The astonishing number of new and interesting forms of Cyrtida which I found in the rich collection of the Challenger (beginning from 1876), and mainly in the Radiolarian ooze of the Central Pacific (Stations 263 to 274), enabled me to give in my Prodromus, in 1881, a greatly enlarged and amended system of this important group. I separated there the Spyroidea (= Zygocyrtida), and the Botryodea (= Polycyrtida) from the true Cyrtoidea by restricted definition, pointing out the essential differences in the structure of the cephalis in these groups of Cyrtellaria. The latter name, as here used, is therefore identical with the "Cyrtida" of my Monograph. In the Prodromus I divided the true Cyrtida (p. 426) into five subfamilies and thirty tribes, corresponding to the differences in the number of the shell-joints and of the radial apophyses, and in the shape of the closed or open mouth. These groups are here retained, but reduced to four families and twenty-four subfamilies, since the Tetracyrtida are better united with the Stichocyrtida (compare below).
Richard Hertwig in his work Organismus der Radiolarien (1879, pp. 74 to 86) gave the first accurate description of the finer structure of the central capsule of the Cyrtida, and pointed out their character as true Monopylea, with porochora and podoconus, and the peculiar shape of its nucleus. He also published excellent figures of some interesting new species.
O. Bütschli, 1882, in his valuable paper entitled: "Beiträge zur Kentniss der Radiolarien-Skelette, insbesondere der der Cyrtida" (Zeitschr. für wiss. Zool., vol. xxxvi. p. 485) made an attempt at a natural classification of the Cyrtida, which he derived from the Spyroidea or Zygocyrtida. As already mentioned above, we cannot accept this essay as the foundation of a true natural system, since the affinities of the Cyrtellaria (and of the Nassellaria as a whole) are far more complicated and difficult than Bütschli supposed. His views were supported by accurate observations only on the structure of the fossil Cyrtoidea of Barbados; these, however, represent the minority only of the genera, and many interesting and important forms (mainly of true "Monocyrtida") remained unknown to Bütschli. A great part, however, of his observations are very useful, and his remarks on comparative morphology are very suggestive.
The Cyrtoidea may be divided into families and subfamilies according to three different principles, viz., (1) the number of joints into which the shell is divided by transverse strictures; (2) the number of radial apophyses which arise from the shell; (3) the shape of the basal mouth, which is either open or closed by a lattice-plate. At present every attempt of classification in this large group must be more or less artificial, since the affinities of the numerous smaller and larger groups are extremely complicated, and the ontogeny, the only sure guide in this phylogenetical labyrinth, is perfectly unknown. It seems therefore the most convenient to employ for our artificial classification, first, the number of shell-joints, second, the radial structure, and third, the shape of the mouth.
A. The number of joints into which the shell is divided by transverse constrictions, serves here for the distinction of four primary groups or suborders of the Cyrtoidea, viz., (1) Monocyrtida with one joint; (2) Dicyrtida with two joints; (3) Tricyrtida with three joints; and (4) Stichocyrtida with four or more joints. In my Prodromus (1881, p. 426) I divided the latter group into Tetracyrtida (with four joints), and Stichocyrtida (with five or more joints); but these two groups may be united, since the fourth and all the succeeding joints are of rather indifferent shape and of little morphological value. The three first joints, however, are usually very different and possess a high morphological importance, so that we distinguish the first joint as cephalis, the second as thorax, and the third as abdomen. The uppermost transverse constriction, which separates the two first joints, cephalis and thorax, is the collar stricture and is usually caused by an internal fenestrated septum, the cortinar septum. The second constriction, which separates the second and third joints (thorax and abdomen) is called the lumbar constriction. The following constrictions (in the Stichocyrtida) are indifferent and of little morphological interest, and require therefore no peculiar designation.
B. The radial structure, indicated by radial apophyses arising from the shell, offers three principal differences, according to which the whole group of Cyrtoidea may be divided into three large groups or sections, viz., (1) Pilocyrtida, or Cyrtoidea triradiata, with three radial apophyses; (2) Astrocyrtida, or Cyrtoidea multiradiata, with numerous radial apophyses (four to nine or more); and (3) Corocyrtida, or Cyrtoidea eradiata, without external radial apophyses. The majority of Cyrtoidea are Pilocyrtida, with three radial apophyses, which are probably homologous to the three primary feet of the Plectoidea and of Cortina (therefore "cortinar feet"). The Astrocyrtida, or the Cyrtoidea with a variable number of radial apophyses (at least four to six) may be derived from the Pilocyrtida by interpolation of secondary or interradial apophyses between the three primary or perradial apophyses. The Corocyrtida, however, or the Cyrtoidea without external radial apophyses, may have originated by reduction and loss of the latter, either from the Pilocyrtida or from the Astrocyrtida.
C. The shape of the basal mouth in the Cyrtoidea exhibits two essential differences only, viz., (1) the terminal mouth of the shell is a simple wide opening in the Stomocyrtida, or (2) the terminal mouth is closed by a lattice-plate, in the Clistocyrtida. As these two different cases occur in all the twelve families, which we have distinguished according to the differences in the number of joints and in the radial structure, we get altogether twenty-four subfamilies which are synoptically arranged in the following table:—
| Synopsis of the four sections, twelve families and twenty-four subfamilies of CYRTOIDEA. | PILOCYRTIDA. Cyrtoidea triradiata. (Three radial apophyses.) | ASTROCYRTIDA. Cyrtoidea multiradiata. (Four to nine or more apophyses.) | COROCYRTIDA. Cyrtoidea eradiata. (No radial apophyses.) | |||
|---|---|---|---|---|---|---|
| Mouth of the shell. | Aperta. | Clausa. | Aperta. | Clausa. | Aperta. | Clausa. |
| MONOCYRTIDA. (Cyrtoidea monothalamia). | Tripocalpida. | Phænocalpida. | Cyrtocalpida. | |||
| Archipilida. | Archiperida. | Archiphormida. | Archiphænida. | Archicorida. | Archicapsida. | |
| DICYRTIDA. (Cyrtoidea dithalamia). | Tripocyrtida. | Anthocyrtida. | Sethocyrtida. | |||
| Sethopilida. | Sethoperida. | Sethophormida. | Sethophænida. | Sethocorida. | Sethocapsida. | |
| TRICYRTIDA. (Cyrtoidea trithalamia). | Podocyrtida. | Phormocyrtida. | Theocyrtida. | |||
| Theopilida. | Theoperida. | Theophormida. | Theophænida. | Theocorida. | Theocapsida. | |
| STICHOCYRTIDA. (Cyrtoidea polythalamia). | Podocampida. | Phormocampida. | Lithocampida. | |||
| Stichopilida. | Stichoperida. | Stichophormida. | Stichophænida. | Stichocorida. | Stichocapsida. | |
| Synopsis of the four sections, twelve families and twenty-four subfamilies of CYRTOIDEA. | PILOCYRTIDA. Cyrtoidea triradiata. (Three radial apophyses.) | |
|---|---|---|
| Mouth of the shell. | Aperta. | Clausa. |
| MONOCYRTIDA. (Cyrtoidea monothalamia). | Tripocalpida. | |
| Archipilida. | Archiperida. | |
| DICYRTIDA. (Cyrtoidea dithalamia). | Tripocyrtida. | |
| Sethopilida. | Sethoperida. | |
| TRICYRTIDA. (Cyrtoidea trithalamia). | Podocyrtida. | |
| Theopilida. | Theoperida. | |
| STICHOCYRTIDA. (Cyrtoidea polythalamia). | Podocampida. | |
| Stichopilida. | Stichoperida. | |
| ASTROCYRTIDA. Cyrtoidea multiradiata. (Four to nine or more apophyses.) | ||
| Mouth of the shell. | Aperta. | Clausa. |
| MONOCYRTIDA. (Cyrtoidea monothalamia). | Phænocalpida. | |
| Archiphormida. | Archiphænida. | |
| DICYRTIDA. (Cyrtoidea dithalamia). | Anthocyrtida. | |
| Sethophormida. | Sethophænida. | |
| TRICYRTIDA. (Cyrtoidea trithalamia). | Phormocyrtida. | |
| Theophormida. | Theophænida. | |
| STICHOCYRTIDA. (Cyrtoidea polythalamia). | Phormocampida. | |
| Stichophormida. | Stichophænida. | |
| COROCYRTIDA. Cyrtoidea eradiata. (No radial apophyses.) | ||
| Mouth of the shell. | Aperta. | Clausa. |
| MONOCYRTIDA. (Cyrtoidea monothalamia). | Cyrtocalpida. | |
| Archicorida. | Archicapsida. | |
| DICYRTIDA. (Cyrtoidea dithalamia). | Sethocyrtida. | |
| Sethocorida. | Sethocapsida. | |
| TRICYRTIDA. (Cyrtoidea trithalamia). | Theocyrtida. | |
| Theocorida. | Theocapsida. | |
| STICHOCYRTIDA. (Cyrtoidea polythalamia). | Lithocampida. | |
| Stichocorida. | Stichocapsida. | |
The cephalis, or the first shell-joint of the Cyrtoidea, is in the majority homologous with the cephalis of the Spyroidea, from which it differs in the reduction of the sagittal ring and the absence of the corresponding sagittal constriction; its cavity is therefore simple, not bilocular. Its homology with the original cephalis of the Spyroidea cannot be doubted, when its base exhibits the typical basal pores of the Semantida. But in many cases these are wanting, and in a great number of Cyrtoidea (mainly of Monocyrtida) there is more or less evidence that the original cephalis is lost, and that the real first joint is the thorax, the original second joint. At present it is quite impossible to distinguish between the former and the latter shells, and therefore in the following descriptions the first joint is always named cephalis and the second thorax. In future, when the affinities of the Cyrtoidea become better known, it will be necessary to distinguish the "Archicephalis," or true cephalis of all Spyroidea and of the majority of Cyrtoidea, from the "Pseudocephalis" or the false cephalis of the minority (e.g., of many Monocyrtida aperta, Archipilida, Archiphormida, Archicorida, &c.).