7. Streblonia pulvinulina, n. sp.
Shell top-shaped or flatly conical, with thirty to forty chambers of slowly increasing size, the tenth chamber about three times as broad as the first. Breadth of the shell exceeds three times the height. Pores circular, subregular, very small, about one-third as broad as the bars; about thirty in the breadth of the tenth chamber. (Resembles closely Pulvinulina.)
Dimensions.—Breadth of the shell 0.25, height 0.08.
Habitat.—Pacific, central area, Station 263, depth 2650 fathoms.
Definition.—Streblonida with simple, spherical, subspherical, or lentelliptical primordial chamber, beginning the screw-like series of spirally ascending chambers. Surface covered with radial spines.
The genus Streblacantha differs from the nearly allied Streblonia only in the covering of radial spines, and bears therefore the same relation to it as Hastigerina has to Globigerina amongst the similar calcareous Polythalamia.
1. Streblacantha siderolina, n. sp. (Pl. 49, figs. 8, 8a).
Streblonia siderolina, Haeckel, 1883, MS.
Shell flatly conical, with fourteen to sixteen nearly hemispherical chambers of gradually increasing size, the tenth chamber about six times as broad as the first. Breadth of the shell nearly equal to the height. Pores subregular, circular, hexagonally framed, twice as broad as the bars; about nine pores on the breadth of the tenth chamber. Surface covered with numerous short conical radial spines, one-fourth to one-sixth as long as the diameter of the shell.
Dimensions.—Breadth of the shell 0.15, height 0.17.
Habitat.—South Pacific, Station 298, depth 2225 fathoms.
2. Streblacantha calcarina, n. sp.
Shell conical, with sixteen to eighteen roundish chambers of gradually increasing size, the tenth chamber about three times as broad as the first. Breadth of the shell about one and a half times the height. Pores irregular, roundish. Surface covered with numerous strong conical radial spines, about half as long as the diameter of the shell.
Dimensions.—Breadth of the shell 0.24, height 0.17.
Habitat.—South Pacific, Station 285, depth 2375 fathoms.
3. Streblacantha hastigerina, n. sp.
Shell nearly spherical, clustered, with nine to eleven nearly spherical chambers of rapidly increasing size, the tenth chamber about six times as broad as the first. Breadth of the shell nearly equal to the height. Pores subregular, circular, of about the same breadth as the bars. Surface bristly, covered with numerous very thin and long, needle-shaped radial spines, longer than the diameter of the shell. (Resembles closely Hastigerina.)
Dimensions.—Breadth of the shell 0.18, height 0.15.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
Definition.—Streblonida with trizonal lentelliptical medullary shell (composed like Larnacilla of three elliptical dimensive girdles surrounding one simple central primordial chamber). From this begins a screw-like series of spirally ascending chambers. Surface smooth or thorny, without radial spines.
The genus Streblopyle presents externally the same appearance and contour as Streblonia, and is composed like this of a variable number of chambers, ascending screw-like around the axis of the spiral shell. The first or primordial chamber, however, in which the growth begins, is in Streblonia a simple spherical shell, but in Streblopyle a trizonal shell or Larnacilla-shell (compare above, p. 600). The chambers are very incompletely separated, and comparatively much larger, their number much smaller than in Streblonia. The structure in the species of this genus is difficult to understand.
1. Streblopyle helicina, n. sp. (Pl. 49, fig. 9).
Shell helicoid, one and a third times as high as broad, with eight to twelve incomplete semizonal chambers, ascending spirally from the lateral half girdle of the lentelliptical medullary shell, octopyle-shaped, and enveloping it in three to four spiral turnings. The height of the whole cortical shell equals nearly five times the height of the trizonal medullary shell. Pores irregular, roundish. Surface of the shell rough or nearly smooth. (This species seems to be nearly allied to Spironium octonium.)
Dimensions.—Breadth of the spiral cortical shell 0.18, height 0.24; breadth of the medullary shell 0.04, height 0.05.
Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.
2. Streblopyle spirulina, n. sp.
Shell egg-shaped or nearly spherical, about as high as broad, with eight to nine semizonal chambers, ascending spirally from the subspherical trizonal medullary shell, and enveloping it in four to five turnings. The sixth chamber twice as broad as the trizonal medullary shell. Pores irregular, roundish. Surface of the shell thorny.
Dimensions.—Breadth of the spiral cortical shell 0.27, height 0.25; breadth of the medullary shell 0.05, height 0.06.
Habitat.—Pacific, central area, Station 265, depth 2900 fathoms.
Phorticida, Haeckel, 1881, Prodromus, p. 464.
Definition.—Larcoidea with quite irregular monothalamous shell, representing irregular modifications of an original lentelliptical latticed shell; the irregular cortical shell encloses a regular or subregular, lentelliptical or trizonal medullary shell.
The family Phorticida comprises a small number of Larcoidea in which a subregular, trizonal, lentelliptical medullary shell is enclosed by an irregular simple or spongy cortical shell. The lattice-work of the latter is sometimes simple and complete, at other times incomplete, with open gates (as in the Pylonida), sometimes also spongy. Its form is always more or less irregular, roundish, often dimply or tuberous; different from most other Larcoidea.
The medullary shell is constantly a regular or subregular Larnacilla-shell, composed of three elliptical latticed girdles of different sizes, perpendicular one to another. This leaves no doubt that the Phorticida are true Larcoidea. The connection of it with the cortical shell is rarely effected by radial beams, commonly by two opposite latticed wings, which are identical with the lateral halves of the transverse girdle in the Pylonida diplozonaria (Amphipyle, Tetrapyle). Often also between this transverse and a second (lateral) girdle there remain large open gates, so that the affinity between these Phorticida and the Pylonida cannot be doubted. In other cases these gates become closed, so that they more nearly approach the Larnacida. From both families they differ by the irregularity of the papillate or tuberous cortical shell. The network is more or less irregular, its surface often thorny, but never covered with symmetrically disposed radial spines.
The central capsule is lentelliptical, encloses the medullary shell, and is enveloped by the cortical shell, as in the nearly allied Pylonida and Larnacida, of which the Phorticida may be regarded as irregular aberrant forms.
| Cortical shell simply latticed, | 313. Phorticium. |
| Cortical shell spongy, | 314. Spongophortis. |
Definition.—Phorticida with irregular cortical shell of simple lattice-work, enclosing a lentelliptical Larnacilla-shaped medullary shell.
The genus Phorticium comprises all Phorticida in which the irregular cortical shell is formed by simple lattice-work, not by spongy framework. We can divide this genus into two subgenera: in Phortopyle (as in the Pylonida) the lattice-work of the cortical shell exhibits large openings or gates; in Phortolarcus these gates are perfectly closed by network; the former may be regarded as abnormal or irregular Pylonida, the latter as modifications of Larnacida.
Definition.—Lattice-work of the irregular cortical shell incomplete, with large openings or gates.
1. Phorticium pylonium, n. sp. (Pl. 49, fig. 10).
Cortical shell irregular, roundish, about three times as large as the enclosed lentelliptical, regular, Larnacilla-shell, connected with it by some radial beams and irregularly latticed girdles; between these remain four to eight large open gates of irregular roundish form and size; and these gates are the same as in Tetrapyle and Octopyle. This very variable species may be regarded as a monstrosity of those genera of Pylonida; it is very common, but all individuals are more or less unequal; some specimens approach to some common species of Tetrapyle. The surface of the shell is more or less spiny.
Dimensions.—Diameter of the irregular cortical shell 0.12 to 0.18; length of the lentelliptical medullary shell 0.05 to 0.06, breadth 0.035 to 0.045.
Habitat.—Cosmopolitan; Mediterranean, Atlantic, Pacific, &c., common, surface and in various depths.
2. Phorticium spironium, n. sp.
Cortical shell irregular, roundish, tuberous, about four times as large as the enclosed subregular Larnacilla-shell, connected with it by some irregular radial beams, and by opposite, spirally begining, irregularly latticed girdles, comparable to those of Spironium; between them remain six to twelve large open gates of irregular size and form. Surface rough. The resemblance to some forms of Spironium makes it probable that this species is a deformity or monstrosity of that genus.
Dimensions.—Diameter of the irregular cortical shell 0.12 to 0.2, of the lentelliptical medullary shell 0.03 to 0.06.
Habitat.—Pacific, central area, Stations 270 to 274, surface, and in various depths.
Definition.—Lattice-work of the irregular cortical shell complete, without large openings or gates.
3. Phorticium deforme, n. sp.
Cortical shell irregular, roundish or longish, three times as large as the enclosed subregular, lentelliptical Larnacilla-shell, connected with it by two opposite latticed wings (the halves of the transverse girdle of Tetrapyle). Network of the cortical shell irregular, dense, perfectly closed, without larger openings or gates. Surface thorny. (May be regarded as a monstrous form of Larnacalpis.)
Dimensions.—Diameter of the irregular cortical shell 0.15 to 0.18, of the medullary shell 0.04 to 0.06.
Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.
4. Phorticium abnorme, n. sp.
Cortical shell irregular, roundish, tuberous, with five to ten quite irregular or nearly hemispherical protuberances, which resemble the cupolas of Zonarida. The regular lentelliptical Larnacilla-shell is one-third to one-fourth as large as the enclosing cortical shell, and is connected with it by some irregular radial beams. Lattice-work completely closed, without gates. Surface spiny. (May be regarded as an anomalous form of Zonidium; as in the other species of this variable genus, the individuals are very unequal.)
Dimensions.—Diameter of the irregular cortical shell 0.12 to 0.2, of the medullary shell 0.03 to 0.05.
Habitat.—Atlantic and Pacific, tropical zone, surface, and at various depths.
Definition.—Phorticida with irregular cortical shell of spongy framework, enclosing a lentelliptical Larnacilla-shaped medullary shell.
The genus Spongophortis differs from Phorticium in the spongy framework of the cortical shell. This encloses the inner Larnacilla-shaped medullary shell either directly, or both shells are separated by a hollow interval, and connected either by radial beams or by latticed lamellæ. Perhaps both these subgenera might be better separated as genera.
Definition.—Spongy cortical shell immediately enclosing the lentelliptical medullary shell, without hollow interval.
1. Spongophortis spongiosa, n. sp.
Cortical shell irregular, roundish, rough or tuberous, composed of loose spongy framework, which immediately envelops the lentelliptical central Larnacilla-shell; the diameter of the former becomes about five to six times as large as that of the latter.
Dimensions.—Diameter of the spongy cortical shell 0.15 to 0.25, of the trizonal medullary shell 0.03 to 0.04.
Habitat.—Pacific, central area, Station 274, surface.
Definition.—Spongy cortical shell separated by a hollow interval from the lentelliptical medullary shell.
2. Spongophortis radiosa, n. sp.
Cortical shell irregular, roundish, four to five times as large as the enclosed lentelliptical Larnacilla-shell, with which it is connected by ten to twenty irregularly disposed radial beams. Spongy framework compact, about as thick as the medullary shell. Surface covered with numerous short, bristle-shaped, radial spines.
Dimensions.—Diameter of the spongy cortical shell 0.15 to 0.2, of the trizonal medullary shell 0.035 to 0.045.
Habitat.—South Atlantic, Station 332, depth 2200 fathoms.
3. Spongophortis larnacilla, n. sp. (Pl. 49, figs. 11a-11d).
Cortical shell irregular, roundish, tuberous, three to four times as large as the enclosed lentelliptical Larnacilla-shell, connected with it by two opposite latticed wings (the halves of the transverse girdle of Tetrapyle). Spongy framework compact, about half as thick as the medullary shell. Surface rough. (May be regarded as an abnormal Tetrapyle or Larnacalpis, with an irregular spongy cortical shell.)
Dimensions.—Diameter of the spongy cortical shell 0.16 to 0.2, of the trizonal medullary shell 0.04 to 0.06.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
Soreumida, Haeckel, 1881, Prodromus, p. 464.
Definition.—Larcoidea with quite irregular polythalamous shell, composed of a variable number of chambers, aggregated without any definite order. Primordial chamber either simple or Larnacilla-shaped.
The family Soreumida contains a small number of Larcoidea, different from most other Sphærellaria in the complete irregularity of the polythalamous shell, which is composed of a variable number of roundish chambers or subspherical latticed shells, aggregated in the form of an irregular heap. We can distinguish in this family only two genera, with very different structure of the central medullary shell or the first chamber beginning the growth; and these correspond to the two subfamilies of the nearly allied Streblonida (p. 704). In Soreuma (as in Streblonia) the first or primordial chamber, from which the growth begins, is like the others, a simple spherical or irregular roundish lattice-shell. In Sorolarcus, however (as in Streblopyle), the first or primordial chamber is a trizonal or Larnacilla-shell. It is not improbable that the former originated phylogenetically from Streblonia, the latter from Streblopyle, by loss of the original spiral order of growth. But it is also possible that these groups have no nearer relation. Among the calcareous Foraminifera a very similar form is represented by Acervulina and its allies.
The general form of the whole shell in the Soreumida is sometimes more egg-shaped or lentelliptical, at other times even subspherical, occasionally quite irregular, tuberous, or clustered. The number of the aggregated chambers is very variable, in Sorolarcus between ten and thirty, in Soreuma ascending to fifty to eighty, sometimes from one hundred to one hundred and fifty and more. Their size is sometimes nearly equal, at other times very different, their form commonly very irregular, roundish, but sometimes also subspherical or egg-shaped. The network of the shell is also commonly irregular, with roundish pores of different sizes. The surface is usually smooth or rough, rarely covered with radial spines.
The central capsule is not known, as I observed only a few skeletons of this family.
| Primordial chamber of the shell simple, subspherical or roundish, | 315. Soreuma. |
| Primordial chamber of the shell trizonal or Larnacilla-shaped, | 316. Sorolarcus. |
Definition.—Soreumida with numerous chambers, aggregated without any regularity around one simple, spherical or subspherical, central chamber.
The genus Soreuma contains those Soreumida in which no trace of any regular structure is found, but all the chambers of the irregular shell are without any order, aggregated around a simple spherical or subspherical central chamber or medullary shell. Soreuma may have originated either from Sorolarcus by loss of the central Larnacilla-shell or from Cenolarcus by irregular apposition of new chambers around the lentelliptical central chamber or simple Larcoid-shell. Some species seem to exhibit a transition to Sorolarcus. Owing to the absolute irregularity of the polythalamous shell Soreuma resembles Acervulina among the Foraminifera.
Definition.—Shell without radial spines.
1. Soreuma irregulare, n. sp. (Pl. 49, fig. 12).
Shell irregular, clustered, or tuberous, composed of a large number (one hundred and twenty to one hundred and fifty or more) of irregular, roundish chambers of very different sizes, the largest four to five times as broad as the smallest. Network very delicate, with very small roundish pores, to five times as broad as the bars. Surface thorny.
Dimensions.—Diameter of the shell 0.3, of the single chambers 0.02 to 0.08.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
2. Soreuma acinosum, n. sp.
Shell irregularly lentelliptical, with different growth in the three dimensions, composed of a large number (forty to sixty or more) of irregular, roundish chambers of very different sizes, the largest six to eight times as broad as the smallest. Pores subregular, circular, twice as broad as the bars. Surface smooth.
Dimensions.—Length of the shell 0.21, breadth 0.17, height 0.13; diameter of the largest chambers 0.03, of the smallest 0.004.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
3. Soreuma subglobosum, n. sp.
Shell nearly spherical, composed of a variable number (twelve to fifteen or more) of irregularly aggregated subspherical chambers of nearly equal size. Pores subregular, circular, twice as broad as the bars. Surface thorny.
Dimensions.—Diameter of the shell 0.25, of the largest chambers 0.04, of the smallest 0.005.
Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.
4. Soreuma acervulina, n. sp.
Shell quite irregular, cloddy, or tuberous, composed of twenty to thirty (or more) roundish chambers of almost uniform size, the largest twice to three times as broad as the smallest. Pores irregular, roundish. Surface smooth.
Dimensions.—Diameter of the shell 0.18 to 0.24, of the largest chambers 0.06, of the smallest 0.02.
Habitat.—South Pacific, Station 295, depth 1500 fathoms.
Definition.—Shell with radial spines.
5. Soreuma spinosum, n. sp.
Shell quite irregular, cloddy, or tuberous, composed of thirty to forty subspherical chambers of nearly the same size. Pores subregular, circular, twice as broad as the bars; on the equator of each chamber six to eight pores. Surface thorny, covered with irregularly scattered conical radial spines, about as long as the diameter of the chambers.
Dimensions.—Diameter of the shell 0.17 to 0.25, of the chambers 0.04.
Habitat.—North Pacific, Station 241, depth 2300 fathoms.
6. Soreuma setosum, n. sp.
Shell nearly spherical, composed of sixty to seventy (or more) irregular, roundish chambers of very different sizes, the largest five to six times as broad as the smallest. Pores irregular, roundish. Surface bristly, covered with very numerous, long and thin, bristle-shaped radial spines, about as long as the diameter of the shell.
Dimensions.—Diameter of the shell 0.28, of the chambers 0.005 to 0.03.
Habitat.—Pacific, central area, Station 271, depth 2425 fathoms.
Definition.—Soreumida with numerous chambers, aggregated irregularly around a trizonal medullary shell or Larnacilla-shell.
The genus Sorolarcus comprises those Soreumida in which the heap of irregularly aggregated chambers encloses a central trizonal medullary shell, by which they demonstrate clearly their descent from Larnacida or Pylonida. The lentelliptical medullary shell exhibits quite the same characteristic structure as that of Larnacilla, being composed of three elliptical latticed girdles, perpendicular one to another. In some species also the beginning of a second system of girdles is clearly indicated, so that there can be no doubt as to their derivation from Amphipyle or Tetrapyle.
Definition.—Shell without radial spines.
1. Sorolarcus larnacillifer, n. sp. (Pl. 49, fig. 13).
Shell irregular, clustered, or tuberous, composed of twenty to thirty irregular, roundish chambers of very different size, the largest four to eight times as broad as the smallest, aggregated without order around a central, lentelliptical, Larnacilla-shaped medullary shell. Pores irregular, roundish, twice to four times as broad as the bars. Surface smooth or a little spiny.
Dimensions.—Diameter of the whole shell 0.18, of the central Larnacilla-shell 0.05.
Habitat.—Pacific, central area, Station 266, depth 2750 fathoms.
2. Sorolarcus tetrapylifer, n. sp.
Shell irregularly roundish, clustered, composed of ten to twelve irregular rather long chambers of almost uniform size, the largest twice as broad as the smallest, aggregated without order around a central shell of the structure of Tetrapyle, which encloses an inner trizonal Larnacilla-shell of half the size. Pores irregular, roundish, twice to four times as broad as the bars. Surface spiny.
Dimensions.—Diameter of the whole shell 0.25, of the outer (Tetrapyle-like) medullary shell 0.12, of the inner (Larnacilla-like) shell 0.06.
Habitat.—Pacific, central area, Station 272, depth 2600 fathoms.
Definition.—Shell with radial spines.
3. Sorolarcus terminalis, n. sp.
Shell nearly spherical, composed of fifteen to eighteen irregularly aggregated roundish chambers of nearly equal size; in the centre a lentelliptical Larnacilla-shell. Surface covered with numerous thin, bristle-like radial spines, somewhat longer than the shell.
Dimensions.—Diameter of the whole shell 0.21, of the central Larnacilla-shell 0.05.
Habitat.—North Pacific, Station 244, depth 2900 fathoms.
vel Actipylea, vel Acanthometrea (Pls. 129-140).
Acantharia, Haeckel, 1881.
Actipylea, Haeckel, 1882.
Acanthometrea, Hertwig, 1879.
Panacantha, Haeckel, 1878.
Definition.—Radiolaria with simple membrane bounding the central capsule, which is everywhere perforated by innumerable fine pores (disposed either equally or symmetrically). Extracapsulum without phæodium. Skeleton centrogenous (its growth proceeding from the centre), acanthinic (organic, not siliceous). Fundamental form originally spherical.
The legion Acantharia vel Actipylea, to the extent here defined, was constituted by me, 1878, in my Protistenreich (p. 102) under the name "Panacantha." A more accurate definition of this group was given in 1879 by Hertwig under the name Acanthometrea. Both names were replaced by me, 1881, in my Prodromus (pp. 421, 465) by the more convenient name Acantharia. This legion comprises all those Radiolaria which were first described by Johannes Müller, 1858, as Acanthometrae, and also an important part of his Haliomma. In my Monograph (1862, pp. 371-424) I disposed them in three families, Acanthometrida, Diploconida, and Dorataspida.
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 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 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 (c1 to c4 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 a1 to a4, the polar spines of the southern circle by the characters e1 to e4. In the first perradial meridian plane lie a1 and a3, e1 and e3, in the second a2 and a4, e2 and e4.
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 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 b1 to b4, and the four southern or capricornal spines by the characters d1 to d4. 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 b1 and b3, d1 and d3, in the second b2 and b4, d2 and d4.
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 (c1, c3) and the appertaining four polar spines (a1, a3, e1, e3) 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 (a2, a4, e2, e4). In some Hexalaspida (Hexonaspis and Hexacolpus) the six spines of the hydrotomical plane become so preponderant that 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 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.