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.

5. Litholophus ligurinus, Haeckel.

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.

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.

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).

As we derive Acanthochiasma from Acanthometron by concrescence in pairs of the twenty radial spines, the Müllerian law of Icosacantha must be employed also to the ten diametral spines of the former, therefore two of them are equatorial, four tropical, and four polar spines. In the same way we may employ the new law of disposition found in the thirty-two radial spines of Actinastrum equally to the sixteen diametral spines of Chiastolus, which we derive from the former, four of them are equatorial, eight tropical, and four polar spines (compare above, p. 732).

The Central Capsule of the Chiastolida is spherical, and exhibits in general the same shape as in the Acanthonida, and specially in the Astrolonchida. Of course every diametral spine pierces the capsule twice, at two points diametrically opposed. In some species of Acanthochiasma the central capsule is formed very late, so that it seems often to be absent. A very accurate description of the capsule and its nucleus, as well as of the calymma and the pseudopodia, is given by Richard Hertwig in his Organismus der Radiolarien (1879, pp. 10-18). The pseudopodia are very numerous, and sometimes bear reddish granules.

Synopsis of the Genera of Chiastolida.

Genus 321. Chiastolus,^[361] n. gen.

Definition.—Chiastolida with sixteen diametral spines, derived from thirty-two radial spines opposite and grown together in pairs.

The genus Chiastolus, hitherto known only by a single observed specimen, comprises the Chiastolida with sixteen diametral spines, which are loosely crossed in the centre of the body. Four of these spines are equatorial, four polar, and eight tropical. We derive Chiastolus from Actinastrum by concrescence of every two opposite spines in one axis of the body (compare above, pp. 729, 732).

1. Chiastolus amphicopium, n. sp. (Pl. 129, figs. 3, 3a, 3b).

Spines in the central part cylindrical, spirally convoluted in a very peculiar manner, broadened towards both ends, strongly compressed, two-edged; the broadest parts are the two truncated distal ends, five to seven times as broad as the thinnest central part, which is placed between two spindle-shaped intumescences. These cochleary central parts of the sixteen spines seem to be resting one upon another. Each spine (composed of two opposite equal radial spines) has nearly the form of a double oar. The single observed specimen (preserved in glycerine) exhibited a most regular disposition of the thirty-two spines (grown together in pairs in the centre). The diameter of the dark non-transparent spherical central capsule equalled one-fifth to one-fourth of the total length of the double spines. When the soft parts of the body were destroyed by sulphuric acid, the sixteen single spines were suddenly dispersed.

Dimensions.—Length of the sixteen double spines 0.5, distal breadth (of the truncated ends) 0.05, central breadth 0.01; diameter of the central capsule 0.12.

Habitat.—South-east Pacific (near Juan Fernandez), Station 297, surface.

Genus 322. Acanthochiasma,^[362] Krohn, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 810.

Definition.—Chiastolida with ten diametral spines, derived from twenty radial spines opposite and grown together in pairs.

The genus Acanthochiasma with a small number of common species, comprises the Chiastolida with ten diametral spines, which are loosely crossed in the centre of the body. I could distinguish only four species, two of which are cosmopolitan and very widely distributed. In all four species the diametral spines are quite simple, cylindrical; only in one species distinguished by a spiral winding or torsion in the middle part, where they are crossed one to another. We derive Acanthochiasma from Acanthometron by concrescence of every two spines opposite in one axis of the body.

1. Acanthochiasma krohnii, Haeckel.

Spines needle-shaped, cylindrical, very thin and long, of equal breadth in their whole length, distinguished by a high degree of elasticity. Central capsule colourless or yellowish-white, transparent. Granules of the sarcode colourless.

Dimensions.—Length of the spines 0.5 to 1.0, breadth 0.001 to 0.002.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Indian, Pacific, very common.

2. Acanthochiasma rubescens, Krohn.

Spines cylindrical, of equal breadth in their whole length, not very elastic, pointed at the two ends. Central capsule intransparent, reddish, with violin-shaped concretions. Granules of the sarcode red coloured.

Dimensions.—Length of the spines 0.2 to 0.6, breadth 0.004 to 0.006.

Habitat.—North Atlantic, Madeira, Krohn; Lanzerote, Haeckel.

3. Acanthochiasma fusiforme, Haeckel.

Spines spindle-shaped, from the thicker central part thinned towards the two thin conical ends, perfectly straight and smooth, rigid, inelastic. Central capsule non-transparent, brown.

Dimensions.—Length of the spines 0.3 to 0.5, breadth in the central part 0.006 to 0.009.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific.

4. Acanthochiasma spirale, n. sp.

Spines spindle-shaped, tapering from the thicker central part towards the two thin conical ends, rigid, inelastic; their central part is spirally convoluted in a very peculiar cochlea-like manner, as in Chiastolus amphicopium (Pl. 129, figs. 3a, 3b). The ten spines are propped one upon another by the central screw. Central capsule dark, opaque.

Dimensions.—Length of the spines 0.2 to 0.3, breadth of the central spiral part 0.01.

Habitat.—Central Pacific, Station 266, surface.

Suborder II. ACANTHONIDA, Haeckel (Pls. 130-132).

Acanthonida, Haeckel, 1881, Prodromus, p. 465.

Definition.—Acanthometra with twenty radial spines, disposed according to the Müllerian or Icosacanthan law in five zones each of four spines.

Family XXXVI. Astrolonchida, Haeckel (Pl. 130).

Astrolonchida, Haeckel, 1881, Prodromus, p. 465.

Definition.—Acanthometra with twenty radial spines of nearly equal size and similar form, disposed according to the law of the Icosacantha. No lattice-shell.

The family Astrolonchida, the first and oldest of the Acanthonida, is no doubt the ancestral stock not only of this suborder but also all Acanthophracta, i.e., of all Icosacantha or all Acantharia in which twenty radial spines are regularly disposed according to the Müllerian law, forming five zones each of four alternating spines (compare above, p. 717). The Astrolonchida differ from the Acanthophracta in the absence of a complete lattice-shell, from the other two families of Acanthonida (the Quadrilonchida and Amphilonchida) in the equal size and similar form of all the spines. Probably this equality is nowhere quite perfect, since in all Icosacantha the central bases of the twenty spines exhibit originally certain slight differences of form and junction, effected by the regular disposition itself. But setting aside this slight difference, only recognisable by means of a very accurate investigation of the central junction (and in thinner spines often not at all recognisable), the twenty spines of the Astrolonchida appear perfectly equal. Therefore the four equatorial spines are not distinguished from the sixteen other spines, as is constantly the case in the two following families.

The number of genera (eleven) and of species (seventy-six) in the Astrolonchida is far larger than in the five other families of Acanthometra, and requires a distinction into three different subfamilies. (A) In the Zygacanthida the form of the radial spines is quite simple, without apophyses or transverse processes; (B) in the Phractacanthida each spine bears two opposite apophyses (rarely two longitudinal rows of these opposite apophyses); (C) in the Stauracanthida each spine bears a cross of four apophyses, opposite in pairs (rarely four longitudinal crossed rows of apophyses, opposite in pairs). The Phractacanthida and Stauracanthida appear as two divergent branches of the pedigree, derived independently from the common ancestral stock of Zygacanthida.

In the Zygacanthida, constantly devoid of apophyses, we can distinguish only three genera, characterised by the different fundamental form of the radial spines; these are:—(1) Acanthometron, with cylindrical or conical spines (without edges); (2) Zygacantha, with compressed and two-edged spines; (3) Acanthonia, with four-edged, prismatic or pyramidal spines. The transverse section of the spines is in the first case circular, in the second elliptical or lanceolate, in the third square. All the different forms of spines, which we find in the numerous Acantharia, may be reduced to these three forms, and among these the second and third are derived from the first.

The development of apophyses or of lateral transverse processes (wanting in the Zygacanthida) is of the greatest value for the further differentiation of the Acantharia. For from the Phractacanthida (with two opposite apophyses on each spine) we must derive the Phrastaspida, the common ancestral stock of the Diporaspida (and therefore also the Belonaspida, Hexalaspida, Diploconida, and Phractopeltida). On the other hand the Stauracanthida (with four crossed apophyses on each spine) have produced the Stauraspida, or the ancestral group of the Tessaraspida and Sphærocapsida. From all these Acanthophracta, possessing a complete lattice-shell, the Astrolonchida differ in the absence of such a complete shell. Also in the few cases in which the apophyses become latticed (Doracantha among the Phractacanthida, and Phatnacantha among the Stauracanthida), the lattice-plates of the neighbouring spines never meet with their edges, as is the case in all Acanthophracta. But in a phylogenetic as well as in an ontogenetic sense the former are the ancestral stock of the latter.

The Central Capsule in the Astrolonchida is commonly spherical, sometimes with twenty roundish elevations or conical papillæ, extending radially to the basal half of the radial spines. The calymma is voluminous, and forms around the radial spines conical or cylindrical "jelly-sheaths," which are connected with the spines by coronas of Myophrisca (or of the bodies formerly called "Gallert-cilien," afterwards recognised as "contractile Filamente").

Synopsis of the Genera of Astrolonchida.

Subfamily 1. Zygacanthida, Haeckel.

Definition.—Astrolonchida with twenty simple radial spines, without apophyses or lateral transverse processes.

Genus 323. Acanthometron,^[363] J. Müller, 1855, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 229.

Definition.—Astrolonchida with simple cylindrical or needle-shaped radial spines, without edges and without apophyses; their transverse section is circular.

The genus Acanthometron, with the restricted definition here given, is the most simple form of all Acanthonida, and may be regarded as the common ancestral form not only of this suborder but also of all Acanthophracta, in general of all Icosacantha, or all Acantharia in which twenty radial spines are regularly disposed after the Müllerian law (p. 717). In the wider sense, given originally to Acanthometra by Johannes Müller, its discoverer, this genus comprised all Acantharia constituting here our order "Acanthometra" (Radiolaria without lattice-shell, with radial spines united in the centre). In my Monograph (1862, p. 375) I restricted this genus to those "Acanthometrida" in which twenty simple spines of equal size (and without apophyses) are supported one upon another in the centre, and I separated as Astrolithium those forms in which they are grown together in the centre. But this difference now appears not so important, and I restrict here the genus Acanthometron (not Acanthometra) to those most simple forms in which the simple radial spines are cylindrical or conical, without edges.

Subgenus 1. Acanthometrella, Haeckel.

Definition.—Spines at the central base without leaf-cross, united by the opposed triangular faces of their pyramidal bases, resting one upon another.

1. Acanthometron elasticum, Haeckel.

Spines cylindrical, very thin and long, needle-shaped, at the central base four-sided pyramidal, without leaf-cross. Distal apex conical. The spines are very elastic, of nearly equal thickness in their whole length. Central capsule quite pellucid, colourless, with a variable number of yellow pigment-bodies (xanthellæ?).

Dimensions.—Length of the spines 0.3 to 0.6, breadth 0.001 to 0.002.

Habitat.—Cosmopolitan, very common in all warmer seas; Mediterranean, Atlantic, Indian, Pacific, surface.

2. Acanthometron cylindricum, n. sp. (Pl. 130, fig. 2).

Spines cylindrical, thick and long, at the central base thickened with a pear-shaped knob, and with very small central fulcral pyramid, without leaf-cross. Distal apex rounded or truncated. Central capsule opaque, filled with red pigment-bodies.

Dimensions.—Length of the spines 0.4 to 0.8, breadth 0.01 to 0.015.

Habitat.—Central Pacific, Stations 266 to 274, surface.

3. Acanthometron fuscum, J. Müller.

Spines very thin and long, in the proximal half cylindrical, in the distal half conical, gradually thinned towards the simple conical apex. Central base a small four-sided pyramid, without leaf-cross. Central capsule opaque, filled with brown pigment-bodies.

Dimensions.—Length of the spines 0.2 to 0.4, breadth 0.002 to 0.004.

Habitat.—Mediterranean (Cette, Portofino, Messina).

4. Acanthometron bulbosum, Haeckel.

Spines very thin and long, cylindrical, with simple needle-shaped apex; in the basal part conical, towards the centre much thickened, with a short four-sided fulcral pyramid, without leaf-cross. Central capsule opaque, filled with brown pigment-bodies.

Dimensions.—Length of the spines 0.3 to 0.5, breadth in the middle and outer part 0.001, in the basal part 0.008 to 0.012.

Habitat.—Mediterranean; Atlantic, Station 354, surface.

5. Acanthometron dolichoscion, Haeckel (Pl. 129, figs. 6-8).

Spines very long and thin, cylindrical, in the thickened outer half about twice as thick as in the thin inner half, at the simple apex compressed, two-edged. Central base little thickened, with a small four-sided pyramid, without leaf-cross. Central capsule transparent, whitish or yellowish.

Dimensions.—Length of the spines 0.6 to 0.8, breadth in the proximal part 0.004, in the distal part 0.008.

Habitat.—Mediterranean (Messina, Portofino), surface.

6. Acanthometron conicum, n. sp.

Spines short and thick, conical, gradually thinner from the thick conical base towards the simple apex. Central part of the base with a large fulcral pyramid, but without leaf-cross. Central capsule transparent, colourless (?).

Dimensions.—Length of the spines 0.08 to 0.12, basal breadth 0.02.

Habitat.—North Pacific, Station 244, surface.

7. Acanthometron pellucidum, J. Müller.

Spines cylindrical, very thin and long, needle-shaped, nearly of equal thickness in their whole length. Central base with a small fulcral pyramid, without leaf-cross. Distal apex bifid or bifurcate, with two thin parallel teeth. Central capsule pellucid, colourless, with yellow pigment-bodies. (Differs from Acanthometron elasticum mainly in the bifid apex.)

Dimensions.—Length of the spine 0.1 to 0.3, breadth 0.002 to 0.004.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

8. Acanthometron wageneri, Haeckel.

Spines cylindrical, in the thickened outer half about twice as broad as in the thin inner half. Central base a little thickened, with large fulcral pyramid, but without leaf-cross. Distal apex bifid, with two divergent, often denticulated teeth. Central capsule pellucid, with yellow pigment-bodies. (Differs from Acanthometron dolichoscion mainly in the bifid apex.)

Dimensions.—Length of the spines 0.2 to 0.5, breadth in the inner part 0.003, in the outer 0.006.

Habitat.—Adriatic Sea; Triest, Wagner; Corfu, Haeckel, surface.

Subgenus 2. Phyllostaurus, Haeckel, 1862, Monogr. d. Radiol., p. 381.

Definition.—Spines at the central base with a broad leaf-cross, composed of four prominent triangular lamellæ; the meeting edges of the neighbouring lamellæ are propped one upon another in such a manner that there are formed twenty-two hollow pyramidal spaces or compartments (compare p. 721).

9. Acanthometron siculum, Haeckel.

Spines elongate, conical, tapering gradually from the thick base towards the simple distal apex. Conical circular base supported by a basal leaf-cross of double the breadth. Central capsule yellowish-brown, opaque.

Dimensions.—Length of the spines 0.3 to 0.4, basal breadth 0.008 to 0.012, leaf-cross 0.024.

Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, surface.

10. Acanthometron catervatum, Haeckel.

Spines cylindrical, nearly of equal breadth throughout their whole length. Apex either simple, conical, or bifid. Base with a large leaf-cross, four to six times as broad as the spine itself. Central capsule transparent, yellow. The Atlantic specimens have much longer spines than those figured from the Mediterranean, but are otherwise not different. Therefore I have changed the inconvenient name brevispinum into catervatum.

Dimensions.—Length of the spines 0.1 to 0.3, breadth 0.005 to 0.008; basal leaf-cross 0.02 to 0.032.

Habitat.—Mediterranean (Messina); North Atlantic, Station 354, Gulf Stream (Færöe Channel) in great abundance, John Murray, surface.

Subgenus 3. Astrolithium, Haeckel, 1860, Monatsber. d. k. preuss. Akad. d. Wiss. Berlin, p. 810.

Definition.—Spines in the basal part grown perfectly together, so that the whole skeleton forms a single piece of acanthin; a star with twenty equal rays.

11. Acanthometron bulbiferum, n. sp. (Pl. 130, fig. 1).

Spines needle-shaped, cylindrical, very thin and long, with simple apex; suddenly thickened and forming a broad regular cone at the central base; all twenty conical bulbs have their broad bases grown together and forming a central icosahedron of acanthin; from its twenty faces arise the circular bases of the cones in regular disposition.

Dimensions.—Length of the spines 0.2 to 0.5, breadth 0.001 to 0.003; height of the basal cones 0.06, basal breadth of them 0.02.

Habitat.—Central Pacific, Stations 265 to 274, surface.

12. Acanthometron bifidum, Haeckel.

Spines needle-shaped, cylindrical, of equal breadth in their whole length. Distal apex bifid, with two parallel straight teeth. Central bases of all twenty spines grown perfectly together and forming a central sphere of acanthin. Central capsule brown opaque.

Dimensions.—Length of the spines 0.2, breadth 0.003.

Habitat.—Mediterranean (Messina), Haeckel, surface.

Genus 324. Zygacantha,^[364] J. Müller, 1858, Abhandl. d. k. Akad. d. Wiss. Berlin, p. 51.

Definition.—Astrolonchida with simple, compressed, and two-edged radial spines, without apophyses; their transverse section is elliptical or rhomboidal.

The genus Zygacantha comprised in the original definition of J. Müller only a single species, Zygacantha furcata, distinguished from the other Acanthometra by forked spines with two long parallel teeth. It seems now advisable to unite in this genus all those Astrolonchida in which the simple spines are two-edged, compressed, or leaf-shaped. The term Zygacantha may be conceived as the general expression of the important fact, that in all Icosacantha the twenty spines are opposite in pairs.

Subgenus 1. Zygacantharium, Haeckel.

Definition.—Spines at the central base without leaf-cross and without hollow pyramidal compartments, united by the opposed triangular faces of their pyramidal bases, resting one upon another.

1. Zygacantha lanceolata, Haeckel.