4. Osculosa or Merotrypasta.—The subclass Osculosa or Merotrypasta includes the two legions Monopylea or Nassellaria, and Cannopylea or Phæodaria, which agree in the following constant and important characters:—(1) The Central Capsule is originally monaxon (ovoid or spheroidal) and retains this ground-form in most of the species. (2) The Membrane of the central capsule possesses a single large principal aperture (osculum) at the basal pole of the vertical main axis. (3) The Pseudopodia radiate from a stream of sarcode which passes out from the central capsule only on one side, namely, through the principal aperture. (4) The Equilibrium of the floating body is monostatic or unistable, since the two poles of the principal axis are always more or less different from each other. (5) The Ground-forms of the skeleton are, therefore, for the most part grammotypic (centraxon) or zygotypic (centroplan), rarely spherotypic. The two legions of the Osculosa are distinguished chiefly by the principal opening (osculum) being closed by a porous plate (porochora with its podoconus) in the Nassellaria (or Monopylea), and by a radiate cover (operculum with its astropyle) in the Phæodaria (or Cannopylea).
5. The four Legions of Radiolaria.—The four principal groups of Radiolaria, to which we have given the name "legions," are natural units, since the most important peculiarities in the structure of the central capsule are quite constant within the limits of the same legion, and since all the forms in the same legion may be traced without violence to the same phylogenetic stem. The four legions are, however, related to each other, in so far as they all exhibit those characters which distinguish the Radiolaria from other Protista. The two which compose the Porulosa (§ [3]) seem somewhat more nearly related to each other than to the two which make up the Osculosa (§ [4]). When, however, the attempt is made to bring them all into a phylogenetic relationship, it undoubtedly appears that the Spumellaria (or Peripylea) are the primitive stem, out of which the other three have been developed as independent branches. All three have been derived, probably independently, from the most ancient stem-form of the Spumellaria, the spherical Actissa.
6. Peripylea or Spumellaria.—Those Radiolaria which we call "Peripylea" on account of the constitution of their central capsule, or "Spumellaria" on account of the nature of their skeleton, are separated from the other three legions of the class by the combination of the following constant characters:—(1) The Membrane of the central capsule is single and evenly perforated all over by innumerable fine pore-canals, but without any larger principal opening (osculum). (2) The Nucleus always lies centrally in the Spumellaria monozoa and is serotinous, for it divides only at a later period into the nuclei of the spores; in the Spumellaria polyzoa it is precocious, and divides early into many small nuclei. (3) The Pseudopodia are exceedingly numerous and distributed evenly over the whole surface of the central capsule. (4) The Calymma contains no phæodium. (5) The Skeleton is seldom wanting, is never centrogenous, and is always siliceous. (6) The Ground-form of the central capsule is originally spherical (often modified); that of the skeleton is also spherical or, in the majority of cases, derived in different ways from the sphere.
7. Actipylea or Acantharia.—These Radiolaria which we call "Actipylea" on account of the constitution of their central capsule, or "Acantharia" from the formation of their skeleton, are separated from the other three legions by the combination of the following constant characters:—(1) The Membrane of the central capsule is single and perforated by numerous fine pore-canals, which are regularly distributed in series or groups, but without a larger principal opening (osculum). (2) The Nucleus is always excentric and generally precocious, since it divides early by a peculiar process of budding into numerous small nuclei. (3) The Pseudopodia are very numerous and distributed regularly in groups (or series united into a network). (4) The Calymma contains no phæodium. (5) The Skeleton is generally present, always centrogenous, and composed of acanthin. (6) The Ground-form of the central capsule is originally spherical (often modified), that of the skeleton polyaxon (often modified).
8. Monopylea or Nassellaria.—Those Radiolaria which we call "Monopylea" from the formation of their central capsule, or "Nassellaria" from the nature of their skeleton, are distinguished from the other three legions of the class by the combination of the following constant characters:—(1) The Membrane of the central capsule is single, and has only one large principal opening (osculum) at the basal pole of the vertical main axis; this osculum is closed by a perforated lid (porochora or operculum porosum) from which there arises within the central capsule a peculiar cone of threads or pseudopodia (podoconus). (2) The Nucleus is usually excentric and is always serotinous, since it only divides at a comparatively late period into spore-nuclei. (3) The Pseudopodia are not very numerous and arise by division of a single stem or bundle of threads of sarcode, which issues from the porochora. (4) The Calymma contains no phæodium. (5) The Skeleton (very rarely absent) is never centrogenous, but always extracapsular and siliceous. (6) The Ground-form of the central capsule is always monaxon (with a vertical allopolar main axis), originally ovoid, often modified; that of the skeleton is also generally monaxon, often modified (triradial or bilateral).
9. Cannopylea or Phæodaria.—Those Radiolaria which we call "Cannopylea" from the constitution of their central capsule, or "Phæodaria" on account of their peculiar phæodium, are distinguished from the other three legions by the combination of the following characters:—(1) The Membrane of the central capsule is double, consisting of a strong outer and delicate inner capsule, and has only one principal opening (osculum) at the basal pole of the vertical main axis; this osculum is closed by a radiate cover (astropyle or operculum radiatum), from the centre of which arises an external tubular spout (proboscis). Occasionally a few small accessory openings (parapylæ) are present besides the principal opening. (2) The Nucleus lies centrally or subcentrally in the capsule (in the vertical main axis), and is serotinous, inasmuch as it only divides at a late period into spore-nuclei. (3) The Pseudopodia are usually very numerous and arise from a thick sarcomatrix, formed by the spreading out of a thick stem of sarcode, which issues from the astropyle. (4) The Calymma always contains a phæodium or peculiar voluminous excentric mass of pigment. (5) The Skeleton (very rarely absent) is never centrogenous, always extracapsular and formed of a silicate of carbon. (6) The Ground-form of the central capsule is always monaxon (with a vertical allopolar main axis) and generally spheroidal; that of the skeleton is very varied.
10. Synopsis of the Subclasses and Legions:—
| First Subclass. | Second Subclass. | ||
Porulosa vel Holotrypasta. Central capsule originally spherical, without osculum or principal opening, with innumerable fine pores. | Osculosa vel Merotrypasta. Central capsule originally monaxon, with an osculum at the basal pole of the vertical main axis. | ||
| Legion I. Spumellaria. (Peripylea). | Legion II. Acantharia. (Actipylea). | Legion III. Nassellaria. (Monopylea). | Legion IV. Phæodaria. (Cannopylea). |
| Central capsule originally spherical, homaxon. | Central capsule originally spherical, homaxon. | Central capsule originally ovoid, monaxon. | Central capsule always spheroidal, monaxon. |
| Capsule-membrane single, pores innumerable, distributed all over. | Capsule-membrane single, pores numerous, regularly distributed. | Capsule-membrane single, a porous area (porochora) at the oral pole of the main axis. | Capsule-membrane always double, an astropyle (with radiate operculum) at the oral pole of the main axis. |
| Nucleus central, originally spherical (usually dividing late). | Nucleus excentric, (usually dividing early). | Nucleus excentric, near the aboral pole (dividing late). | Nucleus always spheroidal, in the main axis (dividing late). |
| Skeleton absent or siliceous, never centrogenous. | Skeleton always of acanthin, always centrogenous. | Skeleton siliceous, usually monaxon, extracapsular. | Skeleton of a silicate, always extracapsular. |
| Calymma always without phæodium. | Calymma always without phæodium. | Calymma always without phæodium. | Calymma always with phæodium. |
| First Subclass. | |
|---|---|
Porulosa vel Holotrypasta. Central capsule originally spherical, without osculum or principal opening, with innumerable fine pores. | |
| Legion I. Spumellaria. (Peripylea). | Legion II. Acantharia. (Actipylea). |
| Central capsule originally spherical, homaxon. | Central capsule originally spherical, homaxon. |
| Capsule-membrane single, pores innumerable, distributed all over. | Capsule-membrane single, pores numerous, regularly distributed. |
| Nucleus central, originally spherical (usually dividing late). | Nucleus excentric, (usually dividing early). |
| Skeleton absent or siliceous, never centrogenous. | Skeleton always of acanthin, always centrogenous. |
| Calymma always without phæodium. | Calymma always without phæodium. |
| Second Subclass. | |
Osculosa vel Merotrypasta. Central capsule originally monaxon, with an osculum at the basal pole of the vertical main axis. | |
| Legion III. Nassellaria. (Monopylea). | Legion IV. Phæodaria. (Cannopylea). |
| Central capsule originally ovoid, monaxon. | Central capsule always spheroidal, monaxon. |
| Capsule-membrane single, a porous area (porochora) at the oral pole of the main axis. | Capsule-membrane always double, an astropyle (with radiate operculum) at the oral pole of the main axis. |
| Nucleus excentric, near the aboral pole (dividing late). | Nucleus always spheroidal, in the main axis (dividing late). |
| Skeleton siliceous, usually monaxon, extracapsular. | Skeleton of a silicate, always extracapsular. |
| Calymma always without phæodium. | Calymma always with phæodium. |
11. Individuality of the Radiolaria.—Like other Protozoa the Radiolaria are unicellular organisms, the whole fully developed organisation of which falls under the category of a single cell, both morphologically and physiologically. Since this view is based upon the composition of the individual body out of two different morphological elements, nucleus and protoplasm, it is at once justified in the case of the majority of Radiolaria, in which the plasmatic body encloses only a single nucleus (the so-called "Binnen-Bläschen"); such is the case in all the Spumellaria monozoa, Nassellaria and Phæodaria. This aspect of the case might appear doubtful in those Radiolaria in which the simple primary cell-nucleus divides early into numerous small secondary nuclei, as is the case in the Spumellaria polyzoa and most Acantharia. Strictly speaking, the multinucleate central capsule should in such cases be regarded as a syncytium; but since the individual unity of the unicellular organism is as clearly defined in these precocious multinuclear Radiolaria as in the ordinary serotinous forms, the former must be considered unicellular Rhizopods just as are the latter. This mode of regarding the case is the more necessary, inasmuch as the early division of the nucleus has no further influence upon the organisation. Just as in many other classes of the Protista there are monozootic (solitary) and polyzootic (social) forms, so also in the Radiolaria there are in addition to the ordinary monozootic or monobious forms certain families in which colonies or cœnobia are formed by the association of individuals; this distinction may be expressed by the terms "Monocyttaria" and "Polycyttaria."