPUMELLARIA; we may therefore call the former ACTIPYLEA (in opposition to the latter, as PERIPYLEA).

_The Nucleus_ of the ACANTHARIA is constantly excentric, whilst it is originally constantly central in the SPUMELLARIA. This excentric position is a necessary consequence of the centrogenous development of the radial spines. Probably connected with this peculiarity is the other, that the nucleus assumes a peculiar, complicated structure, and that in the greater number of ACANTHARIA it becomes cleft very early, and that this cleavage is effected by a peculiar kind of gemmation, first detected and very accurately described by R. Hertwig (compare his Organismus d. Radiol., 1879, pp. 10-24). However, in the young ACANTHARIA the nucleus is constantly simple, and in a certain number of species its cleavage takes place late (as in the greater number of SPUMELLARIA).

_The Endoplasm_, or the intracapsular sarcode, exhibits in the greater number of ACANTHARIA a more or less distinct radial arrangement; but this is often concealed by the different enclosed products of the endoplasm--oil-globules, vacuoles, red or different coloured pigment-granules, crystals, &c. Often it encloses a variable number of "yellow cells" (becoming green by mineral acids) to be considered as symbiotic xanthellæ.

_The Calymma_ or the jelly-veil, including the central capsule, in the ACANTHARIA is more or less voluminous, and commonly envelops the skeleton perfectly. In its surface is sometimes developed a peculiar network of "supporting fibres." A very peculiar product are the remarkable "Myophrisca" of the #Acanthometra#, which are wanting in the #Acanthophracta#; they were first detected by Johannes Müller, and figured as "Cilien-Kränze," afterwards explained by Hertwig as "contractile Fäden," similar to muscular fibrillæ (compare below).

_The Matrix_, placed between the calymma and central capsule, in the majority of the ACANTHARIA is a rather thin layer of granular exoplasm.

_The Pseudopodia_ arising from it are not so numerous as in the SPUMELLARIA, and not so equally disposed over the whole surface. Also their tendency to ramify, anastomose, and form networks seems to be much less developed. Commonly they are simple or little ramified. In many cases (and perhaps everywhere) there may be distinguished two different kinds of pseudopodia:--(1) Axopodia, or permanent pseudopodia (with axial filaments?), piercing the wall of the central capsule, and arising from the central mass of endoplasm; and (2) Collopodia, or variable pseudopodia (without axial filaments), arising outside the capsule from the matrix of extracapsular sarcode or from the {725}exoplasm on the surface of the calymma. These and other differentiations seem to indicate that the pseudopodia in the ACANTHARIA are more highly developed than in the SPUMELLARIA, and justify the denomination of the former as "Actipylea."

_Synopsis of the Orders and Suborders of_ ACANTHARIA.

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I. ACANTHOMETRA.
Skeleton composed only of acanthinic radial spines not forming a
complete lattice-shell.
-------------------------------------------------------------------------
Radial spines in variable and indefinite number,
disposed irregularly, 1. #Actinelida#.

Radial spines constantly twenty, disposed regularly
after the Müllerian law of Icosacantha, 2. #Acanthonida#.
-------------------------------------------------------------------------
II. ACANTHOPHRACTA.
Skeleton composed of twenty acanthinic radial spines (disposed after
the Müllerian law) and of a spherical or variously shaped complete
lattice-shell.
-------------------------------------------------------------------------
Radial spines all twenty of equal size; shell and
central capsule spherical, 3. #Sphærophracta#.

Radial spines of different sizes; shell and central
capsule ellipsoidal, discoidal, or heteromorphous, 4. #Prunophracta#.

----

Order III. ACANTHOMETRA, Johannes Müller, 1855.

_Acanthometra_, J. Müller, 1855, Monatsber. d. k. preuss. Akad. d. Wiss.
Berlin.
_Acanthometrida_, Haeckel, 1862, Monogr. d. Radiol., p. 371.
_Acanthometrea_, R. Hertwig, 1879, Organismus d. Radiol., p. 133.
_Acanthonida et Litholophida_, Haeckel, 1881, Prodromus, pp. 465, 469.

_Definition._--ACANTHARIA without complete latticed shell.

The order #Acanthometra#, the third order of Radiolaria, comprises all those ACANTHARIA in which the acanthinic skeleton is only composed of radial spines arising from one common central point, but never forms a complete latticed shell. By the absence of such a latticed or fenestrated shell the #Acanthometra# differ principally from the nearly allied #Acanthophracta#, the second order of ACANTHARIA, which constantly possess such a complete shell.

Johannes Müller, who first detected and described the #Acanthometra# (in 1855, _loc. cit._), defined them as follows:--"Radiolaria without shell, with siliceous radial spines" (1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 46). He described and figured eighteen species of them, disposed in four genera (_Acanthometra_ with fifteen species, and _Zygacantha_, _Lithophyllium_, _Lithoptera_, each with a single species). Among those eighteen species, however, were two "Acanthometræ cataphractæ," appertaining to the following order, the #Acanthophracta#.

In my Monograph (1862, p. 371) all true #Acanthometra# were united into a single family, Acanthometrida, with the following definition:--"Skeleton composed of a number of radial spines, piercing the central capsule and united in its centre, without {726}latticed shell." In the majority of them I observed that the skeleton did not consist of silex, but of a very peculiar organic substance, which I called "acanthin." At that time I divided the family Acanthometrida into four subfamilies:--(1) Acanthostaurida, (2) Astrolithida, (3) Litholophida, (4) Acanthochiasmida. The two former now represent the suborder #Acanthonida#, the two latter the suborder #Actinelida#. The number of genera which I distinguished in my Monograph amounted to nine, the number of species to fifty. By the rich collections of the Challenger this number is so much increased that we can here describe twenty-seven genera and one hundred and sixty species.

Richard Hertwig in his work on the Organismus der Radiolarien (1879, pp. 6-25) adopted my family Acanthometrida, and gave a very accurate description of its anatomical structure. He confirmed my observations that the radial spines of this family are never hollow, but solid, and that their chemical substance is not silex, but the organic matter "acanthin." He found that the simple nucleus of the Acanthometrida is commonly very early cleft, and that the peculiar brushes of filaments on the calymma, described by Johannes Müller and by me as "Gallert-cilien," are peculiar "contractile filaments," comparable to the "muscle-fibrillæ" of some Infusoria, or the "Myophan-filaments" (Myophrisca).

The order #Acanthometra# is here divided into two different suborders of very unequal extent and value, the #Actinelida# and #Acanthonida#. The first may be regarded as the common ancestral stock, not only of the second, but of all ACANTHARIA. In the small group of #Actinelida# the number of radial spines is variable and commonly indefinite, often very large (more than a hundred); they are therefore Adelacantha. The second suborder, the #Acanthonida#, comprise by far the greatest part of the order, and possess constantly twenty radial spines, regularly disposed after the Müllerian law; they are therefore (like all #Acanthophracta#) Icosacantha (compare above, p. 717).

The #Actinelida# possess constantly simple radial spines, without any apophyses; their form is commonly very simple and primitive. This suborder comprises three small but very different families, the Astrolophida, Litholophida, and Chiastolida. The first family, the Astrolophida, is the original ancestral group. A large and variable, commonly indefinite number of radial spines is here united in the centre of the spherical central capsule and radiating within a spherical space. In the second family, the Litholophida, a small and variable number of radial spines (between ten and twenty) is united in the apex of a conical central capsule and radiating within the quadrant or octant of a spherical space. In the third family, the Chiastolida, a variable number of radial spines is grown together by pairs, in such a manner that every two opposite spines (placed originally in one axis of the spherical central capsule) forms a single "diametral spine"; all these diametral spines are not united in the centre of the central capsule but only crossed loosely near the centre.

{727}The #Acanthonida#, the second suborder of #Acanthometra#, embraces by far the greatest number in this order, viz., all those forms in which twenty radial spines are regularly disposed after the Müllerian law--Icosacantha (compare above, p. 717). The radial spines of this suborder are either simple or provided with transverse processes (either two opposite or four crossed apophyses). They are commonly united in the middle of the central capsule by their opposed basal ends, forming small pyramids; the meeting triangular faces of the neighbouring pyramids being propped one upon another. Above these small basal pyramids often arises a basal leaf-cross formed by four broad triangular leaves or wings with straight edges; the meeting thin edges of the neighbouring spines serve for strengthening the basal junction and form hollow pyramidal spaces or compartments, filled with the contents of the central capsule (compare p. 721). The suborder #Acanthonida# comprises three different families, the Astrolonchida, Quadrilonchida, and Amphilonchida. The first family, the Astrolonchida, comprises by far the greater number of the #Acanthonida#; those genera in which all twenty spines are perfectly equal or nearly equal in size and form. In the second family, the Quadrilonchida, the four equatorial spines are much larger (and often also of another form) than the sixteen other spines (often also the eight tropical larger than the eight polar spines). The third family, the Amphilonchida, is distinguished by the preponderating development of only two opposite equatorial spines, which are much larger (and often also of another form) than the eighteen other spines.

_Synopsis of the Suborders and Families of_ #Acanthometra#.

-------------------------------------------------------------------------
Suborder I. ACTINELIDA.
Number of the radial spines variable, either more or less than twenty,
commonly disposed irregularly and not according to the Müllerian law.
-------------------------------------------------------------------------
Radial spines very numerous (thirty to a hundred
or more), radiating from a common centre
within a spherical space, 1. ASTROLOPHIDA.

Radial spines between ten and twenty, radiating
from one common point within a sphere-quadrant, 2. LITHOLOPHIDA.

Radial spines of variable number; every two
opposite spines grown together in the centre;
therefore numerous diametral spines are
crossed freely in the centre, 3. CHIASTOLIDA.
-------------------------------------------------------------------------
Suborder II. ACANTHONIDA.
Number of the radial spines constantly twenty, disposed regularly
according to the Müllerian law.
-------------------------------------------------------------------------
All twenty radial spines nearly equal, and of
the same size and form, 4. ASTROLONCHIDA.

Four equatorial spines much larger than (and
often also of different form from) the sixteen
other spines, 5. QUADRILONCHIDA.

Two opposite equatorial spines (or principal
spines) much larger than (and often also of
different form from) the eighteen other
spines, 6. AMPHILONCHIDA.

{728}----

Suborder I. ACTINELIDA, Haeckel, 1882.

_Definition._--#Acanthometra# with a variable number of radial spines, which are commonly irregularly disposed, not according to the Icosacantha.

Family XXXIII. #ASTROLOPHIDA#, Haeckel.

_Astrolophida_, Haeckel, 1881, Prodromus, p. 469.

_Definition._--#Acantharia# with a variable number of simple radial spines, radiating within a spherical space from one common central point which is the centre of the spherical central capsule. No lattice shell.

The family #Astrolophida# comprises the simplest and the most primitive forms among all ACANTHARIA, and may therefore be regarded as the common ancestral stock of this whole legion or subclass of Radiolaria. The acanthinic skeleton is composed of a variable number of quite simple radial spines, which are united in the centre of the spherical central capsule and radiate, piercing its walls and the surrounding jelly-veil, within a spherical space.

The first observed form of this family is the ancestral genus _Actinelius_, two different species of which I detected in 1864 in the northern Mediterranean, at Villafranca, near Nice (compare Zeitschr. f. wiss. Zool., 1865, Bd. xv. p. 364, Taf. xxvi. fig. 4). Three other species of the same genus were afterwards found by me in the Challenger collections. Whilst in this _Actinelius_ all radial spines are of the same size, a new nearly allied genus, _Astrolophus_ (with two species), differs from it by the different size of the radial spines, a small number of very large spines being intermingled with a very large number of small spines. In these two genera, _Actinelius_ and _Astrolophus_ (the true "Astrolophida" _sensu strictiori_), the number of the radial spines is quite indeterminable and their arrangement quite irregular and variable.

A third remarkable genus, _Actinastrum_, differs from these two genera in the definite number and regular order of thirty-two radial spines, and may therefore perhaps better represent a peculiar family, Actinastrida. In this genus (of which two species were observed) the thirty-two radial spines are disposed in such a regular manner that they lie in four meridian planes, and that their distal ends fall into five parallel zones. These five zones and these four planes are the same as we find in all Icosacantha (compare above, p. 717). Also the constant twenty spines of these latter are present in _Actinastrum_; but their number is here enlarged by twelve other spines missing in the Icosacantha; four of these are secondary or interradial equatorial spines, lying opposite in pairs between the four primary or perradial equatorial spines; and eight are perradial tropical spines, lying between the eight interradial tropical spines. Therefore the {729}distal ends of the thirty-two radial spines are disposed regularly in five parallel zones, and while two zones (the two polar) contain only the points of every four spines, three zones (the single equatorial and the two tropical) contain the points of every eight spines. The four meridian planes are in _Actinastrum_ the same as in the Icosacantha, crossed in the spineless axis at angles of 45°. But in the Icosacantha each of the two perradial meridian planes contains six radial spines (two equatorial and four polar), each of the two interradial meridian planes only four tropical spines. Whereas in _Actinastrum_ each of the two primary or perradial meridian planes contains ten spines (two equatorial, four tropical, and four polar), each of the two secondary or interradial meridian planes six spines (two equatorial and four tropical). We find therefore altogether thirty-two radial spines in three orders; eight equatorial, sixteen tropical, and eight polar spines.

Only one other genus of Radiolaria exhibits the same characteristic disposition of thirty-two radial spines as _Actinastrum_, and this is _Chiastolus_; but here the two opposite spines of each pair are grown together and form one diametral spine; and the sixteen diametral spines are crossed in the centre of the capsule. In _Actinastrum_, as in _Astrolophus_ and _Actinelius_, the central ends or bases of all the spines are pyramidal, and the triangular faces of the neighbouring spines rest one upon another (as in the greater number of #Acanthonida#). The form of the radial spines in all Astrolophida is quite simple, without lateral processes or apophyses; chiefly cylindrical, more rarely compressed, two-edged or quadrangular.

The central capsule in all Astrolophida is spherical, and in the younger specimens contains a single large concentric and lobed nucleus, but in the older specimens a large number of small nuclei. The surrounding jelly-veil or calymma seems commonly to envelop the spines perfectly. The piercing pseudopodia radiate everywhere between the spines, and are very numerous and thin. The circulating granules in them are sometimes red (_Actinelius purpureus_).

_Synopsis of the Genera of Astrolophida._

Radial spines of indefinite {Spines of equal size, 317. _Actinelius_.
number and of irregular {
disposition. {Spines of unequal size, 318. _Astrolophus_.

Radial spines thirty-two, disposed regularly
in five parallel zones, 319. _Actinastrum_.

Genus 317. _Actinelius_,[357] Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv. p. 364.

_Definition._--#Astrolophida# with a variable and undetermined number of simple radial spines, all of equal size, united in the centre of the spherical central capsule.

{730}The genus _Actinelius_ comprises the most simple and primitive forms among all ACANTHARIA, and may be regarded as the common ancestral stock of this whole legion. The spherical central capsule is pierced by numerous simple radial spines of equal size, the pyramidal bases of which are supported one upon another with their triangular faces in the centre of the capsule. The number and position of the spines are quite indefinite and variable. We may derive _Actinelius_ either from _Actissa_ (#Colloidea#) by development of acanthinic radial spines, or directly from _Actinosphærium_ (Heliozoa) by formation of a central capsule.

Subgenus 1. _Actinelarium_, Haeckel.

_Definition._--Radial spines cylindrical, conical, or spindle-shaped, their transverse section circular.

1. _Actinelius primordialis_, n. sp. (Pl. 129, fig. 1).

Spines sixty to eighty or more, cylindrical, at the distal end thickened, spindle-shaped. Apex simple. Base a small slender pyramid. Central capsule yellow. Granules of the sarcode colourless.

_Dimensions._--Length of the spines 0.3 to 0.4, breadth in the distal part 0.02, in the basal part 0.008.

_Habitat._--Central Pacific, Stations 265 to 274, surface.

2. _Actinelius purpureus_, Haeckel.

_Actinelius purpureus_, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv.
p. 364, Taf. xxvi. fig. 4.

Spines thirty to forty or more, cylindrical, very thin, a little thinner towards both ends. Apex simple. Base a small sulcate pyramid. Central capsule opaque, purple. Granules of the sarcode also purple.

_Dimensions._--Length of the spines 0.2 to 0.3, breadth 0.002.

_Habitat._--Mediterranean (Nice), Haeckel.

Subgenus 2. _Actinelidium_, Haeckel.

_Definition._--Radial spines compressed, two-edged; their transverse section elliptical or lanceolate.

3. _Actinelius protogenes_, n. sp.

Spines fifty to sixty, compressed, two-edged, gradually broadened towards the truncated distal end. Basal or proximal end thin, pyramidal. The spines of this species are similar to those of {731}_Actinastrum pentazonium_ (p. 733) and of _Chiastolus amphicopium_ (Pl. 129, fig. 3), but much more numerous, smaller, and not regularly disposed. These latter two Actinelida must be separated on account of the regular disposition of the thirty-two spines.

_Dimensions._--Length of the spines 0.2, basal breadth 0.008, distal breadth 0.02.

_Habitat._--South Pacific, Station 165, surface.

Subgenus 3. _Actinelonium_, Haeckel.

_Definition._--Radial spines quadrangular, prismatic, or pyramidal, their transverse section square.

4. _Actinelius pallidus_, Haeckel.

_Actinelius pallidus_, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv.
p. 364.

Spines eighty to one hundred and twenty or more, quadrangular, prismatic, of equal breadth throughout their whole length. Apex simple, truncate or pyramidal. Base a four-sided slender pyramid. Central capsule pale yellowish. Granules of the sarcode colourless.

_Dimensions._--Length of the spines 0.2 to 0.3, breadth 0.005.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific.

5. _Actinelius polyacanthus_, n. sp.

Spines two hundred to three hundred or more, quadrangular, pyramidal, gradually thinned towards the simple apex. Base a small three-sided pyramid. Central capsule opaque.

_Dimensions._--Length of the spines 0.12 to 0.18, basal breadth 0.012.

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

Genus 318. _Astrolophus_,[358] Haeckel, 1881, Prodromus, p. 469.

_Definition._--#Astrolophida# with a variable and undetermined number of simple radial spines of different sizes (large and small spines intermingled), which are united in the centre of the spherical central capsule.

The genus _Astrolophus_ differs from the nearly allied ancestral genus _Actinelius_ only in the unequal size of the numerous radial spines. In both observed species very numerous small spines are intermingled with a small number of large spines, and between them numerous spines of medium size. The small spines fill up the hollow spaces between the basal parts of the large spines.

{732}1. _Astrolophus stellaris_, n. sp.

Radial spines from one hundred to two hundred, of very different sizes, but of similar form; about sixteen to twenty very large spines, forty to fifty of medium size, and one hundred to one hundred and twenty much smaller. All spines cylindrical in the greater part of their length, with simple apex, gradually thickened towards the central part, conical, without edges. The base itself is a slender pyramid with four to eight edges.

_Dimensions._--Length of the largest spines 0.3 to 0.4, of the majority 0.1 to 0.2, of the smallest 0.05 to 0.1.

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

2. _Astrolophus solaris_, n. sp. (Pl. 132, figs. 12_a_, 12_b_).

Radial spines from two hundred to three hundred, of very different sizes, but of similar form; about twenty to thirty very large spines, sixty to eighty of medium size, and one hundred and twenty to one hundred and fifty much smaller. All spines cylindrical in the greater part of their length, with simple apex, gradually thickened and four-edged towards the central base. The base itself is a slender pyramid with four to eight edges; partly the faces, partly the edges of these basal pyramids rest one upon another, the points of the larger spines meeting in the centre.

_Dimensions._--Length of the largest spines 0.4 to 0.5, of the majority 0.2 to 0.3, of the smallest 0.1 to 0.16; basal thickness of the largest spines 0.015.

_Habitat._--South-east Pacific (off Juan Fernandez), Station 296, surface.

Genus 319. _Actinastrum_,[359] n. gen.

_Definition._--#Astrolophida# with thirty-two simple radial spines, regularly disposed within four meridian planes in such an order that their distal ends fall into five parallel zones. Central ends of the thirty-two spines supported one upon another in the centre of the spherical central capsule.

The genus _Actinastrum_ differs from the two preceding genera in the definite number and order of the thirty-two radial spines, which are disposed in a very remarkable manner. Twenty radial spines are disposed after the Müllerian law of Icosacantha (compare above, p. 717). The remaining twelve spines are four equatorial spines lying in the two secondary meridian planes, and eight tropical spines lying in the two primary meridian planes. We have therefore together eight equatorial, sixteen tropical, and eight polar spines (compare above, p. 729).

{733}1. _Actinastrum legitimum_, n. sp.

All thirty-two radial spines of equal size and similar form, cylindrical, conical at the distal end, at the central base pyramidal. Central capsule pellucid, colourless.

_Dimensions._--Length of the radial spines 0.3, breadth 0.004.

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

2. _Actinastrum pentazonium_, n. sp.

All thirty-two radial spines of equal size and similar form, compressed, two-edged, gradually becoming broader and thinner from the pyramidal central base towards the truncated distal end. Central capsule dark, opaque. (Compare the similar _Chiastolus amphicopium_, Pl. 129, fig. 3.)

_Dimensions._--Length of the radial spines 0.2, breadth at the base 0.005, at the distal end 0.02.

_Habitat._--South Pacific (west coast of Patagonia), Station 302, surface.

Family XXXIV. LITHOLOPHIDA, Haeckel.

_Litholophida_, Haeckel, 1862, Monogr. d. Radiol., p. 401.

_Definition._--ACANTHARIA with a variable number of simple radial spines radiating within a conical space (or within the quadrant of a sphere) from one common central point, which is the apex of the conical central capsule. No lattice-shell.

The family #Litholophida#, represented only by a single genus, _Litholophus_, differs from all other ACANTHARIA in the remarkable fact that the common point, from which the radial spines arise, is not the geometrical central point of the whole body, but is quite excentric in position, the apex of the conical or pyramidal central capsule. Therefore the spines form together a kind of brush or broom.

When I founded the family Litholophida in my Monograph (1862, p. 401) I knew only a single species, _Litholophus rhipidium_, observed very frequently in Messina. Another species, _Litholophus ligurinus_, was afterwards (1864) found by me at Nice. Six other species were detected in the preparations of the Challenger, some of them very frequent. All these eight species of _Litholophus_ are very nearly allied, and exhibit only slight differences in the form and number of the radial spines; their mode of excentric connection and the structure of the peculiar soft body is everywhere the same.

The radial spines in all observed Litholophida possess the form of the genus _Acanthonia_, _i.e._, they are quite simple, four-sided prismatic or quadrangular, with square transverse section; their four edges are sometimes smooth, at other times elegantly denticulate, commonly more or less prominent or wing-shaped. In the greater number of species they are very long and of nearly equal breadth, prismatic; in some species they are more pyramidal, thinned towards the distal end; the latter {734}is commonly truncated or broken off, sometimes pyramidal. The central end is everywhere thinned, more or less pyramidal, and the neighbouring spines are propped one upon another by the triangular faces of their small basal pyramids. A slight pressure is sufficient to destroy their connection.

The number and disposition of the radial spines seem to be variable and irregular, but require further researches. In four of the observed eight species I found constantly ten spines, in two other species from ten to twenty (commonly twelve or sixteen), and in two species twenty or more. A certain order or disposition of the spines within the conical space in which they radiate could nowhere be ascertained.

When I first observed _Litholophus_, I supposed that it might only be a mutilated or altered form of an _Acanthonia_. Afterwards, observing many specimens with ten spines, I was led to the suggestion that they were produced by self-division of an _Acanthonia_, and that the number of the spines in each half of the body might be afterwards doubled. But this suggestion seems to be refuted by the fact that in no other genus of the numerous ACANTHARIA is self-division observed, and that many hundreds of _Litholophus_ which I observed exhibit quite constantly only a single form of radial spines, that of _Acanthonia_--simple quadrangular spines without any apophyses.

Genus 320. _Litholophus_,[360] Haeckel, 1862, Monogr. d. Radiol., p. 401.

_Definition._--#Litholophida# with a variable number of quadrangular diverging radial spines, united with pyramidal bases in the apex of the conical central capsule.

The genus _Litholophus_, the only one of this family, exhibits the peculiarities just described, but might more nearly be defined as a typical "genus" by the quadrangular form of the radial spines, identical with those of _Acanthonia_.

The central capsule of _Litholophus_ is constantly conical or pyramidal, commonly opaque, of a dark brownish or reddish colour; it contains many small nuclei. It envelops the basal half of all radial spines in such a manner that their basal parts are united in its apex, and their distal parts pierce the rounded base of the conical capsule (Pl. 129, fig. 2).

The calymma or the jelly envelope of the central capsule is only developed at its base, where the spines radiate; at the conical mantle of the capsule it is very thin. The spines seem to be perfectly enclosed in the calymma and connected with it by the same contractile retinacula or "myophrisca" which we observe in the #Acanthonida#. The pseudopodia arise only from the rounded base of the conical capsule, and radiate between the spines, piercing the calymma, diverging within the conical space occupied by the fascicle of spines.

{735}Subgenus 1. _Litholopharium_, Haeckel.

_Definition._--Ten radial spines.

1. _Litholophus decimalis_, n. sp.

Ten radial spines, four-sided prismatic, with prominent smooth edges, of equal breadth throughout their whole length.

_Dimensions._--Length of the spines 0.2 to 0.3, breadth 0.006.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, surface.

2. _Litholophus pyramidalis_, n. sp.

Ten radial spines, four-sided pyramidal, with prominent smooth edges, gradually thickened from the small pyramidal base towards the truncated distal end.

_Dimensions._--Length of the spines 0.3 to 0.4, breadth in the basal part 0.002, in the middle part 0.006, in the distal part 0.012 to 0.02.

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

3. _Litholophus decapristis_, n. sp. (Pl. 129, fig. 2).

Ten radial spines, four-sided prismatic, with prominent, elegantly denticulated edges, of equal breadth in their whole length.

_Dimensions._--Length of the spines 0.2 to 0.4, breadth 0.008.

_Habitat._--Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

4. _Litholophus decastylus_, n. sp.

Ten radial spines, four-winged pyramidal, with broad and thin, elegantly denticulated edges, gradually thickened from the small pyramidal base towards the truncated distal end.

_Dimensions._--Length of the spines 0.2 to 0.3, breadth in the basal part 0.002, in the middle part 0.005, in the distal part 0.015.

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

Subgenus 2. _Litholophidium_, Haeckel.

_Definition._--Number of the radial spines variable, between ten and twenty, commonly twelve to sixteen.

{736}5. _Litholophus ligurinus_, Haeckel.

_Litholophus ligurinus_, Haeckel, 1865, Zeitschr. f. wiss. Zool., Bd. xv.
p. 366.

Spines of variable number, from eleven to twenty, commonly twelve to sixteen, four-sided prismatic, with smooth thin edges, of equal breadth in their whole length or a little thinner towards the proximal end.

_Dimensions._--Length of the spines 0.3, breadth 0.005.

_Habitat._--Mediterranean (Nice); Central Pacific, Station 274, surface.

6. _Litholophus rhipidium_, Haeckel.

_Litholophus rhipidium_, Haeckel, 1862, Monogr. d. Radiol., p. 402, Taf.
xix. fig. 6.

Spines of variable number, from eleven to twenty, commonly twelve to sixteen, four-sided prismatic, with distantly denticulated edges, of equal breadth in their whole length or a little thinner towards both ends.

_Dimensions._--Length of the spines 0.3, breadth 0.006.

_Habitat._--Mediterranean (Messina); North Atlantic, Station 352, surface.

Subgenus 3. _Litholophonium_, Haeckel.

_Definition._--Number of the radial spines twenty (or more?).

7. _Litholophus fasciculus_, n. sp.

Spines constantly (?) twenty, four-sided prismatic, with smooth prominent edges, nearly of equal breadth throughout their whole length.

_Dimensions._--Length of the spines 0.3 to 0.5, breadth 0.008.

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

8. _Litholophus penicillus_, n. sp.

Spines constantly twenty (or more?), four-sided prismatic, with distantly denticulated edges, gradually thickened from the small pyramidal base to the middle part, of equal breadth in the distal half.

_Dimensions._--Length of the spines 0.3 to 0.4, breadth 0.005 to 0.007.

_Habitat._--North Pacific, Station 252, surface.

Family XXXV. #CHIASTOLIDA#, Haeckel.

_Acanthochiasmida_, Haeckel, 1862, Monogr. d. Radiol., p. 402.

_Definition._--ACANTHARIA with a variable number of simple radial spines, which are grown together in pairs (two opposite spines of each pair representing together a single diametral spine). Diametral spines crossed loosely in the centre of the spherical or irregular roundish central capsule. No lattice-shell.

{737}The family #Chiastolida# (or Acanthochiasmida) differs from all other ACANTHARIA in the peculiar mode of the radial spines; these grown together in pairs in the centre of the body, so that every two spines opposite in one axis of the body form together one single diametral spine. All diametral spines are loosely crossed in the middle of the central capsule, or connected by a peculiar screw-like winding, but not united firmly.

I established the family Acanthochiasmida in my Monograph (1862, p. 402) upon the single genus _Acanthochiasma_ (with three species), in which only ten diametral spines are constantly found; I derived these from the twenty radial spines of the common #Acanthometra#, supposing that every two opposite spines of the latter (lying in one axis) were grown together in the centre, whilst the intimate connection of the twenty radial spines in the common centre was dissolved. This opinion was afterwards confirmed by Richard Hertwig, who observed _Acanthochiasma_ intact in the living state. Although two species of this genus are cosmopolitan and very common, the number of species is very small; I could add to those three older known forms only a single new species.

Another genus of this family, _Chiastolus_, was observed by me only in a single specimen but it is extremely interesting. It has sixteen diametral spines, disposed quite regularly after the same law of the thirty-two spines of _Actinastrum_ which we described above (compare above, p. 729). Therefore we cannot doubt that the former is derived from the latter in the same way, every two opposite radial spines (of one axis) being grown together to form a single diametral spine. As we place _Acanthometron_ (with twenty spines) and _Actinastrum_ (with thirty-two spines) in two different families, it would perhaps be more convenient to separate also _Acanthochiasma_ and _Chiastolus_ as representatives of two different families--Acanthochiasmida (with ten diametral spines) and Chiastolida (with sixteen diametral spines).