Chapter IV: The Skeleton (2)

A. The distinction between medullary and cortical shells was originally
based in my Monograph (1862, p. 50) upon the topographical relation of
the lattice-shells to the central capsule, inasmuch as I regarded all
intracapsular shells as medullary, all extracapsular as cortical.
Hertwig, however (1879, p. 122), rightly pointed out that this
distinction is unpractical, "because the same lattice-shell in the same
species may lie within or without the central capsule, according to the
size of the latter." He proposes, therefore, to restrict the term
medullary shell to the innermost, and to call all the others cortical; a
course which seems justified by the special significance of the primary
innermost lattice-shell ("as the point of origin of the radial spines").
But in most #Sphærellaria# which form three or more concentric shells,
the two innermost, which lie near together within the {lxxxvi}central
capsule, are very different in size and dictyosis from all the others
which lie outside, and are separated by wider interspaces (compare Pls.
17, 24, 29-32, 40, &c.). In these cases it appears better to regard the
two inner as inner and outer medullary shells, and all the others as
cortical shells. The character of the dictyosis in the intracapsular and
extracapsular shells is often so different that I have made it the basis
of separation of _Thecosphæra_ and _Rhodosphæra_ among the Liosphærida
(p. 60), of Elatommatida and Diplosphærida among the Astrosphærida (p.
208), &c.

B.--R. Hertwig (1879, L. N. 33, pp. 40, 123) separates the true
(simultaneously formed) "cortical shells" (_e.g._, of _Actinomma_,
_Cromyomma_) from the arachnoid "siliceous networks" (_e.g._, of
_Diplosphæra_ and _Arachnosphæra_) which are formed by the successive
union of tangential apophyses of the radial spines. Whether this
principle is right in the theory or not, it cannot be carried out
practically. Compare also Pl. 25, fig. 4.

130. _Dictyosis or Lattice Formation of the Skeleton._--In the great majority of Radiolaria the dictyosis or formation of lattice-work, and especially the formation of a variously-shaped "lattice-shell," plays such an important part that the whole class has long been popularly known in Germany by the name "lattice animalcules" ("Gitterthierchen" or "Gitterlinge") (_Protista dictyota_). The old name Polycystina also (1838), although referring only to the SPUMELLARIA and NASSELLARIA, is derived from the lattice-work of the siliceous skeleton. The extremely various forms in which this is manifested furnish the means of distinguishing species. The specific conformation of the skeletal lattice-work is usually caused by the special disposition of the sarcodictyum (§ 94), whose exoplasmatic threads become silicified or (in the ACANTHARIA) converted into bars of acanthin. In many cases, however, the form of the lattice is mainly dependent upon the situation and form of the radial spines or of special processes from them. With respect to their origin, two varieties of lattice may be distinguished--simultaneous and successive. _Simultaneous dictyosis_ occurs especially in the simple lattice-shells of the #Sphærellaria# and PHÆODARIA, where, at a given moment ("dictyotic moment") the _whole_ lattice of the shell is excreted on the surface of the calymma. _Successive dictyosis_, on the other hand, is found more particularly in the lattice-shells of the ACANTHARIA (and in the concentric cortical shells of many #Sphærellaria#), which develop from the separate lattice-plates formed by the apophyses of the radial spines, and hence not at the same moment. The lattice-shells of the #Cyrtellaria#, which gradually grow out from a sagittal ring or a basal tripod, arise by successive dictyosis.

131. _Dictyosis of the Spumellaria._--Siliceous lattice-structures are wanting in the first section of the SPUMELLARIA, the #Collodaria#, but in the second section, #Sphærellaria#, they are developed in extraordinary variety of details. In spite of this extreme richness in different forms, the lattice-shells of the SPUMELLARIA may all be derived from one and the same primitive ground-form, a simple lattice-sphere with regular hexagonal meshes (_Phormosphæra_, p. 61, Pl. 12, figs. 9-11; _Heliosphæra_, &c.). {lxxxvii}The siliceous bars which bound these regular and subregular meshes are at first exceedingly then and filiform; afterwards they become thicker or spread out laterally, whence the meshes often become round with a hexagonal frame (Pl. 12, fig. 5; Pl. 28, fig. 1). If the latter vanish, a lattice-shell with simple circular meshes is formed. Very commonly the regular form of the meshes or pores becomes more or less irregular, polygonal, or roundish. Hence, in general, four different principal forms of dictyosis may be distinguished among the SPUMELLARIA; viz. (1) regular or subregular _hexagonal_ meshes; (2) regular or subregular _circular_ meshes; (3) irregular _polygonal_ meshes; (4) irregular _roundish_ meshes. The three latter forms are to be regarded as secondary, derived from the primary first form. In those SPUMELLARIA which possess several concentric lattice-shells enclosed one within another, either these have all the same form of dictyosis, or the lattice-work of the innermost primary shell is different from that of the outer secondary shells (Pls. 19, 20); sometimes these latter also differ more or less among themselves (§ 129).

132. _Dictyosis of the Acantharia._--The lattice-structures of the ACANTHARIA differ essentially from those of other Radiolaria in several particulars. Firstly, they consist not of silica but of acanthin (§ 102); secondly, they are always secondary formations, usually developed from transverse processes of the primary centrogenous radial spines; thirdly, their formation is not simultaneous (at the same time over the same shell), but successive (proceeding from the individual radial spines tangentially towards the middle of the intervals); fourthly, the configuration of the network is due to the relative position of the spines and the mode of union of their transverse apophyses. Since they are at right angles to the spines, and since the branches of the apophyses are at right angles to them, the original ground-form of their dictyosis is a lattice-work with quadrangular meshes; these are often quite regular and square (Pl. 130, figs. 5, 6; Pl. 136, figs. 2, 9, &c.); more commonly they are rectangular or irregularly quadrangular (Pl. 131, fig. 10; Pl. 133, figs. 2, 3, &c.). In the majority of the ACANTHARIA the quadrangular form of the meshes passes over into an irregularly polygonal or roundish one (Pls. 137, 138). Very often the primary meshes of the lattice-shells, which immediately surround the radial spines, are larger and more regular ("aspinal pores"), whilst the numerous secondary meshes between them are smaller and irregular ("coronal pores"; Pl. 135, figs. 1-4, &c.).

133. _Dictyosis of the Nassellaria._--The siliceous lattice-structures of the NASSELLARIA are formed on the whole like those of the SPUMELLARIA, with which they were formerly united under the name "Polycystina." In this group also there may be distinguished as two main forms the regular and irregular. In the NASSELLARIA the regular lattice-structures generally exhibit hexagonal or circular meshes, whilst the irregular are either polygonal or roundish; the irregular forms are, however, much more abundant than the {lxxxviii}regular, and a further distinction from the SPUMELLARIA consists in the fact that the primary skeletal elements, from which the lattice is secondarily developed, exercise a predominant influence upon their form. These primary elements in the majority of the NASSELLARIA are to be seen in two morphologically most important structures:--first, the _primary sagittal ring_, which embraces the central capsule in the median plane (§ 124); and secondly, the _basal tripod_ (§ 125), whose three diverging rays proceed from the base of the central capsule, whilst commonly a fourth vertical ray supports the dorsal side of latter (compare Pls. 81-91, p. 892). In the majority of the NASSELLARIA these two primary elements appear in combination, whilst in others only one of them is recognisable. In addition there occur numerous monaxon lattice-shells in which neither of these elements can be recognised, but a simple ovoid lattice-shell (cephalis) alone forms the whole skeleton or its primary part (Pl. 51, fig. 13; Pl. 98, fig. 13). The great difficulty in the morphological interpretation and phylogenetic derivation of the NASSELLARIAN skeleton lies in the fact that each of these three elements--the primary sagittal ring, the basal tripod, and the latticed cephalis--may form the whole skeleton by itself or be combined with one or both of the others (p. 893). Even nearly related or at all events very similar forms may differ very greatly in this respect. With regard to the manifold forms of their dictyosis it follows that it is partly dependent upon one of the two first elements, partly independent. In the #Plectellaria# (or those NASSELLARIA which do not possess a complete lattice-shell) the lattice-work is usually irregular and arises by union of the ramifications, which proceed either from the primary sagittal ring (Pls. 81, 82, 92-94) or from the basal tripod (Pl. 91). In the #Cyrtellaria# (or NASSELLARIA with a complete lattice-shell, Pls. 51-80), on the other hand, the lattice-work is sometimes regular, sometimes irregular, being often very different in the different joints of a segmented shell (Pl. 72); a great part of it arises independently of the two chief morphological elements, and develops according to laws similar to those which regulate the dictyosis of the SPUMELLARIA.

134. _Dictyosis of the Phæodaria._--The lattice-structures of the PHÆODARIA, which consist of a silicate of carbon (§ 102), are on the whole not developed in such variety as those of the other Radiolaria, but exhibit several essentially different types of structure, not reducible to a common primitive type of lattice-work. In one portion of this legion there occurs an ordinary simple lattice-work (as in SPUMELLARIA and NASSELLARIA), with solid trabeculæ; of these the Castanellida (Pl. 113) and Concharida (Pls. 123-125) have usually regular or subregular, circular meshes, sometimes hexagonally framed; the Orosphærida (Pls. 106, 107) large irregular polygonal meshes with thick trabeculæ, the Sagosphærida (Pl. 108) large triangular meshes with thin filiform trabeculæ. The Challengerida (Pl. 99) are characterised by a very delicate regular lattice-work, with minute hexagonal pores, like a Diatomaceous frustule. The Medusettida (Pls. 118-120) {lxxxix}show a peculiar alveolar structure, numerous small compartments being enclosed between two parallel plates. In the Circoporida (Pls. 114-117) and Tuscarorida (Pl. 100) the opaque porcellanous shell has a peculiar cement structure (§ 104), and the lattice-structure is confined for the most part to characteristic rings of pores at the base of the hollow tubes, which arise from the shell. The most peculiar lattice-work, however, appears in the segmented shell of the Aulosphærida (Pls. 109-111) and Cannosphærida (Pl. 112). In the former the large meshes of the lattice-work are usually subregular and triangular, in the latter polygonal; the trabeculæ are hollow cylinders, filled with jelly, and containing usually a central axial thread. In each nodal point of the lattice, in which three or more tangential tubes meet, these are separated by stellate or astral septa.

135. _Radial Spines of the Skeleton._--The skeleton in the great majority of Radiolaria is armed with radial spines, which are of great importance in the development of their general form and of their vital functions. From a morphological point of view the number, arrangement, and disposition of the spines is usually the determining factor as regards the general form of the skeleton. Physiologically they discharge distinct functions, as organs of protection and support; they act also, like the tentacles of the lower animals, as prehensile organs, since their points, lateral branches, barbed hooks, &c. serve to hold fast nutritive materials. In general main-spines and accessory spines may be distinguished in most Radiolaria; the former are of pre-eminent importance in determining the figure of the skeleton; the latter are merely appendicular organs. The main-spines present such characteristic and important differences in the various legions of Radiolaria that they must be considered separately.

136. _Radial Spines of the Spumellaria._--The radial spines, which exhibit most manifold variations in the large order #Sphærellaria#, present characteristic differences in its four suborders. In the #Sphæroidea# their number and disposition serve for the separation into families (p. 59); the Cubosphærida (Pls. 21-25) always possess six radial main-spines, which stand opposite to each other in pairs and lie in three diameters of the shell, which are at right angles to each other and correspond to the axes of the regular crystallographic system. The Staurosphærida (Pl. 15) have four spines, which form a regular cross and stand opposite to each other in pairs, in two axes at right angles. The Stylosphærida (Pls. 13-17) show only two main-spines, which are opposed to each other in the vertical main axis of the body. Finally, the Astrosphærida (Pls. 18-20, 26-30) are characterised by a larger and variable number of radial spines (eight, twelve, twenty or more), sometimes regularly, sometimes irregularly arranged. Among the other #Sphærellaria# the #Prunoidea# (Pls. 13-17, 39, 40) are most allied to the Stylosphærida with two opposite main-spines; the #Discoidea# (Pls. 31-47), on the other hand, to the Staurosphærida with four crossed spines; there exist, however, #Discoidea# with two opposite, three marginal, or numerous radial main-spines; it is {xc}characteristic of this suborder that they all usually lie in the horizontal median plane of the lenticular shell, arising from its equatorial margin. The #Larcoidea# (Pls. 9, 10, 49, 50) show a great variety in the number and arrangement of their radial main-spines, which in the different families of this suborder stand in direct causal relation to the various forms of growth of the shell; usually the primary main-spines lie either in the three different dimensive axes, at right angles to each other, whose differentiation is characteristic of the lentelliptical Larcoid shell (§§ 34, 122) or in definite diagonal axes, which cut the former obliquely. The radial spines of the SPUMELLARIA are _never_ united in the centre of the body, but arise separately from the surface of the primary central lattice-shell (medullary shell), more rarely from one of the secondary (cortical) shells, which enclose it. Their form is originally three-edged (sometimes pyramidal, sometimes prismatic); the cause of this is to be found in their origin from the nodal points of the lattice-shell, whose meshes are primitively hexagonal; hence three trabeculæ unite in each nodal point, and are produced into three edges of the spine. Very commonly, however, the spines are round (conical or cylindrical), more rarely polygonal. The three edges are often delicately toothed, not unfrequently spirally twisted around the axis of the spine (Pl. 21, figs. 1, 12).

137. _Radial Spines of the Acantharia_.--The radial spines of this legion have a much greater significance than in the other three classes of Radiolaria, since here alone they are the primary determining factors in the skeletal structure, and grow outwards from the middle of the central capsule. This centrogenous origin of the radial spines is as characteristic of the ACANTHARIA as their chemical constitution, which is not siliceous but acanthinic (§ 102). Furthermore, their form is in most cases so peculiar that even an isolated ACANTHARIAN spine can be generally distinguished from one belonging to either of the other three legions. In the great majority of the ACANTHARIA (all #Acanthonida# and #Acanthophracta#) twenty radial spines are constantly present, which, disposed according to a definite geometrical law, make up the skeleton (compare § 110 above and p. 717). The twenty spines are generally simply apposed to each other in the centre (either by the surfaces or the edges of their pyramidal base); more rarely they are completely united and form a single star-like piece of acanthin (_Astrolithium_). Very rarely (_Acanthochiasma_) each two opposite spines are united so that ten diametric bars cross in the middle of the central capsule. Whilst in the great majority of ACANTHARIA these twenty radial spines are present, the small group #Actinelida# is characterised by the possession of an inconstant, often very large number, sometimes over one hundred. Among these #Actinelida# are probably to be found the stem-forms of the whole legion. The variously modified spines of the ACANTHARIA may be grouped in three main categories: (1) round (cylindrical or conical); (2) four-edged (prismatic or pyramidal); (3) two-edged (leaf- or sword-shaped). The latter very commonly bear two {xci}opposite transverse processes, the former four crossed ones. By ramification and union of these apophyses arise the lattice-shells of the #Acanthophracta# (excepting the Sphærocapsida).

138. _Radial Spines of the Nassellaria._--The radial spines in this legion show as great a variety in their form as in the SPUMELLARIA, and, as in them, are solid, siliceous bars, usually three-edged (prismatic or pyramidal), or round (cylindrical or conical); more seldom they are polygonal in section. The great majority of the NASSELLARIA are, however, distinguished by a triradial structure, three primary radial bars diverging from the base of the central capsule (usually from the centre of the porochora); there is usually in addition a fourth apical spine, which passes upwards vertically or obliquely on the dorsal aspect of the central capsule. These three or four typical radial spines of the NASSELLARIA may be derived with great probability from the basal tripod of the #Plectoidea# (_Plagoniscus_, _Plectaniscus_, &c., Pl. 91); and since this tripod is very characteristically combined in _Cortina_ and _Cortiniscus_ with the primary sagittal ring of the #Stephoidea#, the three typical rays may be generally designated "cortinar feet," in contradistinction to the other radial processes of the NASSELLARIAN skeleton. One of the three descending basal feet ("pes caudalis," Pls. 91-95, C) is always unpaired, and lies in the vertical median plane (or sagittal plane), just as does the vertically directed apical spine, which originally forms the dorsal bar of the sagittal ring, and is produced upwards into the "apical horn," (marked _a_ on the plates). The other two basal feet are paired, and diverge right and left, forwards and downwards ("pedes pectorales," _p.p._). Six-rayed NASSELLARIA, in which three secondary (interradial) feet are intercalated between the three primary (perradial) cortinar feet, are less common than the three-rayed forms. In some groups the number rises still higher, nine, twelve, or even more secondary feet being intercalated between the three primary. Besides, accessory radial spines may be developed on different parts of the shell, which have sometimes a definite relationship to the typical radial spines, sometimes not. Their form and ramification are very various (Pls. 51-98).

139. _Radial Spines of the Phæodaria._--The radial spines of the PHÆODARIA are very clearly distinguished from those of other Radiolaria by the fact that they are usually hollow tubes, rarely solid bars. As a rule, the tubes are cylindrical, often slightly fusiform or conical, their siliceous wall is very thin, and their lumen filled with jelly; a fine thread of silica usually runs in the axis, and in several families is connected by fine transverse threads with the wall of the tube (Pl. 110, figs. 4, 6; Pl. 115, figs. 6, 7). The peculiar family Medusettida is characterised by a very remarkable segmentation of the hollow spines (Pls. 118-120). Each tube is divided by a series of septa into chambers, which communicate by a central or excentric opening in each septum, an arrangement resembling the siphuncle of the chambered Cephalopod shells. The number and {xcii}arrangement of the radial tubes in most PHÆODARIA is indefinite and very variable; only in a few families is the number constant in each species and genus, and the disposition regular. The Medusettida (Pls. 118-120) resemble the NASSELLARIA, inasmuch as equal radial feet diverge from the base of their shell, sometimes three in number (_Cortinetta_, Pl. 117, fig. 9), sometimes four (_Medusetta_, Pl. 120, figs. 1-4), sometimes six (_Gazelletta_); _Gorgonetta_ is specially distinguished by the possession of six ascending and six descending spines regularly alternating (Pl. 119). The Tuscarorida (Pl. 100) usually have three or four equidistant feet. The Circoporida (Pls. 115-117), on the other hand, rather approach the #Sphæroidea#, their spherical or regular polyhedral shell having a definite number of tubular radial spines, which arise at regular intervals from their angles; _Circoporus_ has six, _Circospathis_ nine, _Circogonia_ twelve, and _Circorrhegma_ twenty radial tubes. Very rarely the tubes of the PHÆODARIA are angular, usually they are round, more or less cylindrical, though they are often bifurcated or even ramified, and exhibit a great wealth of the most delicate appendages; siliceous hairs, bristles, spines, barbed or anchor-like hooks, spathillæ, brushes, circlets, &c. (compare Pls. 99-128).

140. _Main-Spines and Accessory Spines._--As accessory spines (Paracanthæ) we oppose to the main-spines (Protacanthæ), just described, all those processes which have no determining influence upon the formation of the skeleton as a whole, but are to be regarded as secondary constituents of the skeleton, or appendicular organs of inferior significance. They are developed in the utmost variety, sometimes as hairs or bristles, sometimes as thorns or clubs, either straight or curved (often zigzag), smooth or barbed; sometimes standing vertically upon the shell, or directed towards the centre, sometimes obliquely, or rising at a definite angle. In those SPUMELLARIA whose lattice-shell consists of several concentric spheres, the accessory spines generally arise from the outermost, the main-spines, on the contrary, from the innermost. In the NASSELLARIA, multifarious forms of accessory spines are especially developed in the order #Plectellaria#. In the PHÆODARIA they are often furnished with delicate appendages, _e.g._, anchor-hooks, spathillæ, coronets, &c. Among the ACANTHARIA the accessory spines which arise from the surface of the shell in the #Acanthophracta# are very characteristic. They are not radially disposed (like the similar superficial spines of the SPUMELLARIA), but parallel to the radial main-spines from whose transverse processes they arise. Since in all these #Acanthophracta# the twenty radial main-spines are opposite to each other in pairs, all the accessory spines (often several hundred) are parallel to ten different regularly disposed axes of the lattice-shell (Pls. 134-138).

The skeletons of the Radiolaria, in addition to the general relations
which have been discussed above, present numerous and important special
differences in the various larger and smaller groups. These are indicated
in detail in the descriptions of the legions, orders, and families in the
systematic portion of this Report.

{xciii}BIOGENETICAL SECTION.

A SKETCH OF OUR KNOWLEDGE OF THE DEVELOPMENT OF THE RADIOLARIA IN THE YEAR 1884.

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