128. The Conchoid Skeleton.—By the name "Conchoid skeletons" are distinguished the bivalved lattice-shells which occur exclusively in the legion Phæodaria; they are quite characteristic of the Phæoconchia or Phæodaria bivalvia, which embrace three families:—Concharida (Pls. [123]-[125]), Cœlodendrida (Pls. [121], [122]), and Cœlographida (Pls. [126]-[128]). The two valves of the lattice-shell of the Concharida are simple, hemispherical, or boat-shaped, whilst in the Cœlodendrida and Cœlographida tubes grow out from them, which branch and usually give rise by anastomosis to a second external bivalved shell. In all Phæoconchia the two valves are so disposed about the central capsule that an open slit remains between them, into which open the apertures of the central capsule; and since all these Phæodaria conchoidea are Tripylea, with three typical openings in the central capsule, and since the two lateral accessory openings lie at either side of the aboral pole, and the unpaired main-opening at the oral pole of the main axis, it follows that the two valves are to be regarded as dorsal and ventral as in the Brachiopoda (not right and left as in the Lamellibranchiata). The dorsal and ventral valves are usually equal, but in a portion of the Concharida they present constant differences. In this family the two valves are attached to each other by their free edges, just as in the bivalved Mollusca and Diatoms; and these edges may either be smooth (Conchasmida, Pl. [123], figs. 1-6), or dentate (Conchopsida, Pls. [124], [125]); the valvular connection of the latter is sometimes strengthened by a special ligament which unites the two valves at the aboral pole (Pl. [123], figs. 8, 9). The form of the valve is sometimes hemispherical, sometimes boat-shaped, with a sagittal keel.
129. Medullary and Conical Shells.—In all Radiolaria whose skeleton consists of a double shell or of two concentric lattice-shells united by radial bars, an inner medullary shell (testa medullaris) and an outer cortical shell (testa corticalis) may be distinguished (see note A, below). The medullary shell is usually to be regarded as a primary, the cortical as a secondary structure. Such double shells occur among the Spumellaria in the Dyosphærida (Pls. [19], [20]), as well as in many Prunoidea (Pls. [39], [40]), Discoidea (Pls. [33], [34]), and Larcoidea (Pls. [9], [10]); among the Acantharia only in the family Phractopeltida (Pl. [133]); among the Nassellaria only in very few Cyrtoidea (e.g., Periarachnium, Pl. [55], fig. 11), and finally among the Phæodaria in the Cannosphærida (Pl. [112]) as well as in part of the Cœlodendrida (Pl. [121]) and Cœlographida (Pls. [127], [128]). In most cases (if not always?) the cortical shell arises by the growth of radial spines from the surface of the medullary shell; these become united at equal distances from the centre by transverse apophyses, the surface of the secondary calymma furnishing the basis for their secretion (§ [85]). Nevertheless, it seems that in many Sphærellaria the formation of the whole cortical shell proceeds simultaneously (at a definite dictyotic period) like that of the primary medullary shell (see note B). Whilst in the Phæodaria, Acantharia, and Nassellaria, at most two concentric shells are formed, in many Spumellaria their number increases continuously with additional growth; in many Sphærellaria it rises to four, eight, or even more, as well as in many Discoidea (if the concentric, peripherally disposed rings of chambers be regarded as incomplete flattened shells). In these cases either only the innermost primary lattice-shell is to be styled "medullary shell," or at most the two innermost (inner and outer medullary shells), all the others being cortical.
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 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.). 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 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]) 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.