195. Phæocystina and Phæocoscina.—Whilst the malacoma of all Phæodaria possesses the characteristics of the legion, and hence justifies the assumption of a monophyletic origin, the skeleton, on the other hand, shows in the different groups such manifold and fundamental variations that a polyphyletic origin of the latter is indubitable. Different Phæodinida have commenced the formation of the skeleton independently, and it has progressed in different directions. In the Phæocystina it remained incomplete and led to the formation of various Beloid skeletons, whilst the Phæocoscina developed complete lattice-shells. Both of these divisions too are to be regarded as polyphyletic, since the skeletal forms of the different groups cannot be derived without violence from a common primitive form.
196. Phæocystina with a Beloid Skeleton.—The order Phæocystina includes all Phæodaria which have no complete lattice-shell; it contains, firstly, the skeletonless Phæodinida (the common stem-group of the legion), and secondly, the Phæacanthida, or Phæodaria with a Beloid skeleton (§ [115]). The latter are divisible into several very different groups (at least two or three) which are probably different in origin. The Aulacanthida (Pls. [102]-[105]) form radial tubes which perforate the calymma, their proximal end resting upon the surface of the central capsule, whilst the distal extremity projects freely outwards. The skeleton of the Cannorrhaphida, on the other hand, is composed of many separate portions which are never radially arranged but are either placed tangentially to the surface of the calymma or scattered irregularly in its gelatinous mass. Furthermore, in the three subfamilies of which this family is composed, the individual skeletal portions are so different that they have probably arisen independently of each other; in the Cannobelida they form cylindrical tangential tubes (Pl. [101], figs. 3-5), in the Catinulida flat basin or cap-like structures (Pl. [117], fig. 8), in the Dictyochida hollow rings, from which small pyramids are developed by unilateral formation of lattice-work (Pl. [101], figs. 9-14; Pl. [114], figs. 7-12).
197. Phæosphæria with a Sphæroid Skeleton.—The order Phæosphæria includes those Phæodaria which possess a spherical (sometimes slightly modified) lattice-shell without the characteristic aperture of the Phæogromia. They have probably arisen independently of these, though they may have been derived from the Castanellida by loss of the shell-aperture, which was present originally. The four families which we have distinguished among the Phæosphæria, are so different in the structure of their lattice-shell that their phylogenetic connection is doubtful. In the Orosphærida (Pls. [106], [107]) and the Sagosphærida (Pl. [108]) the whole lattice-shell consists of a single piece and is unjointed (without astral septa); in the former it is very firm and massive, with thick laminated trabeculæ and polygonal meshes; in the latter it is very delicate and brittle, with filiform trabeculæ and large triangular meshes. On the other hand, the voluminous shell of the Aulosphærida (Pls. [109]-[111]), and of the Cannosphærida (Pl. [112]), is characterised by a very peculiar system of joints; it is composed of numerous separate cylindrical tubes, which are placed tangentially and united at the nodes by stellate partitions or astral septa. The Cannosphærida possess further a simple central Cyrtoid shell, connected with the outer jointed shell by hollow radial trabeculæ. Since many Aulosphærida possess rudiments of such centripetal trabeculæ it is possible that these latter have been derived from the former by the loss of the central Cyrtoid shell; the formation of this monaxon shell perhaps indicates descent from the Phæogromia (Castanellida).
198. Phæogromia with a Cyrtoid Skeleton.—That order of the Phæodaria which we designate Phæogromia, contains many very different forms, all agreeing in the possession of a Cyrtoid skeleton, or a monaxon lattice-shell, which has a large aperture at one pole of its vertical main axis (§ [123]). This Cyrtoid skeleton is sometimes ovoid or conical, sometimes lentiform or helmet-shaped, sometimes polyhedral or almost spherical. Although the principle of its structure is simple and often like that of the Monocyrtida among the Nassellaria, yet the structure of the wall and of the apophyses is so different in the various groups of the Phæogromia, that the order is probably polyphyletic, and its Cyrtoid shells have arisen independently of each other. Only in the Castanellida (Pl. [113]) does the shell-wall usually consist of simple lattice-work; in the Challengerida, on the other hand (Pl. [99]), it has an extremely fine Diatom-like structure; in the Medusettida (Pls. [118]-[128]) a peculiar alveolar structure, and in the Circoporida (Pls. [114]-[117]) and Tuscarorida (Pl. [100]) it possesses a characteristic porcellanous constitution (with tangential spicules in a porous cement-mass); in the latter of these groups the surface is smooth, in the former peculiarly tabulate; the two families have also different stem-forms.
199. Phæoconchia with a Conchoid Shell.—The order Phæoconchia (Pls. [121]-[128]) is separated not only from all other Phæodaria, but also from all other Radiolaria, by the possession of a bivalved shell resembling that of a Lamellibranch; the two valves of this Conchoid skeleton are to be interpreted as dorsal and ventral (§ [128]). Probably these bivalved shells are independent products, but possibly they may have been formed by the bisection of a simple spherical lattice-shell; in the former case the Phæoconchia would be directly descended from the Phæodinida, in the latter from the Castanellida. The three families which we have distinguished among the Phæoconchia, probably constitute a connected stem, the most primitive group of which are the Concharida (Pls. [123]-[125]). From these the Cœlodendrida (Pls. [121], [122]) have next arisen by the formation of a "galea" upon the apex of each valve, and the growth of hollow tubes from this helmet-like structure. Finally, the Cœlographida (Pls. [120]-[128]) have been developed from the Cœlodendrida by the formation of a basal nasal tube (rhinocanna) from each galea, and the formation of a median or paired frenulum, which connects the opening of the nasal tube with the apex of the galea. In the Cœlodendrida, as well as in the Cœlographida, there are two different subfamilies, of which the more primitive (Cœlodorida, Cœlotholida) have free branches from the hollow radial tubes, whilst the more recent (Cœlodrymida, Cœloplegmida) form an outer bivalved shell by anastomosis of the branches of the tubes.
200. The Fundamental Biogenetic Law.—The causal connection between ontogeny and phylogeny, which finds its most precise statement in the fundamental biogenetic law, holds in general for the Radiolaria as for all other organisms. In order to furnish direct proof of this, however, a complete empirical knowledge both of individual and of palæontological development would be necessary. In both these directions, as has been shown in the foregoing chapters, our knowledge of the Radiolaria is very incomplete and fragmentary, but still we are able to convince ourselves indirectly of the validity of the law as applied to Radiolaria by the aid of comparative anatomy. This is now so fully known to us (§§ [1]-[140]) that we are able not only to draw a complete and satisfactory picture of their morphology, but also to arrive at most important conclusions regarding the ontogeny and phylogeny of the individual groups. As regards the formation of the multiform skeleton of the Radiolaria, most of the ontogenetic series of forms, with which we have become acquainted by comparative anatomy, are of palingenetic nature; that is, they are primarily due to inheritance and thus of direct phylogenetic significance. On the other hand, among the ontogenetic phenomena of the Radiolaria, as far as they have yet been investigated, only very few are cenogenetic, that is, brought about by adaptive modification and without direct significance as regards phylogeny.